Cnemidophorus Background and Classification

Teiid whiptail lizards of the genus Cnemidophorus range widely in the New World, extending from the northern United States southward to Argentina, and occupy many diverse ecological communities. However, while exhibiting this extensive distribution, their greatest diversity occurs in North America, where they are a conspicuous component of the herpetofauna of the arid and semiarid regions of the southwestern U.S. and Mexico. By conservative count, there are approximately 50 species known (for recent summaries see Maslin and Secoy, 1986; Wright, 1993), with new species continuing to be found (e.g., Markezich et al., 1997; Rocha et al., 1997, 2000).

Because of their abundance and conspicuous nature, whiptails are an ecologically important squamate lizard clade, which is reflected by the great number of ecological and life history studies conducted on this group (reviewed in Wright and Vitt, 1993). Cnemidophorus has been (and continues to be) one of the most extensively studied genera of lizards, third only to Sceloporus and Anolis (Dunham et al., 1988). Besides their abundance and geographic proximity to North American biologists, one of the reasons whiptails have been so intensively studied is the occurrence of parthenogenetic all-female species (of interspecific hybrid origin; see below) within this diverse clade. Approximately one-third of the described species are unisexual, with the majority of these all-female species occurring in the southwestern U.S. and northern Mexico (Wright, 1993). Diploid and triploid unisexual species have evolved many times in Cnemidophorus, in each instance the switch from sperm-dependent to sperm-independent reproduction occurring in one generation in an F₁ interspecific hybrid (for reviews, see Darevsky et al., 1985; Dessauer and Cole, 1989; Moritz et al., 1989a, 1992a; Darevsky, 1992; Cole and Dessauer, 1995), and dynamic hybridization presently occurs in nature (e.g., Walker et al., 1989; Dessauer et al., 2000; Taylor et al., 2001). Consequently, whiptail lizards are used broadly in research, particularly in reproductive biology, population genetics, physiological ecology, and evolutionary biology, often with emphasis on the instantaneous, multiple and independent origins of parthenogenetic cloning.

The species of Cnemidophorus are currently allocated to six species groups (table 1). Based on external morphology and karyology, these groups were erected by Lowe et al. (1970), who modified Burt's (1931) arrangement. All except the lemniscatus group are confined to North and Central America. The lemniscatus group is largely a South American radiation, with only a single species (C. lemniscatus) extending into Central America. Two of the northern Cnemidophorus species groups (cozumela and tesselatus) are composed entirely of parthenogenetic species. The origins of the unisexual species in both of these groups involve hybridization between bisexual species from different species groups (i.e., sexlineatus group ;ts deppii group = cozumela group; sexlineatus group ;ts tigris group = tesselatus group). The lemniscatus and sexlineatus groups each possess bisexual and unisexual species. However, unlike the aforementioned completely unisexual groups, the unisexuals in the lemniscatus and sexlineatus groups are derived exclusively from hybridizations between species within their respective groups (intragroup hybridizations).

Higher-Level Relationships and Cnemidophorus Monophyly

While Cnemidophorus has been extensively studied and much is known about its biology, ecology, and natural history, the specific phylogenetic placement of Cnemidophorus within the Teiidae, as well as the higher-level relationships within Cnemidophorus, has received little attention. Presch (1974) provided osteological evidence that the macroteiids consisted of two major groups: Teiini (including Ameiva, Cnemidophorus, Dicrodon, Kentropyx, and Teius) and Tupinambini (including Callopistes, Crocodilurus, Dracaena, and Tupinambis). Within the Teiini, Ameiva, Cnemidophorus, and Kentropyx shared the most similarities, leading Presch to hypothesize that these three taxa were more closely related to each other than any were to Dicrodon or Teius. However, there were no derived osteological characters provided to resolve the relationships among Ameiva, Cnemidophorus, and Kentropyx. Informally we refer to these three very similar taxa as the cnemidophorines.

Using external morphology and intuition, Burt (1931) was the first to hypothesize higher-level relationships within Cnemidophorus (fig. 1A). Because members of the South American lemniscatus group shared some characteristics with other South American teiids (e.g., Ameiva), Burt (1931) postulated that the lemniscatus group was the most primitive lineage within Cnemidophorus. The ancestor of the North American groups was hypothesized to have been derived from the lemniscatus group, with this lineage giving rise to the deppii (excluding C. hyperythrus) and sexlineatus groups. Burt (1931) also proposed that his tesselatus group (including the as-yet-to-be-described tigris group) and hyperythrus groups were derived from the sexlineatus group.

Based on karyology, external morphology, and knowledge of the existence of unisexual species, Lowe et al. (1970) modified the higher-level classification and hypothesized relationships within Cnemidophorus. The evolutionary scenario (fig. 1B) proposed by Lowe et al. (1970) was largely influenced by their assumption that the chromosomes of vertebrates evolve primarily by means of Robertsonian centric fusion, thus resulting in the reduction of diploid chromosome number. Members of the deppii group possess the highest diploid number (2n = 52) within Cnemidophorus. Given this, Lowe et al. (1970) suggested that the deppii group (including the cozumela group) represented the most “primitive” lineage within Cnemidophorus, possessing a karyotype essentially identical to that of the hypothesized ancestor of Cnemidophorus. Such a conclusion differed from Burt (1931), who suggested that the lemniscatus group was ancestral to the remaining Cnemidophorus species groups. Lowe et al. (1970) postulated that the lemniscatus group evolved from a deppii-like ancestor, requiring only a single centric fusion to derive the lemniscatus group karyotype (2n = 50) from the deppii group/ancestral karyotype. The sexlineatus and tigris groups were proposed to be sister taxa, with their common ancestor being derived from a deppii-like ancestor (via three centric fusions).

Based on mitochondrial DNA restriction site data, Moritz et al. (1992a) provided the first explicit phylogenetic analysis of higher-level relationships within Cnemidophorus. In that study, C. lemniscatus was used to root the resulting phylogeny. This outgroup choice was based on Burt (1931) and the fact that the greatest observed genetic distances were between C. lemniscatus and the remaining Cnemidophorus species (see also Dessauer and Cole, 1989). Moritz et al. (1992a) provided strong support for a sister group relationship between the sexlineatus and tigris groups, corroborating the hypothesis of Lowe et al. (1970). These mitochondrial data also supported the placement of the deppii group as the sister taxon to the sexlineatus group + tigris group clade. Monophyly of the deppii and sexlineatus groups was also supported by Moritz et al. (1992a). However, because of the limited sampling, these conclusions could only be considered preliminary. Even so, the relatively large estimated sequence divergences between C. lemniscatus and the remaining Cnemidophorus species are suggestive of a relatively basal position for the lemniscatus group. However, this study cannot be viewed as a rigorous test of the basal relationships within Cnemidophorus (e.g., hypotheses of Burt, 1931 vs. Lowe et al., 1970). Such a test would require the inclusion of other closely related teiine taxa (e.g., Ameiva, Kentropyx) as outgroups.

While there have been previous attempts to organize Cnemidophorus into species groups and hypothesize on the interrelationships of these groups, there has never been a rigorous attempt to demonstrate the monophyly of this group of lizards. All previous studies generally assumed that Cnemidophorus was monophyletic, based on the phenetic similarity between Cnemidophorus and other South American teiid lizards (i.e., Ameiva, Dicrodon, Kentropyx, and Teius). Historically, Cnemidophorus has been defined by the absence of presumably derived character states exhibited by these other South American genera (i.e., laterally compressed teeth in Dicrodon and Teius, keeled ventral scales in Kentropyx, basal tongue sheath in Ameiva). The long recognition that Cnemidophorus lacked apomorphies, and earlier hypotheses suggesting that various lineages of Cnemidophorus were independently derived from ancestral South American “stocks” (e.g., Burt, 1931; Lowe et al., 1970) suggest that Cnemidophorus monophyly is in question and should be rigorously tested. Although taxon sampling was limited (ingroup taxa = three Cnemidophorus species groups, Ameiva, and Kentropyx), a phylogenetic study using allozymes by Dessauer and Cole (1989) provided support for Cnemidophorus paraphyly, with the lemniscatus group hypothesized to be more closely related to Kentropyx than to a clade containing the sexlineatus and tigris groups.

Objectives of the Present Study

As the use of Cnemidophorus increases in research and the literature mushrooms, it becomes increasingly important to establish the validity of this taxon as a monophyletic group, if indeed it is. Dessauer and Cole (1989) provided preliminary evidence suggesting Cnemidophorus paraphyly. However, their taxon sampling was limited and/or incomplete (e.g., absence of the deppii group and other critical cnemidophorine lineages). Thus, it is timely to more rigorously examine the phylogenetic relationships between Cnemidophorus and other teiine taxa (Ameiva, Dicrodon, Kentropyx, and Teius), particularly now that the necessary samples are available. The inferred phylogenetic relationships presented below are based on diverse types of data. The bulk of these data are derived from mitochondrial ribosomal RNA (rRNA) genes, but these data are augmented with previously published allozyme data (Dessauer and Cole, 1989; Cole and Dessauer, 1993; Cole et al., 1995; Markezich et al., 1997) and morphological characters traditionally used in Cnemidophorus systematics.

The following questions are addressed in this paper: (1) Is Cnemidophorus a monophyletic group? (2) If not, what nomenclatural changes are needed and appropriate at this time? (3) What are the relationships between Cnemidophorus and the other teiinine genera? (4) Are the traditionally recognized bisexual species groups within Cnemidophorus monophyletic, and what is their relationship to each other? Finally, (5) Do the newly inferred higher-level relationships require reexamination of past hypotheses of chromosomal evolution within Cnemidophorus? In addition we comment briefly on the reticulate phylogeny of unisexual clones of hybrid origin and determination of the maternal ancestor of Kentropyx borckiana, a unisexual species of hybrid origin.

Choice of Taxa

Twenty-seven recognized Cnemidophorus taxa were included in the present study, representing all currently recognized bisexual species groups (deppii, lemniscatus, sexlineatus, and tigris species groups; Wright, 1993). This sample allows a preliminary test of the monophyly of these groups. Also, several additional non-Cnemidophorus teiine species were included in order to test Cnemidophorus monophyly. In all, 41 ingroup taxa (= Ameiva, Cnemidophorus, Dicrodon, Kentropyx, and Teius) were included (appendix 1).

The following five outgroup taxa (successively more distant) were included also: Tupinambis (Teiidae), Pholidobolus (Gymnophthalmidae), Acanthodactylus and Lacerta (Lacertidae), and Eumeces (Scincidae). The relationships of these outgroups to the ingroup are fairly well understood (Estes et al., 1988; Lee, 1998). However, to minimize outgroup assumptions, a global parsimony rooting approach was taken (Maddison et al., 1984), with Eumeces (assumed to be the most distantly related outgroup) being used to root the overall resulting tree(s).

