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A defining event for Mesozoic plant-pollinator interactions is the angiosperm radiation, which extended the reach of pollinating insects during the Early Cretaceous in a brief interval of geologic time. Recent evidence indicates that events beginning in the Early Permian and increasing during the Middle Jurassic provided repeated opportunities for insect feeding on pollen, pollination drops, and reproductive tissues of extinct gymnosperm lineages. Pollination was an associated development. Studies on the detailed mouthpart structure of several fluid-feeding insect lineages indicate targeting of certain tubular features of gymnosperm ovulate organs that previously were considered anomalous and difficult to interpret. One mouthpart type, the long-proboscid condition, consists of elongate, tubular (siphonate) proboscises that accessed surface fluids powered by a cibarial pump, often assisted with a distal proboscis sponging organ. These proboscises were received in ovulate organs through often intricate cupulate integumental tubes, interovular channels, salpinx tubuli, pappus tubules, prolonged micropyles, and a catchment funnel-pipe-micropyle device. These ovulate structures also are consistent with insect access to nutritive rewards, including pollination drops, nectarial secretions, and pollen. Other evidence for pollination includes the entomophilous structure and size of pollen found on insect and plant contact surfaces and in insect guts, nutritional levels of modern pollination drop fluids similar to angiosperm nectar for supporting metabolically high activity levels of aerially active insects, and plant-host outcrossing. While the long-proboscid pollination mode of fluid feeding targeted gymnosperm hosts that deployed unisexual reproductive organs at some distance from each other, either on the same or on different plants, another mode of pollination, that of mandibulate insects, consumed typically solid tissues in compact bisexual strobili, targeting pollen and perhaps pollination drops (adults), and associated sterile tissues (larvae). These two groups of pollinator associations were irretrievably altered as angiosperms diversified during a 35-million-year interval of the Early Cretaceous, evident in three patterns. First was the demise of most pollinator associations that evolved during the preceding 65 million years; second was the lateral transfer of some of these associations onto angiosperms that continue today as relicts; and third was emergence of new pollinator associations with angiosperms.
Over the past two decades there has been remarkable progress in resolving angiosperm phylogeny and patterns of evolution. Analyses of large, primarily plastid molecular data sets have revealed new insights into numerous historically contentious problems of deep-level angiosperm phylogeny, including relationships among “basal angiosperms” (not members of either the eudicot or monocot clades), among clades of Mesangiospermae, and among major clades of eudicots. The same large data sets also have provided evidence for numerous rapid radiations throughout the evolution of angiosperms. The five lineages of Mesangiospermae, as well as most major core eudicot lineages, each likely arose within a narrow range of just a few million years. The rapid radiations in rosids (Rosidae) gave rise to angiosperm-dominated forests, which are also associated with the diversification of ants, beetles, hemipterans, amphibians, and most extant ferns. Ongoing phylogenetic analyses now routinely construct phylogenetic hypotheses encompassing thousands of taxa. Such trees enable us to take a broad phylogenetic perspective on character evolution, community assembly, and conservation. While the wealth of new sequence data continues to transform the study of angiosperm evolution, it also presents major computational and informatic challenges associated with the management and analysis of enormous data sets.
The recycling of transpired water is well known to be an important source of rainfall, particularly in the tropics, and angiosperms have transpiration capacities higher than any other plants throughout evolutionary history. Thus, the evolution and rise to ecological dominance of flowering plants are proposed to have strongly altered climate. Transpiration capacity is closely correlated with leaf vein density, and the average vein density of angiosperm leaves is four times greater than that of all other plants, living or extinct. A rapid transition to high vein densities occurred separately in three or more flowering plant lineages about 100 million years ago. Climate modeling of the impact of this physiological revolution indicates that the tropics would be hotter, drier, and more seasonal in the absence of the angiosperms, and the overall area of tropical rainforest would decline substantially. Because angiosperm diversity is influenced by rainforest area and by precipitation abundance and evenness, the high diversity of angiosperms is partially a product of a positive feedback loop with the climate modifications initiated by the angiosperms themselves. Lineage diversifications among vertebrate and invertebrate animals and nonangiospermous plants in the wake of the angiosperm radiation may be tied to the unprecedented impact of angiosperms on climate.