Molecular Data

DNA Data: Total genomic DNA was isolated from small amounts of liver or erythrocytes (∼100 mg) following the phenol-chloroform extraction protocol of Hillis et al. (1996). Two portions of the mitochondrial genome were amplified using the polymerase chain reaction (PCR) in Perkin-Elmer 2400 or Ericomp TwinBlock thermocyclers. One PCR product was a ∼380 bp fragment from the 12S ribosomal RNA (rRNA) gene. The other PCR product was a ∼500 bp fragment from the 16S rRNA gene. The primers and PCR parameters used to amplify these fragments are described in Reeder (1995). Purification of amplified DNA and automated DNA sequencing were performed following methods described in Wiens and Reeder (1997). The DNA sequences for Acanthodactylus cantoris and Lacerta agilis were obtained from GenBank (accession numbers AF080298, AF080300, AF080344, and AF080346).

The mitochondrial rDNA sequences (appendix 2) were aligned under varying gap costs (opening gap cost of 6, 9, and 12) using the multiple sequence alignment program Clustal W (Thompson et al., 1994). Sequence alignment procedures and parameters are described in Wiens and Reeder (1997). It has been demonstrated that rRNA secondary structure models can be useful in the alignment of these gene sequences (Kjer, 1995; Titus and Frost, 1996). Following the procedure outlined in Wiens and Reeder (1997), rRNA secondary structure information was used to assist in DNA sequence alignment. Regions of sequence were considered alignment-ambiguous if nucleotide positional homologies differed among the different gap cost alignments (Gatesy et al., 1993). Ambiguously aligned regions were excluded from phylogenetic analysis. In all, 1072 nucleotide positions were aligned (491 12S and 581 16S; appendix 2), with 61 positions (25 12S and 36 16S) excluded from phylogenetic analysis. Gaps (= insertion/deletion events) were coded as a fifth character state, as described in Wiens and Reeder (1997). All DNA sequences are deposited in GenBank (accession numbers AY046420–AY046503, AF080344, AF080346, AF080298, and AF080300). Upon request, the PAUP* matrix is available from one of us (T.W.R.).

We followed Dessauer et al. (1996) in using allele-specific oligonucleotide probes to screen multiple individuals of Kentropyx borckiana to determine the maternal ancestor of this unisexual species.

Allozyme Data: Data on 31 phylogenetically informative protein loci (= characters) were scored for 19 taxa of teiid lizards. The entire allozyme database was produced in one laboratory (H.C.D.'s), so there is complete internal consistency across the data set. Data are the alleles detected at individual gene loci. For phylogenetic analysis, each locus was interpreted as the character and the alleles present in a taxon as character states (Buth, 1984). All allozyme characters were analyzed unordered.

The gene loci and codes for phylogenetic analysis of the allozymes are presented in appendix 3. The data were published previously in the following reports: Dessauer and Cole, 1989 (Ameiva, Cnemidophorus, Kentropyx, and Tupinambis); Cole and Dessauer, 1993 (South American Cnemidophorus); Cole et al., 1995 (Kentropyx); and Markezich et al., 1997 (South American Cnemidophorus). However, this is the first report in which all of these data have been cross-correlated, so the individual alleles as specified in this report (appendix 3) will not necessarily bear the same letter designation as in those original papers, some of which were alphabetized only on the basis of the alleles being compared within the individual report.

Methods of collecting, preparing, and storing tissue samples, and methods of conducting protein electrophoresis, identifying loci, and determining allele products present in the various species are detailed in the papers cited above and relevant references therein (also see Dessauer et al., 2000). The data are of discrete characters that could be scored unambiguously. Although most loci for each taxon show no intraspecific variation or polymorphism, some do. In cases where two or more alleles were recorded for a taxon, each allele was recorded as present at that locus for that taxon. We did not attempt to use frequency data (we used only presence or absence of allele character states) because degree of variability varies widely among loci, it can vary geographically, and because sample sizes vary widely among the taxa. For example, we examined only one specimen of Tupinambis teguixin and more than 35 of Cnemidophorus inornatus. The problems associated with geographic variation and sample size are illustrated by Dessauer et al. (2000), who examined more than 650 individuals of Cnemidophorus tigris. We did not try to integrate all of their data on rare alleles into this report.

Morphological Data

Data on the 10 morphological characters were recorded for 42 taxa of teioid lizards (including Pholidobolus and two populations of Kentropyx altamazonica). These taxa include all of the teiids for which DNA sequence data were analyzed. Because of problems with homology assessment, morphological data were not coded for any of the non-teioid taxa. Data were recorded from museum specimens, which are specified in appendix 1 (Specimens Examined).

These characters have historically been useful in recognizing generic and subgeneric species groups within the Teiidae, as suggested by previous authors (Burt, 1931; Lowe et al., 1970; Peters and Donoso-Barros, 1970; Hoogmoed, 1973). While not a large set of characters, we felt it was better to include these traditional characters than to exclude them. It has been demonstrated that even a small number of morphological characters (within the context of a large combined data set largely consisting of molecular characters) can have an effect on a phylogenetic analysis (e.g., Titus and Larson, 1996). The character descriptions, coding, and matrix are presented in appendix 4. All were discrete characters that could be scored unambiguously and for which there was little intraspecific variation.

Phylogenetic Analysis

The mtDNA, allozymic, and morphological data were combined into a single data matrix for phylogenetic analysis. Taxa missing a particular subset of the total data (e.g., allozymes) were coded as missing (?) those data. Phylogenetic analyses were performed with PAUP* 4.0b2 (Swofford, 1999). The heuristic tree search routine was used (with TBR branch swapping and 100 random taxon additions). When multiple shortest trees were discovered, the trees were summarized with a strict consensus tree (Sokal and Rohlf, 1981), thus depicting only those relationships shared among all shortest trees. A character state change was considered to unambiguously support a clade if it was placed along a branch by both ACCTRAN (Farris, 1970) and DELTRAN (Swofford and Maddison, 1987) optimizations.

Initial phylogenetic analyses were performed with uniformly weighted characters (i.e., all character state transformations had a weight of 1, irrespective of data type). However, it is fairly well understood that vertebrate mtDNA exhibits substitution biases (e.g., transitions occurring more rapidly than transversions), and different sites or regions (e.g., third codon positions, stem vs. loop regions) evolve at different rates. Thus, differential weighting of nucleotide substitutions and/or sites may be warranted. Seemingly realistic and justifiable weighting schemes can be devised for the DNA data at hand (e.g., Arevalo et al., 1994; Cunningham, 1997; Wiens et al., 1999). However, philosophical and methodological difficulties arise within the context of a combined phylogenetic analysis (e.g., what weight is applied to morphological characters vs. the differentially weighted nucleotide substitutions?). Also, different genes within a combined analysis may have different substitution properties (e.g., are the best character state transformations of gene A equivalent to those of gene B?). An objective way to differentially weight characters within the context of a combined analysis is to use the a posteriori method of successive approximations (Farris, 1969; Carpenter, 1988). Such a weighting strategy differentially weights all the characters based on their relative degrees of homoplasy. Those characters most consistent with the initial starting tree are given the greatest weights, regardless of data partition (i.e., DNA, allozymes, morphology). In our study, the initial tree(s) for successive weighting was that inferred from a uniformly weighted combined data analysis. Reweighting characters was performed in PAUP*, using the maximum rescaled consistency index (rci; Farris, 1989) (base weight = 100; weights truncated instead of being rounded [as in Hennig86]).

While originally envisioning means of objectively determining character weights, Kluge (1997a, 1997b) has recently argued that all character weighting (a priori and a posteriori) should be rejected. Kluge states that all forms of differential character weighting invoke additional background knowledge about biological processes that are untestable. While such an affirmation regarding the use of biological processes or models of evolution are debated (e.g., Swofford et al., 1996), one should always be cautious of the assumptions that are being made in any phylogenetic analysis. In our study we use successive approximations to test the sensitivity of the most parsimonious unweighted trees(s) to differential character weighting based on inferred levels of homoplasy (Farris, 1969; Kluge, 1997a). Clade stability during successive approximations (= clades congruent with tree(s) based on uniform weighting) instills us with additional confidence for those relationships inferred in the uniformly weighted analysis (Carpenter et al., 1993).

A common criticism or concern of successive approximations is that the final inferred tree may be largely dependent on the initial starting tree from which weights were determined (Swofford et al., 1996). To test how robust inferred clades were to initial starting trees, we generated 20 random trees in MacClade v3.07 (Maddison and Maddison, 1992) and performed successive approximations on each of the random trees. Congruence among the final trees from the 20 completed successive approximation analyses was summarized with a 50% majority-rule consensus tree.

The number of taxa scored for the mtDNA (n = 44) and morphological (n = 43) data far exceeded the number available for the allozyme (n = 19) data. Thus, some taxa are incomplete for a subset of the total combined data. However, these incomplete taxa (missing ∼8% of informative characters) were still included in the phylogenetic analyses (see Wiens and Reeder, 1995; Reeder and Wiens, 1996). Also, two taxa (Cnemidophorus murinus and C. ocellifer) were coded for only the 10 morphological characters (representing ∼3% of the total informative characters), since we lacked tissue samples for molecular analysis. While these highly incomplete taxa (missing ∼97% of informative characters) were included in certain phylogenetic analyses, their impact on tree stability was assessed by bootstrapping (see below) the combined data with and without these two species. The phylogenetic placement of C. murinus is of special significance because it is the type species of Cnemidophorus.

Support for individual clades was assessed by nonparametric bootstrapping (Felsenstein, 1985). Bootstrap analyses were based on 500 heuristic tree searches (with TBR branch swapping). Because of computational constraints, only three random taxon additions per pseudoreplicate were performed in each of the heuristic tree searches. Bootstrapping was performed in both the uniformly weighted and successive approximation analyses. Sullivan et al. (1997) have noted that weighted parsimony analyses often significantly increase bootstrap values (relative to their values in uniformly weighted analyses of the same data). However, because of the inherent properties of parsimony, the elevated bootstrap values in weighted parsimony analyses probably represent overestimates of the amount of support for the inferred clades (Yang et al., 1995; Sullivan et al., 1997). Therefore, we cautiously interpret the bootstrap results of the successively weighted data, and base most of our conclusions of relative support from the unweighted bootstrap analysis. For the uniformly weighted data, clades with bootstrap values of ≥70% were considered strongly supported (following Hillis and Bull, 1993).

Uniformly Weighted Analysis

Phylogenetic analysis of the 317 uniformly weighted phylogenetically informative characters (235 informative characters among teiine taxa) resulted in four shortest trees (L = 1539; CI = 0.39; RI = 0.61). The strict consensus of these four trees is shown in figure 2A. The numbers of unambiguous synapomorphies supporting the unambiguously resolved branches of the strict consensus tree are given in table 2. All inferred teiine clades were supported by unambiguously placed synapomorphies. However, the vast majority of the clades were supported only by mtDNA character state transformations. In all, only four of the 36 teiine clades were unambiguously supported by mtDNA, morphological, and allozymic synapomorphies (table 2), possibly because allozyme data were coded for only 19 taxa.

Monophyly of the Teiidae (excluding Gymnophthalmidae) is not supported by this analysis. However, teiid paraphyly is only weakly supported, with the gymnophthalmid (= microteiid) Pholidobolus being placed with Tupinambis (bootstrap = 57%). Teiini (Clade 1) monophyly is strongly supported (80%) by 11 synapomorphies, with Teius and Dicrodon representing the most basal lineages. Within the teiine clade, 18 of the 34 unambiguously resolved clades are strongly supported (bootstraps ≥70%) by the combined data. Within the Teiini, the cnemidophorine taxa are also supported as a clade (Clade 3). However, cnemidophorine monophyly is only weakly supported (<70%).

While cnemidophorine monophyly is supported, monophyly of Cnemidophorus is rejected. All of the South American Cnemidophorus species (except C. longicaudus) are more closely related to species of other genera of Central and South American cnemidophorines (i.e., Ameiva and Kentropyx) than to the North American species of Cnemidophorus. However, this neotropical clade (Clade 4) is only weakly supported by these data. Within Clade 4 C. lacertoides is weakly placed as the sister species of the remaining taxa. Monophyly of the lemniscatus complex (i.e., C. arenivagus, C. gramivagus, and C. lemniscatus; Clade 10) is strongly supported (100%), with this clade being placed as the sister taxon of a strongly supported Kentropyx (100%; Clade 13). In addition to 12 mtDNA synapomorphies, the lemniscatus complex is also supported by one morphological synapomorphy (basal tongue sheath absent [character state 1.b]). Kentropyx monophyly is supported by 19 synapomorphies: 12 mtDNA, four morphological (keeled ventral scutes [3.b], 14 rows of ventral scutes [4.c], two enlarged anal spurs per side in males [6.c], abruptly enlarged mesoptychial scales [8.c]), and three allozymes. Within Kentropyx, it is equally parsimonious to place K. calcarata as the sister taxon of all remaining Kentropyx, or as the sister species to the K. altamazonica + K. pelviceps clade.

Analysis of these data also rejects the monophyly of Ameiva. Within Clade 4, A. undulata is more closely related to the lemniscatus group + Kentropyx clade than to the small clade containing A. ameiva, A. bifrontata, and A. quadrilineata. Also, the West Indian species (A. auberi and A. chrysolaema) are strongly supported as a clade (98%), but they are distantly related to mainland “Ameiva”. The West Indian clade is weakly placed (55%) as the sister taxon to a large clade containing all of the North American Cnemidophorus (Clade 20) and the South American C. longicaudus.

The monophyly of a large North American clade of Cnemidophorus (Clade 20) is strongly supported (84%) in this analysis by 10 mtDNA synapomorphies. This clade contains the bisexual deppii, sexlineatus, and tigris groups, each of whose monophyly is strongly supported (89%, 80%, and 100%, respectively). Within the North American clade, the deppii group and tigris group are strongly supported (85%) as sister taxa (Clade 21). While deppii group (Clade 22) monophyly is well supported, the inferred relationships within this group are weak, with the Baja California C. hyperythrus being placed as the sister species of the C. deppii + C. guttatus clade of mainland Mexico. The phylogenetic relationships within the tigris group (Clade 24) are well supported, except for the interrelationships among the following three taxa: C. tigris punctilinealis, C. t. aethiops, and the C. t. septentrionalis + C. t. tigris clade.

The monophyly of the sexlineatus group (Clade 29) is strongly supported by eight synapomorphies: six mtDNA, one morphological (enlarged postantebranchial scales [7.c]), and one allozyme. Only two of the seven resolved clades within the sexlineatus group are strongly supported by this analysis. One of these is the clade containing C. gularis gularis, C. g. scalaris, and C. g. septemvittatus (85%; Clade 31), which is weakly placed as the sister taxon to C. costatus costatus. The other strongly supported clade is the sister group relationship between C. inornatus and C. sexlineatus. The C. inornatus + C. sexlineatus clade is supported by 18 or 20 synapomorphies (depending on resolution of C. burti taxa): 13 or 15 mtDNA, one morphological (slightly enlarged postantebrachial scales [7.b]), and four allozymes. The only ambiguity within the sexlineatus group is the phylogenetic affinity of C. burti burti and C. b. stictogrammus (fig. 2A, B). Both of these taxa are weakly placed in a clade containing C. costatus griseocephalus, C. inornatus, and C. sexlineatus. However, it is equally parsimonious to place C. b. burti and C. b. stictogrammus as sister taxa, or to place C. b. burti as the most basal taxon within its clade. And finally, the two C. costatus taxa included (C. c. costatus and C. c. griseocephalus) are not supported as each other's closest relative.

Successive Approximations Analysis

Phylogenetic analysis of the 317 successively weighted phylogenetically informative characters (235 informative characters among teiine taxa) resulted in a single shortest tree (fig. 3; L = 43,281) with a CI of 0.61 and RI of 0.79. The numbers of unambiguous synapomorphies supporting the unambiguously resolved branches of the strict consensus tree are given in table 3. As in the uniformly weighted analysis, all clades are supported by unambiguous synapomorphies, with most clades being unambiguously supported only by mtDNA character state transformations.

Successive weighting (based on the four fundamental phylogenies from the unweighted analysis; two iterations) of these data resulted in a phylogeny that is very similar to the phylogenies inferred in the uniformly weighted analysis, with Cnemidophorus and Ameiva both being paraphyletic. Besides greater resolution in the successive approximations analysis, the only differences between the unweighted and the successive approximations analyses involve the following relationships: (1) Dicrodon and Teius have switched positions, with Dicrodon now being the sister taxon to the remaining teiines; and (2) interrelationships within the sexlineatus group of North America. Within the sexlineatus group, the C. inornatus + C. sexlineatus clade is now the sister taxon to the remaining sexlineatus group species. While the C. inornatus + C. sexlineatus clade still appears to be strongly supported, the number of unambiguously placed synapomorphies supporting this group is about half of that from the uniformly weighted analysis (11 vs. 20). Also, the single morphological synapomorphy (i.e., slightly enlarged postantebrachial scales [7.b]) in the uniformly weighted analysis no longer diagnoses the C. inornatus + C. sexlineatus clade, but instead becomes a synapomorphy of the sexlineatus group as a whole. In the unweighted analysis, enlarged postantebrachial scales (7.c) was a sexlineatus group synapomorphy, with a reversal to slightly enlarged postantebrachial scales (7.b) diagnosing the C. inornatus + C. sexlineatus clade. The two C. burti taxa are now unambiguously supported as sister taxa by a single mtDNA synapomorphy. And finally, while the specific placement of C. costatus griseocephalus has changed relative to the unweighted analysis (figs. 2, 3), this taxon and C. c. costatus are still not each other's closest relatives.

Bootstrap analysis resulted in 25 teiine clades with bootstrap values ≥70% (compared to only 18 clades in the unweighted analysis) (fig. 2 vs. fig. 3). The increase in bootstrap support in the successively weighted analysis is consistent with the results from other recent empirical studies (see Phylogenetic Analysis under Materials and Methods). Ten inferred clades remain weakly supported (bootstrap <70%) in the weighted analysis. These clades may represent the poorest supported relationships of the study. And finally, the level of support for the sexlineatus group (bootstrap = 65%) appears to have decreased in the weighted analysis, relative to its strong support (80%) in the unweighted analysis.

Effects of Initial Starting Tree in Successive Approximations

Twenty random trees were generated, which were 14–32 steps longer than the original four equally parsimonious unweighted trees. Application of successive approximations on these random trees indicated that the initial starting tree did influence the final inferred tree. None of these analyses on random trees resulted in a phylogeny completely congruent with our preferred successive approximations phylogeny (fig. 3). However, when the results of the 20 successive approximation analyses are summarized with a majority-rule consensus tree (fig. 4), it is clear that most of the inferred relationships in figure 3 are being recovered through successive approximations, regardless of the starting tree. Twenty-six clades were recovered in ≥90% of the random tree analyses, and these clades are supported in the preferred successive approximations analysis.

Phylogenetic Placement of Cnemidophorus murinus and Cnemidophorus ocellifer

Because of the lack of tissue, Cnemidophorus murinus and C. ocellifer were coded for only the 10 morphological characters. However, while lacking 97% (307 of 317) of the phylogenetically informative characters, analysis of the complete data set containing C. murinus and C. ocellifer unambiguously places these two species within the teiine phylogeny (figs. 2A, 3). The inclusion of these two species did not alter the previously inferred interrelationships among the other teiine species. Also, the placement of these two species is identical in the uniformly and successively weighted analyses. The most parsimonious placement of C. ocellifer is as the sister species of the large clade (Clade 3; figs. 2A, 3) containing all the remaining cnemidophorines. Cnemidophorus murinus is nested further in the cnemidophorine clade, being placed as the sister species of the lemniscatus complex (Clade 10; figs. 2A, 3). While C. murinus and C. ocellifer are unambiguously placed by the morphological data, these specific placements are weakly supported. In fact, the relative support throughout the phylogeny generally decreases when these taxa are included in a bootstrap analysis. The decrease in tree support is attributed to the largely incomplete nature of the data for C. murinus and C. ocellifer.

“Cnemidophorus” Phylogeny

“Cnemidophorus” Paraphyly: One of the primary goals of this study was to rigorously test Cnemidophorus monophyly and infer the interrelationships among the bisexual species groups. Equal weighting of the combined mtDNA, allozymic, and morphological data resulted in four equally parsimonious phylogenies (strict consensus in fig. 2), and successive weighting resulted in a completely resolved phylogenetic hypothesis (fig. 3) for teiine lizards. The higher-level teiine relationships inferred in these two analyses are essentially identical, with these data not supporting Cnemidophorus monophyly. Such a conclusion should come as no surprise. While Cnemidophorus monophyly has long been assumed, no apomorphies have ever been proposed, and its monophyly has never been explicitly tested. In fact, “Cnemidophorus” (we use quotation marks in reference to the broader paraphyletic group) has historically been defined by the absence of presumably derived character states exhibited by the other teiine teiids (Ameiva, Dicrodon, Kentropyx, and Teius). Our data support at least four distinct clades or lineages of “Cnemidophorus”: (1) North American “Cnemidophorus” clade (deppii, sexlineatus, and tigris species groups) + “C” longicaudus; (2) “C” lacertoides; (3) “C” lemniscatus complex + “C” murinus; and (4) “C” ocellifer.

“Cnemidophorus” lemniscatus Group: Except for “Cnemidophorus” longicaudus and “C” ocellifer (placement based on morphology only), all members of the traditional lemniscatus group are more closely related to other neotropical cnemidophorines (i.e., Ameiva and Kentropyx) than they are to the North American “Cnemidophorus”. Such a conclusion is consistent with the hypothesis put forth by Burt (1931), who proposed that the lemniscatus group was derived from an Ameiva-like “Cnemidophorus” ancestor, although he visualized the lemniscatus group subsequently giving rise to the ancestor of the North American “Cnemidophorus”.

Specifically, “Cnemidophorus” lacertoides and the “C” lemniscatus complex (Clade 10) are placed within a more inclusive clade (Clade 4; figs. 2A, 3) that contains Kentropyx and mainland neotropical Ameiva. However, even within this neotropical clade, the lemniscatus complex does not form a clade with “C” lacertoides. The combined data strongly support a clade containing those lemniscatus group species (i.e., “C” arenivagus, “C” gramivagus, “C” lemniscatus, “C” murinus) that possess anal spurs, and this clade is placed as the sister taxon to Kentropyx. Traditionally “C” lacertoides has been included as a member of the lemniscatus group (Wright, 1993). However, our data do not support a close relationship between these taxa. In fact, the generic assignment of “C” lacertoides has been controversial (Cole et al., 1979), as the species has been alternatively placed in Ameiva (Vanzolini and Valencia, 1966). Note also that our analyses place “C” lemniscatus splendidus and “C” arenivagus as sister taxa (figs. 2A, 3), suggesting that the specific status of “C” lemniscatus splendidus merits reevaluation (Markezich et al., 1997).

Two additional species that have traditionally been placed in the “lemniscatus group” are “Cnemidophorus” longicaudus and “C” ocellifer (Wright, 1993). Our combined analysis places “C” longicaudus as the sister species of the North American “Cnemidophorus” clade. While we find this possible relationship perplexing considering that “C” longicaudus is found in south-central South America, our current analysis suggests this inferred relationship is weakly supported. “Cnemidophorus” ocellifer was scored only for the 10 morphological characters, but it is unambiguously placed as the sister species to all remaining cnemidophorines (Clade 3; figs. 2A, 3). However, like the placement of “C” longicaudus, this specific placement of “C” ocellifer is very weakly supported. Therefore, we do not have great confidence in the placement of these two species, and their inferred relationships will likely change with the addition of new data (Bell and Reeder, unpubl. data).

North American “Cnemidophorus” Clade: Our current study strongly supports the monophyly of a group of North American “Cnemidophorus”, composed of the deppii, sexlineatus, and tigris species groups (each of which is also strongly supported). Such a hypothesis is consistent with Burt (1931), although he had also postulated that the neotropical “lemniscatus group” gave rise to the ancestor of the North American “Cnemidophorus”. Within this clade there appears to be relatively strong support for a sister group relationship between the deppii and tigris species groups. Such a relationship has not been previously proposed. Lowe et al. (1970) hypothesized that the sexlineatus and tigris groups (each of which possesses uniquely derived karyotypes; see Karyotype Evolution Revisited below) were each other's closest relatives, with this clade potentially supported by a single centric fusion. A sexlineatus group + tigris group relationship was also strongly supported by mitochondrial restriction site data in Moritz et al. (1992a). Thus, there appears to be strong conflict between our mitochondrial rDNA sequences and the mitochondrial restriction site data of Moritz et al. (1992a). Since the mitochondrial genome is inherited as a single, nonrecombining unit (Brown, 1981, 1983), these two mtDNA data sets might be expected to yield the same result. Nevertheless, the nature of the restriction sites and the nucleotide gene sequences are quite different data sets, based on different details in the mtDNA.

Based on shared identical karyotypes, Lowe et al. (1970) and Robinson (1973) proposed that the Baja California “Cnemidophorus” ceralbensis and “C” hyperythrus (sensu lato; see Grismer, 1999) were closely related to the mainland Mexico deppii group (“C” deppii, “C” guttatus, and “C” lineatissimus). However, since Lowe et al. (1970) also hypothesized that the deppii group possessed the ancestral karyotype of “Cnemidophorus”, it was possible that the Baja taxa were being placed within the deppii group by the possession of a shared primitive trait. Also, members of the “C” hyperythrus complex (Grismer, 1999) and “C” ceralbensis share a uniquely derived feature (i.e., undivided frontoparietal scale; Walker et al., 1966; Walker and Taylor, 1968), but evidence supporting a specific relationship of the Baja clade to the remaining deppii group taxa has been lacking. The results of our current study corroborate and strongly support a close relationship between the Baja California “C” hyperythrus and the mainland Mexico deppii group, with these data supporting the placement of “C” hyperythrus as the sister taxon to the “C” deppii + “C” guttatus clade.

Species limits within the tigris group are controversial, with recent checklists recognizing anywhere from a single, widespread polytypic species (Wright, 1993) to eight species (Maslin and Secoy, 1986). Also, the phylogenetic relationships among the >20 named taxa (i.e., subspecies and species) are largely unknown, with only the recent molecular study by Radtkey et al. (1997) providing a preliminary hypothesis of relationships for the Baja taxa. The goal of our study was not to rigorously evaluate the relationships within the tigris group. However, even with this limited sampling, significant preliminary results are evident. “Cnemidophorus” tigris maximus (“C” maximus of Maslin and Secoy [1986]) of the Cape Region of Baja California is placed as the sister taxon of all remaining tigris group taxa in our study. This finding is consistent with Radtkey et al. (1997), whose mitochondrial cytochrome b data suggested that a clade of southern Baja California “C” tigris populations (including “C” t. maximus) was the sister taxon to a clade containing the northern Baja taxa and the few non-Baja California populations they studied. Our data also strongly support the placement of “C” t. marmoratus as the sister taxon to the remaining western U.S. “C” tigris taxa. Such a finding is significant, because it demonstrates that the ongoing and evidently unrestricted hybridization between the geographically proximate “C” t. marmoratus and “C” t. punctilinealis (see Dessauer et al., 2000) is between relatively distantly related “C” tigris lineages.

With >20 recognized species, the sexlineatus group is the largest species group within the North American “Cnemidophorus” clade. While we excluded the unisexual taxa and included only seven bisexual species in our study (thus limiting what can be hypothesized regarding sexlineatus group evolution), our results are reasonably congruent with some past hypotheses of sexlineatus group relationships. Our data strongly support a clade containing “C” inornatus and “C” sexlineatus. However, the placement of this clade within the sexlineatus group is ambiguous. The uniformly weighted analysis weakly supported its placement within a clade containing “C” burti and “C” costatus griseocephalus (fig. 2), whereas the successively weighted analysis placed the “C” inornatus + “C” sexlineatus clade as the sister taxon to all remaining sexlineatus group taxa (fig. 3). Moritz et al. (1992a) hypothesized a relationship similar to that inferred in the successive weighted analysis, with the “C” inornatus + “C” sexlineatus clade being relatively basal within the sexlineatus group.

Much taxonomic confusion exists within the “Cnemidophorus” gularis complex. Walker (1981a, 1981b) concluded that “C” septemvittatus and “C” scalaris were conspecific (but heterosubspecific) with “C” gularis. More recently, Wright (1993) (without comment) elevated “C” g. gularis to specific status and treated “C” scalaris as a subspecies of “C” septemvittatus (see Crother et al., 2001, regarding taxonomic uncertainty as to correct specific epithet for “C” scalaris/septemvittatus). While our sampling is inadequate to address the species limits problems within the “C” gularis complex, the results of our data analysis are consistent with these three taxa being very closely related. For now, we have followed the taxonomic recommendation of Walker (1981a, 1981b), but acknowledge that additional work is needed in this group.

Within the sexlineatus species group, the species complex involving the polytypic “Cnemidophorus” burti and “C” costatus has also bewildered past “Cnemidophorus” systematists. While we made no rigorous attempt to thoroughly resolve these uncertainties, our data provide new insight into the potential magnitude of the problem. For example, our data do not support the supposedly conspecific “C” costatus costatus and “C” c. griseocephalus as being each other's closest relatives. The placement of “C” c. costatus as being closely related to the “C” gularis complex (figs. 2A, 3) is consistent with the findings of Duellman and Zweifel (1962). Duellman and Zweifel (1962) commented on southern Mexico populations tentatively assigned to “C” costatus. They noted that these populations were similar to “C” c. costatus, but also had attributes likening them to “C” septemvittatus. As for “C” c. griseocephalus, Dessauer and Cole (1989) provided evidence (allozymes) that this taxon was genetically more similar to “C” burti than to “C” c. costatus (misidentified as “C” deppii in Dessauer and Cole, 1989). Without comment, Wright (1993) considered “C” c. griseocephalus to be conspecific with “C” burti (his “C” burti griseocephalus), an action possibly prompted by the data of Dessauer and Cole (1989). The results of this phylogenetic analysis do not support a particularly close relationship of this taxon to “C” burti or “C” c. costatus (figs. 2, 3). Our study further reinforces the complexity of the problems within the “C” burti/costatus complex, and suggests that future endeavors to resolve their species limits may require the consideration of the “C” gularis complex as well.

“Ameiva” Phylogeny

Compared to “Cnemidophorus”, our taxon sampling within Ameiva was not extensive, with two species from each of the following areas: West Indies (A. auberi, A. chrysolaema), Central America (A. quadrilineata, A. undulata), and South America (A. ameiva, A. bifrontata). Even with this limited sampling, our data show that Ameiva is paraphyletic. The West Indian clade is placed as the sister taxon of the clade containing “C” longicaudus and the North American “Cnemidophorus”. The other “Ameiva” species are more closely related to the neotropical “C” lemniscatus group (sensu stricto) and Kentropyx. However, most of these inferred relationships for “Ameiva” are weakly supported, with strong support for only two small clades: “A” auberi + “A” chrysolaema and “A” ameiva + “A” bifrontata. While “Ameiva” phylogeny was not one of the main foci of this study, it is evident that additional phylogenetic studies of “Ameiva” are needed.

Evolution of Tongue Characters Traditionally Used in Cnemidophorine Systematics

Historically, finding characters to diagnose “Cnemidophorus” from “Ameiva” has been problematic. Burt (1931) used attributes of the tongue as the only real basis for differentiating these genera: (1) “Ameiva” possesses a sheath at the base of the tongue (visibly separating it from the glottis) and has the posterior margin of the tongue not forked (or only slightly so); and (2) “Cnemidophorus” lacks a basal tongue sheath and has the posterior margin of the tongue clearly forked (possessing an arrowhead- or heart-shaped tongue, according to Burt [1931]). However, not all species have perfectly fit this scheme, with “C” lacertoides being a species of taxonomic instability. Without comment, Burt (1931) transferred this species to “Ameiva” (leaving one to assume that this species possessed the two lingual characteristics of “Ameiva”). Milstead (1961) and Presch (1971) noted that this species exhibited the “Cnemidophorus” tongue type and recommended that this species be placed back in “Cnemidophorus”, whereas Vanzolini and Valencia (1966) believed the tongue structure was more similar to “Ameiva”. The confusion largely stems from the fact that “C” lacertoides does not perfectly fit the diagnosis developed by Burt (1931). “Cnemidophorus” lacertoides possesses a distinctly forked posterior edge of the tongue (as in other “Cnemidophorus”), but also exhibits the tongue sheath characteristic of “Ameiva”. The results of our phylogenetic analysis shed some light on the evolution of these tongue characters in teiines and help determine which teiines can be diagnosed by derived character states.

The absence of a tongue sheath appears to be the ancestral condition for teiines (absent in the most recent common ancestor of Teiini; Clade 1 of figs. 2, 3). However, the ancestral condition for cnemidophorines (Clade 3 of figs. 2, 3) is ambiguous, with each of the following evolutionary scenarios being equally parsimonious: (1) The absence of a tongue sheath is ancestral for cnemidophorines, with independent origins of a tongue sheath in the ancestor of the neotropical clade (Clade 4 of figs. 2, 3; reversal in “Cnemidophorus” lemniscatus complex) and the West Indian “Ameiva” (Clade 18 of fig. 2; Clade 19 of fig. 3); or (2) presence of tongue sheath is a synapomorphy of cnemidophorines, with independent losses in the “C” lemniscatus complex and the “C” longicaudus + North American “Cnemidophorus” clade (Clade 19 of fig. 2 or Clade 20 of fig. 3). While the evolution of this character among cnemidophorines is largely ambiguous, under both scenarios the “C” lemniscatus complex has secondarily lost the tongue sheath.

A distinctly forked posterior edge of the tongue is the ancestral condition for Teiini, as well as the cnemidophorines. The derived loss of the forking occurred independently at least twice among cnemidophorines: (1) Once in the West Indian “Ameiva” (Clade 18 of fig. 2; Clade 19 of fig. 3); and (2) one or two times among the neotropical cnemidophorines. Within the neotropical cnemidophorine clade, the distinctive forking of the posterior edge of the tongue was either lost once in the ancestor of Clade 5 (figs. 2, 3) or lost twice (independently in Clade 6 and “A” undulata). If the forking was lost only once among the neotropical cnemidophorines, then a reversal must have occurred in the ancestor of the “Cnemidophorus” lemniscatus complex + Kentropyx clade (Clade 9 of figs. 2, 3). Under any of the above evolutionary scenarios, it is apparent that the “diagnostic” distinctly forked posterior edge of the tongue is the plesiomorphic condition for all “Cnemidophorus”. Unfortunately, the basal teiine and cnemidophorine relationships are weakly supported, with some of these currently inferred relationships likely to change with the addition of more data (e.g., placement of “C” longicaudus and the West Indian “Ameiva”; see Taxonomic Implications and Nomenclatural Recommendations). Thus, any future phylogenetic rearrangements will likely require a reassessment of the evolution of these two tongue characters that historically have played an important part in cnemidophorine systematics.

Taxonomic Implications and Nomenclatural Recommendations

One of the main goals of this study was to test “Cnemidophorus” monophyly, as well as the monophyly of the currently recognized bisexual “Cnemidophorus” species groups. Our study demonstrates that “Cnemidophorus” is paraphyletic with respect to “Ameiva” and Kentropyx. Given this result, nomenclatural changes are needed in order to maintain a classification that more accurately reflects the evolutionary relationships within the cnemidophorine clade (Clade 3; figs. 2, 3). Within this large assemblage exists a strongly supported clade that has informally been referred to as the North American “Cnemidophorus” clade (Clade 20 of fig. 2; Clade 21 of fig. 3). This clade contains the monophyletic deppii, sexlineatus, and tigris species groups and their associated unisexual taxa. Except for “C” longicaudus and “C” ocellifer, all other species of the “Cnemidophorus lemniscatus group” (i.e., the “C” lemniscatus complex, “C” murinus, and “C” lacertoides) are more closely related to Central and South American “Ameiva” and Kentropyx than to members of the North American “Cnemidophorus” clade. “Cnemidophorus” ocellifer was weakly placed as the sister species to the large clade containing “Ameiva”, Kentropyx, and all other “Cnemidophorus”, and “C” longicaudus was weakly placed as the sister species of the North American “Cnemidophorus” clade.

If our goal is to recognize monophyletic groups, then widespread nomenclatural change is required. One option would be to classify all cnemidophorine species of Clade 3 (figs. 2, 3) into a single large taxon. In this case, Ameiva Meyer, 1795, would have priority over Cnemidophorus Wagler, 1830, and Kentropyx Spix, 1825. A second option would be to name the more exclusive well supported cnemidophorine clades (within Clade 3) that are morphologically distinct and/or geographically coherent. We do not favor the first alternative for two reasons. First, this former option would subsume long-recognized and morphologically distinctive groups (e.g., Kentropyx) under a single name. Second, we feel that the recognition of a single taxon (i.e., an expanded Ameiva) would obscure the true phyletic diversity within this large and diverse assemblage. Given this, we find it necessary to resurrect Aspidoscelis as the available generic name for species of the North American “Cnemidophorus” clade (Clade 20 of fig. 2; Clade 21 of fig. 3). As the type species of Cnemidophorus is “C” murinus, that generic name remains with the South American taxa (see additional details below).

Aspidoscelis Fitzinger, 1843

Type Species: Lacerta sexlineata Linnaeus, 1758, is the nominal type species.

Etymology: Aspidoscelis was first named by Fitzinger (1843). He merely listed it as a subgenus of Cnemidophorus, with the comment that the type species is Lacerta 6-lineata Linnaeus (= Cnemidophorus sexlineatus). No etymology was presented.

The name probably was derived from two Greek nouns, aspido, meaning “shield”, and scelis, meaning “rib” or “leg”. This seems appropriate, because it could refer to the large scales on the legs and has a meaning similar to that of Cnemidophorus: “=uipped with leggings”.

According to the International Code of Zoological Nomenclature (1999, art. 30), gender of a compound word is that of the final component if it is a noun, so Aspidoscelis is feminine, although Cnemidophorus is masculine. Consequently, in the list of taxa below, we emend the specific and subspecific epithets for agreement with the feminine gender (ICZN, 1999, art. 31.2). Special thanks are due to Darrel Frost for providing these emendations.

Content: The genus Aspidoscelis contains at least 87 currently recognized bisexual and unisexual taxa. The following list of taxa is a blend of those recognized by Grismer, 1999; Maslin and Secoy, 1986; Taylor and Walker, 1996; Walker, 1981a, 1981b; Walker et al., 1997; Wright, 1993; and Wright and Lowe, 1993. Given the complex nature of the interrelationships among the described taxa of the A. burti, A. costata, and A. gularis complexes, many additional evolutionary species may exist within Aspidoscelis. We realize that no two individuals or teams of herpetologists would independently come up with the same list of species and subspecies recognized these days for such a large and complex genus (especially given the insular forms), but this is our best working hypothesis for now.

The Aspidoscelis cozumela Group: A. cozumela; A. maslini; A. rodecki.

The Aspidoscelis deppii Group: A. carmenensis; A. ceralbensis; A. danheimae; A. deppii; A. d. deppii; A. d. infernalis; A. d. schizophora; A. espiritensis; A. franciscensis; A. guttata; A. g. guttata; A. g. immutabilis; A. g. flavilineata; A. hyperythra; A. h. hyperythra; A. h. beldingi; A. lineatissima; A. l. lineatissima; A. l. duodecemlineata; A. l. exorista; A. l. livida; A. picta.

The Aspidoscelis sexlineata Group: A. angusticeps; A. a. angusticeps; A. a. petenensis; A. burti; A. b. burti; A. b. stictogramma; A. b. xanthonota; A. calidipes; A. communis; A. c. communis; A. c. mariarum; A. costata; A. c. costata; A. c. barrancorum; A. c. griseocephala; A. c. huico; A. c. mazatlanensis; A. c. nigrigularis; A. c. occidentalis; A. c. zweifeli; A. exsanguis; A. flagellicauda; A. gularis; A. g. gularis; A. g. colossus; A. g. pallida; A. g. scalaris; A. g. septemvittata; A. g. semifasciata; A. g. semiannulata; A. innotata; A. inornata; A. i. inornata; A. i. arizonae; A. i. cienegae; A. i. chihuahuae; A. i. gypsi; A. i. heptagramma; A. i. juniperus; A. i. llanuras; A. i. octolineata; A. i. pai; A. i. paulula; A. labialis; A. laredoensis; A. mexicana; A. motaguae; A. opatae; A. parvisocia; A. sacki; A. s. sacki; A. s. gigas; A. sexlineata; A. s. sexlineata; A. s. viridis; A. sonorae; A. uniparens; A. velox.

The Aspidoscelis tesselata Group: A. dixoni; A. neomexicana; A. neotesselata; A. tesselata.

The Aspidoscelis tigris Group: A. tigris; A. t. tigris; A. t. aethiops; A. t. disparilis; A. t. marmorata; A. t. maxima; A. t. multiscutata; A. t. pulchra; A. t. punctilinealis; A. t. rubida; A. t. septentrionalis; A. t. stejnegeri; A. t. undulata; A. t. variolosa.

Definition and Diagnosis: Tongue morphology: Basal tongue sheath absent and posterior portion of tongue clearly forked. Scutellation: Smooth ventral scutes; eight rows of ventral scutes across midbody; granular dorsal scales; anal spurs in males absent; mesoptychial scales abruptly enlarged over scales in gular fold, more anterior ones becoming smaller; three parietal scales; three or four supraocular scales on each side.

The above combination of traits distinguishes Aspidoscelis from all other cnemidophorine teiid genera. Aspidoscelis differs from Kentropyx by the absence of keeled ventral scutes and the absence of enlarged anal spurs in males (presence of keeled ventral scutes in Kentropyx is unique among teiids). Aspidoscelis can also be differentiated from all species currently placed in “Ameiva” by the absence of a basal tongue sheath (present in “Ameiva”) and the possession of a distinctly forked posterior portion of the tongue (not clearly forked in “Ameiva”).

Species of Aspidoscelis are easily distinguished from Cnemidophorus murinus and the C. lemniscatus complex by the following attributes: (1) lack of anal spurs in males (present in C. murinus and the C. lemniscatus complex); (2) presence of abruptly enlarged mesoptychial scales, with more anterior scales becoming smaller (somewhat enlarged in C. murinus and the C. lemniscatus complex, with more anterior mesoptychials becoming abruptly enlarged); and (3) presence of three parietal scales (five in C. murinus and the C. lemniscatus complex).

Aspidoscelis differs from “Cnemidophorus” ocellifer by the presence of three parietal scales (five in “C” ocellifer). Also, most species of Aspidoscelis possess slightly to greatly enlarged postantebrachial scales (most species within the A. sexlineata group), whereas the postantebrachials are granular in “C” ocellifer. Species of Aspidoscelis can be differentiated from “C” lacertoides by the following traits: (1) absence of a basal tongue sheath (present in “C” lacertoides); (2) eight rows of ventral scutes across midbody (10–12 in “C” lacertoides); and (3) presence of three parietal scales (five in “C” lacertoides). Also, as in “C” ocellifer, “C” lacertoides possesses granular postantebrachial scales. And finally, Aspidoscelis can be distinguished from “C” longicaudus by the presence of eight rows of ventral scales across midbody (10–12 in “C” longicaudus) and abruptly enlarged mesoptychial scales over the gular fold scales (somewhat enlarged in “C” longicaudus). Some populations of A. tigris have secondarily reduced mesoptychials, thus resembling “C” longicaudus. However, all A. tigris typically have only eight ventral scutes across the midbody.

Distribution: Aspidoscelis occurs throughout most of North America (except Canada), reaching the East and West Coasts of the United States, and ranging south through all of Mexico into Central America. Its southern limit is in extreme northwestern Costa Rica. Range maps for the species groups are provided in Wright (1993).

Comment: Our data place the South American “Cnemidophorus” longicaudus as the sister species of Aspidoscelis, and its inclusion in Aspidoscelis would be consistent with the phylogeny (figs. 2, 3). However, this placement of “C” longicaudus is very weakly supported. We suspect that the true affinities of “C” longicaudus lie with the other “lemniscatus group” species and “Ameiva” in South America. Preliminary sequence data from additional mitochondrial genes (Bell and Reeder, unpubl. data) lend support to this suspicion. Therefore, the exclusion of “C” longicaudus from Aspidoscelis in this paper probably prevents the demonstration of paraphyly of Aspidoscelis in future studies. As currently defined, Aspidoscelis is a strongly supported and geographically coherent clade. Within Aspidoscelis, there is strong support for the monophyly of the deppii, sexlineata, and tigris species groups; thus, we advocate the continued recognition of these informal supraspecific groups formerly associated with “Cnemidophorus”. While our present phylogenetic analysis did not include all of the described bisexual species of the aforementioned species groups, we are confident of their proposed group membership (based largely on karyotypic data; see Karyotype Evolution Revisited), and strongly doubt that their inclusion in future phylogenetic studies will render Aspidoscelis paraphyletic. And finally, the species of the unisexual cozumela and tesselata species groups are also included in the genus Aspidoscelis, as these unisexuals are derived from hybridization events within Aspidoscelis.

The removal of all of the North American taxa from Cnemidophorus leaves only the “lemniscatus group” species within Cnemidophorus (sensu stricto). However, the recognition of Aspidoscelis still does not make Cnemidophorus monophyletic (due to “lemniscatus group” paraphyly). Within Clade 3 (figs. 2, 3) there exists a strongly supported clade that corresponds to the “C” lemniscatus complex (Clade 10). Based on only morphological data, “C” murinus is placed as the sister species of the lemniscatus complex. Also, all males of this clade (”C” murinus + lemniscatus complex) possess two anal spurs (one per side), while all remaining species of the “lemniscatus group” lack anal spurs. Since the type species of Cnemidophorus is C. murinus, Cnemidophorus could be made monophyletic by restricting this name to the strongly supported and morphologically distinct clade containing C. murinus and the C. lemniscatus complex. However, that still leaves us with the problem of what to do with the remaining “lemniscatus group” species lacking anal spurs (i.e., C. lacertoides, C. longicaudus, and the C. ocellifer complex). To maintain Cnemidophorus monophyly, each of these taxa would have to be removed from Cnemidophorus and placed into other taxa (e.g., “Ameiva”).

The phylogenetic placements of Cnemidophorus longicaudus, C. lacertoides, and C. ocellifer suggest erecting monotypic genera for each species. Cnemidophorus ocellifer was originally described and placed in Teius. However, because of the pentadactyl condition of the hind foot of C. ocellifer and its resemblance to Cnemidophorus, Burt (1931) transferred this species to Cnemidophorus. Our current data do not support a close relationship between C. ocellifer and Teius; thus, a new generic name is needed for C. ocellifer (and probably to include the other members of the C. ocellifer complex; see Rocha et al., 1997, 2000). Cnemidophorus longicaudus was originally placed in Ameiva, and C. lacertoides has had an unstable taxonomic history, with C. lacertoides being repeatedly shifted between Cnemidophorus and Ameiva (Burt, 1931; Vanzolini and Valencia, 1966; Cole et al., 1979). Initially, it may appear that the appropriate action would be to return C. longicaudus and C. lacertoides to Ameiva. However, these two species are not closely related, nor are they closely related to any Ameiva. Furthermore, Ameiva is also paraphyletic, so no benefit would result by moving these two Cnemidophorus species from one paraphyletic taxon to another.

As previously mentioned, the phylogenetic placements of C. longicaudus, C. lacertoides and C. ocellifer are weakly supported. Given this, we prefer to tentatively leave these three species within Cnemidophorus, even though such an action renders Cnemidophorus paraphyletic. We feel that a better understanding of the phylogenetic relationships among the South American “Cnemidophorus” species is needed before additional taxonomic changes (e.g., transfer of taxa to existing genera and/or the proposal of new genera) should be made. Ultimately, we suspect that Cnemidophorus will be restricted to those species possessing anal spurs in males (i.e., C. murinus and the C. lemniscatus complex). However, such a conclusion requires that additional data and taxa (i.e., C. ocellifer complex species and additional species of Ameiva) be included in future studies before final taxonomic recommendations are proposed.

Maternal Ancestor of Kentropyx borckiana

As Kentropyx borckiana was known only from female specimens, Hoogmoed (1973) and Gallagher and Dixon (1992) were the first to suggest that it was yet another unisexual teiid species. Gallagher and Dixon (1992) hypothesized that K. borckiana (like other unisexual teiids) was of hybrid origin, with the bisexual K. calcarata and K. striata being the ancestor species. Based upon an extensive analysis of morphology and allozymes, Cole et al. (1995) confirmed that K. borckiana was of hybrid origin involving K. calcarata and K. striata. Cole et al. (1995) were also able to exclude K. altamazonica (a third bisexual species occurring near K. borckiana) of any involvement in the hybridization event giving rise to K. borckiana. While both ancestral species had been determined with confidence, it was not known which of the two bisexual species had been the maternal ancestor.

Karyological (e.g., Cole, 1979) and allozymic (e.g., Cole et al., 1988; Dessauer and Cole, 1989) studies have been successful in determining the ancestral species involved in the hybrid origins of many unisexual lizards. However, such methods could not elucidate which was the maternal and which was the paternal ancestor. With the advent of methods to effectively assay mitochondrial DNA, this maternally inherited molecular marker has been instrumental in elucidating the maternal ancestral species in numerous unisexual teiid species (e.g., Brown and Wright, 1979; Densmore et al., 1989a, 1989b; Moritz et al., 1989b). In our phylogenetic study, the Kentropyx borckiana mtDNA strongly grouped (bootstrap 97–100%) with K. striata. Overall, the single K. borckiana mtDNA sequence differed from the K. striata mtDNA by only 0.9%. Using allele-specific oligonucleotides (see Dessauer et al., 1996) designed for the detection of K. calcarata and K. striata 12S mtDNA, we determined that an additional K. borckiana individual also possesses K. striata-like mtDNA (fig. 5). Thus, we provide strong evidence implicating K. striata as the maternal ancestor of the unisexual K. borckiana.

Unisexual Species: An Overview

Teioid Unisexual Species: There are numerous unisexual species within the Teiidae, and two or more occur among their closest relatives, the microteiids or Gymnophthalmidae. All of the unisexual taxa that have been studied in detail consist of parthenogens with a clonal pattern of inheritance, and they had a hybrid origin. Figure 6 illustrates the reticulate phylogeny of the teioid unisexual species, in which the numbered nodes indicate the following:

1. The Gymnophthalmidae is a diverse and understudied neotropical group. To date, one confirmed and two apparent unisexual lineages have been discovered. In the northern part of its range, Gymnophthalmus underwoodi is a diploid clonal parthenogen of hybrid origin (Cole et al., 1990, 1993; Kizirian and Cole, 1999). However, some Brazilian populations assigned to G. underwoodi are morphologically and genetically distinct and apparently represent a different lineage (Yonenaga-Yassuda et al., 1995), which requires additional research. In addition, Leposoma percarinatum probably is at least one unisexual lineage also (Uzzell and Barry, 1971; Hoogmoed, 1973; Avila-Pires, 1995).

2. Teius suquiensis is known on the basis of more than 160 specimens, all females (Avila and Martori, 1991). No genetic data are available for comparing this taxon with bisexual species of Teius.

3. “Cnemidophorus” cryptus is a diploid clonal parthenogen of hybrid origin (Dessauer and Cole, 1989; Sites et al., 1990). Two clones probably originated from separate F₁ hybrid zygotes (Cole and Dessauer, 1993), although it is not known whether these were produced by the same individual parents or in the same clutch of eggs. The current working hypothesis is that “C” gramivagus and “C” lemniscatus are the two ancestral species (Cole and Dessauer, 1993).

4. “Cnemidophorus” pseudolemniscatus is a triploid clonal parthenogen of hybrid origin, which is hypothesized to have been “C” cryptus ;ts “C” lemniscatus (Dessauer and Cole, 1989; Cole and Dessauer, 1993).

5. Kentropyx borckiana is a diploid clonal parthenogen of hybrid origin (Hoogmoed, 1973; Cole et al., 1995). In this study we have determined that K. striata was the maternal ancestor (see above).

6. Aspidoscelis rodecki and the A. cozumela complex are diploid unisexuals of the cozumela species group. Both taxa are of hybrid origin, with A. deppii and A. angusticeps being the probable bisexual ancestors (Fritts, 1969; Moritz et al., 1992b; Hernandez-Gallegos et al., 1998). Based on mtDNA evidence, Moritz et al. (1992b) determined that A. rodecki and the A. cozumela complex were independently derived from an A. angusticeps maternal ancestor. The A. cozumela complex includes several distinct clonal lineages (Fritts, 1969; Moritz et al., 1992b; Hernandez-Gallegos et al., 1998).

7. The third member of the cozumela species group is an undescribed diploid unisexual species (Moritz et al., 1992b; Wright, 1993). Based on unpublished data, John Wright speculated that Aspidoscelis guttata and A. motaguae were the parental species. Moritz et al. (1992b) provided mtDNA evidence supporting A. motaguae as the maternal ancestor.

8. The Aspidoscelis tesselata complex includes diploid clonal parthenogens that may be derived from more than one F₁ hybrid zygote (Parker and Selander, 1976; Densmore et al., 1989b; Dessauer and Cole, 1989, work in progress). It is not known whether these were produced by the same individual parents or in the same clutch of eggs. Mitochondrial DNA evidence has confirmed A. tigris marmorata as the maternal ancestor (Brown and Wright, 1979; Densmore et al., 1989b; Dessauer et al., 1996). Walker et al. (1997) provided rationale for using the specific epithet tesselata for the unisexual species comprising most of the diploid populations in this complex (contra Wright, 1993). Walker et al. (1997) also recognized the isolated diploid populations in southwest New Mexico and southwest Texas as A. dixoni (originally described by Scudday, 1973; pattern class “F” of Zweifel, 1965).

9. The Aspidoscelis neotesselata complex includes triploid clonal parthenogens of hybrid origin(s) involving three different species of diploid bisexual ancestors (Parker and Selander, 1976; Densmore et al., 1989b; Dessauer and Cole, 1989; Walker et al., 1997), with A. tigris marmorata being the maternal species of the intermediate ancestor, A. tesselata (Densmore et al., 1989b; Dessauer et al., 1996).

10. Aspidoscelis neomexicana is a diploid parthenogen with several clones that may or may not have diverged from a single F₁ hybrid female (Parker and Selander, 1984; Cole et al., 1988; Cordes et al., 1990). Mitochondrial DNA evidence has shown that A. tigris marmorata was the maternal ancestor of this unisexual form (Brown and Wright, 1979; Densmore et al., 1989b; Dessauer et al., 1996).

11. The Aspidoscelis laredoensis complex includes diploid clonal parthenogens of hybrid origin(s) (McKinney et al., 1973; Bickham et al., 1977; Dessauer and Cole, 1989; Abuhteba et al., 2000), with A. gularis being the bisexual maternal ancestor (Wright et al., 1983; Parker et al., 1989).

12. This node in figure 6 represents an intermediate parthenogenetic ancestor(s) of certain triploid taxa (nodes 13–15). The two bisexual species involved in the original hybridization event(s) were Aspidoscelis inornata and A. burti stictogramma (or possibly A. costata barrancorum), with the intermediate ancestor originally occurring in both forms of reciprocal hybridizations. For the unisexual A. flagellicauda, A. opatae, A. sonorae, and A. uniparens complexes, the bisexual A. inornata was the maternal parent of this intermediate ancestor (Densmore et al., 1989a). For the unisexual A. exsanguis and A. velox complex, identity of the intermediate ancestor's maternal species is ambiguous. Based on mtDNA data, Moritz et al. (1989b) hypothesized that either A. burti stictogramma or A. costata barrancorum were equally likely to have been the maternal species of this diploid intermediate ancestor. The intermediate ancestor of A. opatae may or may not still survive today in northeastern Sonora (Dessauer and Cole, 1989; Wright, 1993). If it does, then this diploid species requires a new name (following the recommendations of Cole, 1985; Frost and Wright, 1988), since the original description of A. opatae applies to the triploid populations (node 15; see below). And finally, it has been suggested that diploid individuals of A. velox may exist in some populations of northern New Mexico (Cuellar and Wright, 1992) and southern Utah (Wright, 1993 [based on unpubl. data]). If such individuals and/or populations do occur, then the name A. innotata potentially could be applied to this diploid species. We feel the existence of these diploid A. velox populations need to be verified by investigating the possibility that they actually represent “cryptic” populations of the bisexual A. inornata.

13. Aspidoscelis exsanguis is a triploid clonal parthenogen of hybrid origin in which an intermediate diploid parthenogen (node 12; see above) backcrossed with a third bisexual ancestor. Two similar hypotheses have been proposed for its bisexual ancestors. Based on allozyme data, Good and Wright (1984) hypothesized that A. inornata, A. costata barrancorum, and A. gularis septemvittata were the bisexual ancestors. However, they did acknowledge that A. burti stictogramma was almost equally as likely as A. c. barrancorum. Dessauer and Cole (1989) postulated that A. inornata, A. b. stictogramma, and A. g. scalaris were involved in the hybrid origin of this species. The disagreement between these two studies is partially the result of different population and taxon sampling: (1) The A. g. septemvittata in the two studies were from different populations; (2) A. g. scalaris was not included in Good and Wright (1984); and (3) A. c. barrancorum was not included in Dessauer and Cole (1989). The allozymic similarities among the members of the A. gularis complex and among members of the A. burti/costata complex (Dessauer and Cole, 1989) also contributes to the difficulty of determining the parental species when these bisexuals were involved.

14. The Aspidoscelis flagellicauda and Aspidoscelis sonorae complexes include multiple triploid clones that are of hybrid origin(s) in which an intermediate diploid parthenogen (node 12; see above) backcrossed with its paternal bisexual ancestor, Aspidoscelis b. stictogramma (Dessauer and Cole, 1989).

15. The Aspidoscelis uniparens and Aspidoscelis velox complexes include multiple triploid clones that are of hybrid origin(s) in which an intermediate diploid parthenogen (node 12; see above) backcrossed with one of its bisexual ancestors, A. inornata (Dessauer and Cole, 1989).

In addition, “Cnemidophorus” nativo, a unisexual taxon of the “C” ocellifer complex from Brazil was recently described (Rocha et al., 1997). The genetics, reproduction, mode of origin, and ancestry of this species remain to be investigated in detail.

Below we address the following two issues concerning the evolution of unisexual species: Our ability to know their bisexual ancestors, and the extent and origin of parthenogenetic cloning in vertebrates.

Knowing Ancestors: It is generally accepted that one cannot truly discover ancestors in the process of reconstructing bifurcating phylogenies. We agree that this is generally true. However, in the case of unisexual lineages that are the result of recent hybridization events, it is possible that the derived forms of the ancestral bisexual species still exist. For example, the mitochondrial DNAs of some populations of the bisexual Aspidoscelis tigris marmorata are more closely related to the unisexuals A. neomexicana and A. tesselata than to some other A. t. marmorata populations (Densmore et al., 1989b). Whether or not these individual ancestral demes/populations are extant or extinct, the taxon A. t. marmorata still exists today, with geographic variation. In addition, when an F₁ hybrid female clones herself at an age of one or two years, both of her parents (of different species) may still be alive. However, those individual parents did not change into different species during their lifetimes. Thus, we have indicated ancestors by name for parthenogenetic species in figure 6. However, as noted elsewhere, the indicated ancestral species for some unisexuals is still in question, with additional taxon sampling and/or molecular markers being needed to further resolve these issues.

As yet there is no credible method for estimating ages of parthenogenetic lineages. However, those that have been studied in detail appear to be very young, based on the remarkable integrity maintained in the ancestral genomes they clone. Biogeographic, mitochondrial, and allozyme data suggest that essentially all unisexual Aspidoscelis lineages originated during or since the Pleistocene (Moritz et al., 1989a). These ancestral genomes are often so well preserved that in one analysis of a diploid parthenogen, detailed predictions could be made about the karyotype and protein mobilities of an unknown bisexual ancestor, which was later discovered (Cole et al., 1993). In general, the origin of the unisexual lineages was such a short time ago that their ancestors are best considered as being represented by populations surviving today, although some allele frequencies and distributions may have shifted.

Extent and Origin of Parthenogenetic Cloning in Vertebrates: All genetic data available for unisexual species of lizards indicate that females clone the F₁ hybrid state, excepting rare mutations (Dessauer and Cole, 1986, 1989). One possible exception may exist within Lepidophyma of the Xantusiidae (L. flavimaculatum obscurum; Bezy and Sites, 1987). The phylogeny of the teioid unisexual species (fig. 6) and their perpetuation of the F₁ hybrid state in lineages suggest that there is an instantaneous switch from sperm-dependent to sperm-independent reproduction in one generation of hybridization (Neaves, 1971; Cole, 1975, 1985; Darevsky et al., 1985; Moritz et al., 1989a). Although it is possible that a rare mutation affecting the reproductive mode could occur in an F₁ hybrid individual, the high frequency with which F₁ hybrids have established clones independently suggests a cause-and-effect relationship between hybridization and the origin of parthenogenesis in squamates, perhaps through genetic dysfunction in the control of meiosis in hybrids (Neaves, 1971; Cole, 1975; Moritz et al., 1989a, 1992a; Darevsky, 1992; Cole and Dessauer, 1995). For the unisexual taxa discussed in the literature cited above, this sudden switch happened at least 12 times among F₁ hybrids represented by the diploid nodes of figure 6. The hypothesis of cause-and-effect is also supported by the way hybrid origins of parthenogens span the phylogenetic breadth of the Teiidae and occur broadly in other squamates within the Gymnophthalmidae, Lacertidae, and Gekkonidae, and (if a hybrid origin is demonstrated in the future) possibly also the unisexual Chamaeleoninae, Agaminae, Xantusiidae, and Serpentes (Ramphotyphlops). The ability to produce parthenogenetic hybrids may extend back throughout the >200 million year history of the squamates.

It may appear paradoxical that the ability to spawn parthenogens instantly could have lasted for hundreds of millions of years, yet the recent unisexual clones are among the youngest of all lineages. A unisexual lineage may well experience a brief existence, being more prone to extinction than are bisexual lineages (White, 1970; Maynard Smith, 1978; but see Moritz et al., 1989a). If so, we would expect that throughout the history of the squamates unisexual lineages have originated repeatedly, appearing briefly in certain places in the phylogeny where bisexual taxa underwent interspecific hybridization. These hybridizing species are generally not closely related to one another. Evidence for such a conclusion is supported by two general observations: (1) Hybridization between closely related Aspidoscelis taxa is not uncommon, but unisexual clones are not always produced (e.g., Walker, 1981a, 1981b; Dessauer et al., 2000); and (2) the bisexual ancestral species of unisexual clones are genetically divergent (Cole, 1985; Dessauer and Cole, 1989; Moritz et al., 1992a). Hybridization events may have been most frequent during times of environmental changes and shifting habitats, which could have brought together populations that had been previously isolated from each other. We will never know how many of the squamate taxa known only from fossils were actually unisexual species, but we would not be surprised if it is on the order of 0.5% (as it is today), the number varying with the extent of environmental disturbances. As with other forms of life, we think that more unisexual lineages of vertebrates have gone extinct than survive today.

Successive Approximations and Initial Starting Trees

It has been a major concern of successive approximations that the final inferred phylogeny may be largely dependent on which initial tree(s) is used to successively weight the characters. Our results based on successive approximations on 20 randomly selected trees indicate that the initial starting tree does influence the outcome, with each analysis yielding a slightly different final tree. However, the vast majority of the inferred clades (fig. 4) were also evident in our successive approximations phylogeny (fig. 3). Those clades that were consistently recovered in the random tree successive approximation analyses also represent those groups that are strongly supported by our data (based on bootstrap analysis of the unweighted data). In general, at least for our data, it appears that those relationships that are strongly supported are also robust to starting tree selection during successive approximations. These results give us additional confidence that those relationships that we determined to be strongly supported are not sensitive to the starting tree.

Similar analyses on mitochondrial rDNA sequences in Phrynosoma (Reeder, unpubl.) and mitochondrial ND4 sequences in Australian Sphenomorphus group skinks (Reeder, unpubl.) yield similar results. Recently, Kluge (1997a) has rejected reliability weighting because it invokes some unknown biological process that differentially commits some characters to higher degrees of homoplasy. However, for some molecular data sets it appears that some nucleotide positions (at least in the mitochondrial genome) are evolving at such a rate that they will be highly homoplastic (= large number of changes) on essentially any tree. When this is the case, such characters will be greatly down-weighted in all successive approximation analyses. Discovering this phenomenon does not require any specific knowledge of the biological process affecting the evolution of these characters, only that the characteristics of the data be thoroughly examined. How general these results are to other data sets is unknown. However, if one is to use successive approximations for phylogeny estimation, we recommend that individuals assess the sensitivity of their results to initial starting trees.

Karyotype Evolution Revisited

Considerable new data have appeared since initial hypotheses on karyotype evolution in teiid lizards were presented (Gorman, 1970; Lowe et al., 1970). Now we have karyotypic data for 66% of the taxa in the phylogeny presented here (figs. 2, 3). However, nearly all of the observations are based on standard, Giemsa stained chromosome preparations, in which homology of chromosome arms is not sufficiently clear for unambiguous coding for phylogenetic analysis. Consequently, we excluded the karyotypic data from our analyses, and now we can use the phylogenies (figs. 2, 3) to review hypotheses on karyotype evolution, which are identical for both phylogenies. We begin at the terminal nodes of the North American Aspidoscelis and work backward down toward the base of the teiine phylogeny. Terminology of chromosome shape based on centromere position follows Cole (1970), and we have reinterpreted illustrations of some other authors to be consistent with our terminology and assignment of chromosomes into groups (Set I–Set III), following Lowe et al. (1970). Figure 7 provides a summary and phylogenetic perspective of these major karyotypic events that have occurred during teiine evolution.

The 10 taxa representing the sexlineata species group of Aspidoscelis all share one basic karyotype (Lowe et al., 1970: 131, their fig. 2B). This is consistent with their representing one clade (fig. 7; Clade 29 of fig. 2, or Clade 30 of fig. 3), particularly as their karyotype appears nowhere else in the phylogeny. The Set I chromosomes include only one pair of large metacentric macrochromosomes. These have a subterminal secondary constriction on one arm, the nucleolar organizer region, or NOR (Ward and Cole, 1986), which sets off an elongate satellite. The Set II chromosomes include 12 pairs of smaller macrochromosomes, which are all telocentric to subtelocentric. The Set III chromosomes include 10 pairs of microchromosomes. Sex chromosomes are not recognizable. This karyotype has a diploid number (2n) of 46 and can be referred to as 2n = 46 with 2 + 24 + 20 chromosomes. Only two basic variants are known within the sexlineata group: (1) In A. sexlineata, chromosomes of the fourth largest pair of Set II have a longer short arm than in the other species (fig. 7) (Bickham et al., “1976” [1977]; Cole et al., 1988); and (2) most of the parthenogenetic species of this group are triploids, and many of these perpetuate minor chromosomal mutations through cloning (3n = 69 or so). Consequently, we conclude that the basic sexlineata group karyotype, 2n = 46 with 2 + 24 + 20 chromosomes, was present in the common ancestor of this group.

The six taxa representing the tigris species group of Aspidoscelis also all share one basic, unique karyotype (Cole et al., 1969; Lowe et al., 1970: 134, their fig. 3A), which is consistent with them representing a monophyletic group (Clade 24 of fig. 2, or Clade 25 of fig. 3). There are three pairs of large biarmed Set I macrochromosomes, of which the first pair is metacentric, the second largest pair (also metacentric) has a subterminal NOR (Ward and Cole, 1986) that sets off a dot-like satellite, and the third largest pair is the sex chromosomes (fig. 7) (Cole et al., 1969; Bull, 1978). The male is the heterogametic sex (XY sex determination). The Set II chromosomes include eight pairs of smaller macrochromosomes, which are all subtelocentric to submetacentric. Set III includes 12 pairs of microchromosomes. This karyotype can be abbreviated as 2n = 46 with 6 + 16 + 24 chromosomes, and, considering that it occurs in all of the taxa, must have occurred in the ancestor of this clade.

The three taxa representing the deppii species group of Aspidoscelis also all share one basic karyotype (Lowe et al., 1970: 129, their fig. 1; Robinson, 1973) that is unique to their clade (fig. 7; Clade 22 of fig. 2, or Clade 23 of fig. 3). There are no large Set I macrochromosomes. The Set II chromosomes include 14 pairs of telocentric macrochromosomes, the second or third pair of which has an inconspicuous dot-like satellite distal to a nearly terminal secondary constriction. Set III includes 12 pairs of microchromosomes. This karyotype has 2n = 52 with 0 + 28 + 24 chromosomes, and most likely occurred in the common ancestor of this clade.

There are no karyotypic data for “Cnemidophorus” longicaudus or for “Ameiva” auberi, but there are for “A” chrysolaema (Gorman, 1970: 237, his fig. 4d; De Smet, 1981). “Ameiva” chrysolaema has 2n = 50 with 2 + 24 + 24 chromosomes (fig. 7). De Smet (1981) reported that all macrochromosomes were telocentric, whereas Gorman (1970) showed the largest one to be subtelocentric, as well as two among the Set II chromosomes, the latter of which looks to us as the normal variation one sees from cell to cell among telocentric to subtelocentric chromosomes. Nevertheless, the Set I chromosomes illustrated by Gorman (1970) looked identical, or nearly so, to that which characterizes the lemniscatus species group (sensu stricto) of Cnemidophorus, as do all of the other features of this karyotype. This karyotype is also the same as the basic karyotype hypothesized to be present in the common ancestor of Clade 4 (figs. 2, 3; no data are available for Kentropyx altamazonica, K. pelviceps, “A” bifrontata, “A” quadrilineata, “A” undulata, and Cnemidophorus gramivagus, although lizards designated as “lemniscatus class E” from Brazil may be C. gramivagus [Cole and Dessauer, 1993]).

Kentropyx calcarata, K. striata, K. borckiana, Cnemidophorus arenivagus, C. lemniscatus lemniscatus, C. l. splendidus,”C” lacertoides, and “Ameiva” ameiva all have similar karyotypes of 2n = 50 with 2 + 24 + 24 chromosomes (fig. 7) (Gorman, 1970; Lowe et al., 1970: 131, their fig. 2A; Cole et al., 1979, 1995; and Markezich et al., 1997). All of these species also have a dot-like satellite on the long (or only) arm of the Set I pair of chromosomes. Karyotypic differences among these species are minor, involving only two points: (1) The Set I chromosomes are either telocentric (usually) or subtelocentric; and (2) the Set II chromosomes usually are all telocentric, but some species have one or two subtelocentric chromosomes, and the second largest pair of Set II in “C” lacertoides is uniquely submetacentric. These differences are readily explained by recent derivations, possibly through unequal pericentric inversions or addition of heterochromatin. Because “C” lacertoides, “Ameiva” ameiva, and the three species of Kentropyx karyotyped all share the telocentric Set I pair of chromosomes, we suggest that this is the ancestral state for this clade (Clade 4; figs. 2, 3). This, taken together with the similar karyotypic data for “A” chrysolaema (described above) suggests that the ancestor of Clade 3 (figs. 2, 3) had the following karyotype: 2n = 50 with 2 + 24 + 24 chromosomes (fig. 7), and all of the macrochromosomes were telocentric or subtelocentric.

Given the karyotypic similarities and nearly complete information available for the taxa discussed so far, we now review karyotype evolution with a scenario that begins at the node that represents the common ancestor of Aspidoscelis and “Cnemidophorus” (fig. 7; Clade 3 of figs. 2, 3). That ancestor probably had a karyotype of 2n = 50 with 2 + 24 + 24 chromosomes, and all of the macrochromosomes were telocentric or subtelocentric. Evolution in Clade 4 (figs. 2, 3) and in the lineage leading to “Ameiva” chrysolaema only involved minor changes in centromere positions of from zero to two or three macrochromosomes. This hypothesis predicts that the species in Clade 4 (and “A” auberi and “C” longicaudus) that have not been karyotyped yet will be found to have basically the same ancestral karyotype or one readily derived therefrom (fig. 7).

Evolution in the lineage leading to the Aspidoscelis deppii species group involved the addition of two pairs of Set II telocentric chromosomes while losing the Set I pair, and a change in occurrence of the satellites from being on the largest to the third largest pair. Although other possibilities exist, this could have happened in two steps: (1) unequal pericentric inversion on the ancestral Set I chromosome to produce a large metacentric or submetacentric chromosome with the dot-like satellite on the long arm; (2) centric fission of this derived chromosome, resulting in simultaneous loss of the Set I pair and addition of two pairs of Set II telocentric chromosomes. It is conceivable and parsimonious to suggest that the derived Set I macrochromosome prior to the centric fission just mentioned occurred in the most recent common ancestor of the deppii and tigris groups (fig. 7), and that today this chromosome is represented in A. tigris by the second largest metacentric chromosome that bears the NOR and dot-like satellite. Additional evolution to the karyotype of A. tigris could have involved two centric fusions of Set II chromosomes to form the rest of the Set I condition of tigris plus unequal pericentric inversions and/or addition of heterochromatin on each of the Set II pairs of chromosomes (fig. 7).

Evolution of the karyotype of the Aspidoscelis sexlineata species group from that of their ancestor shared with “Ameiva” chrysolaema involved a change in the position of both the centromere and secondary constriction of the Set I chromosomes, little to no conspicuous changes in Set II, and an apparent loss of two pairs of microchromosomes, which, through translocations, could have become incorporated into other chromosomes (fig. 7). Future analyses of chromosome banding patterns could reveal whether there are arm homologies among the satellite chromosomes of the deppii, tigris, and sexlineata groups, which could indicate whether the Set I pair of biarmed chromosomes with satellites occurred in the ancestor represented by the node that ties together these three groups.

Now we turn to the karyotype evolution in the far more distant past, involving the other macroteiids, including Tupinambis, Dicrodon, and Teius. The most basal lineage among these is that which leads to Tupinambis (figs. 2, 3). The Tupinambis that have been karyotyped to date (Gorman, 1970: 236, his fig. 2d; De Smet, 1981) have been reported to represent two species, T. teguixin and T. nigropunctatus, but details on individuals examined were not always cited; at least we know that Gorman's were from Trinidad and Brazil. These lizards had either 2n = 36 or 38 with 12 macrochromosomes (6 pairs) and either 24 or 26 microchromosomes (fig. 7). The six pairs of macrochromosomes, from largest to smallest, appear as follows: number 1, large metacentric; number 2, large submetacentric with subterminal secondary constriction and dot-like satellite on the long arm; numbers 3 and 4, somewhat smaller, metacentric, similar to each other; number 5, somewhat smaller metacentric; and number 6, significantly smaller subtelocentric. This is or approximates in close detail (but with no. 6 submetacentric) the karyotype hypothesized to have occurred in the common ancestor of Iguania and possibly all lizards (Gorman, 1970, 1973; Paull et al., 1976; Sites et al., 1992). This karyotype is shared by additional South American macroteiids, including Callopistes, Dracaena, and Crocodilurus (which has chromosome no. 6 larger than in the other species and one pair fewer of microchromosomes evident; Gorman, 1970: 236, his fig. 2). Centric fissioning of all of the macrochromosomes in this karyotype would result in a karyotype approximating that of “Ameiva” ameiva (fig. 7; Gorman, 1970), although additional changes in details would have been involved also.

Dicrodon and Teius remain to be discussed, two species of lineages that diverged possibly before, during, or after the extensive fissioning of the large ancestral macrochromosomes as represented in Tupinambis (fig. 7). According to Gorman (1970: 238, his fig. 5a), D. guttulatum has a karyotype of 2n = 56 with 0 Set I chromosomes, 32 telocentric Set II chromosomes, and 24 microchromosomes, so the extensive fissioning of macrochromosomes occurred after it shared a common ancestor with Tupinambis. The karyotype known for Teius is rather similar to that of D. guttulatum, with Teius having 2n = 54 with two large Set I macrochromosomes + 28 Set II macrochromosomes (mostly telocentric) + 24 microchromosomes (Gorman, 1970: 238, his fig. 5b; Hernando, 1994). The Set I chromosome is submetacentric and appears similar to chromosome number 2 of the ancestral state as represented in Tupinambis, so this could be one ancestral biarmed chromosome that was not yet fissioned in the common ancestor of Teius and Dicrodon, or it could represent a new centric fusion that occurred in Teius. If the former, this chromosome became fissioned in both the Dicrodon and between the Teius and Ameiva clades (fig. 7).