Eudicots and their currently recognized major subclades are characterized as to floral structure (including some “embryological” features) based on ca. 3000 original publications. A new classification of nucelli is presented. In particular, the distinction between tenuinucellar and incompletely tenuinucellar ovules has proven to be useful for the characterization of larger subclades. Tenuinucellar ovules characterize Gentianales, and largely Lamiales of lamiids, and Asteraceae of campanulids, thus all of the most diversified subclades of asterids. In contrast, incompletely tenuinucellar ovules are present in basal asterids and the less diversified subclades of lamiids and campanulids. Interestingly, Santalales and Caryophyllales, which have been tentatively classified in the asterid lineage s.l., have relatively thin nucelli. Also, the presence of endosperm haustoria or the particular differentiation of the embryo suspensor characterizes some larger subclades such as Fabales and Lamiales or core Saxifragales, respectively. The expanded malvids (with inclusion of Crossosomatales, Geraniales, and Myrtales) are supported by floral features. For several features that are potential key innovations, because they characterize strongly diversified clades (e.g., pentamerous flowers in core eudicots or monosymmetric flowers in Lamiales), it is shown that they are also sporadically present in species-poor clades more to the base of the phylogenetic tree. This indicates that what appear as key innovations in certain groups may be present in latent form long before their manifestation as a key innovation. Thus, it is possible that such key innovations are not always apomorphies in a strict sense (e.g., monosymmetric flowers). This conforms with results of evo-devo research, in which genetic structures connected with floral features were found in clades in which the floral features are not present.
The order Poales comprises a substantial portion of plant life (7% of all angiosperms and 33% of monocots) and includes taxa of enormous economic and ecological significance. Molecular and morphological studies over the past two decades, however, leave uncertain many relationships within Poales and among allied commelinid orders. Here we present the results of an initial project by the Monocot AToL (Angiosperm Tree of Life) team on phylogeny and evolution in Poales, using sequence data for 81 plastid genes (exceeding 101 aligned kb) from 83 species of angiosperms. We recovered highly concordant relationships using maximum likelihood (ML) and maximum parsimony (MP), with 98.2% mean ML bootstrap support across monocots. For the first time, ML resolves ties among Poales and other commelinid orders with moderate to strong support. Analyses provide strong support for Bromeliaceae being sister to the rest of Poales; Typhaceae, Rapateaceae, and cyperids (sedges, rushes, and their allies) emerge next along the phylogenetic spine. Graminids (grasses and their allies) and restiids (Restionaceae and its allies) are well supported as sister taxa. MP identifies a xyrid clade (Eriocaulaceae, Mayacaceae, Xyridaceae) sister to cyperids, but ML (with much stronger support) places them as a grade with respect to restiids graminids. The conflict in resolution between these analyses likely reflects long-branch attraction and highly elevated substitution rates in some Poales. All other familial relationships within the order are strongly supported by both MP and ML analyses. Character-state mapping implies that ancestral Poales lived in sunny, fire-prone, at least seasonally damp/wet, and possibly nutrient-poor sites, and were animal pollinated. Five subsequent shifts to wind pollination—in Typhaceae, cyperids, restiids, Ecdeiocoleaceae, and the vast PACCMAD-BEP clade of grasses—are significantly correlated with shifts to open habitats and small, inconspicuous, unisexual, and nectar-free flowers. Prime ecological movers driving the repeated evolution of wind pollination in Poales appear to include open habitats combined with the high local dominance of conspecific taxa, with the latter resulting from large-scale disturbances, combined with tall plant stature, vigorous vegetative spread, and positive ecological feedback. Reproductive assurance in the absence of reliable animal visitation probably favored wind pollination in annuals and short-statured perennials of Centrolepidaceae in ephemerally wet depressions and windswept alpine sites.
This paper reviews monocot flower structure and gynoecium development and evaluates these data to clarify the evolutionary history of the monocot flower. Despite some congruence between molecular and morphological data regarding the delimitation and phylogenetic relationships of monocots, there is currently no universally accepted view on the morphology of the ancestral monocot flower, reflecting a high degree of parallelism in monocot floral evolution. We focus on two character suites that encompass the key features of monocot flowers: (1) the typical monocot groundplan of trimerous-pentacyclic flowers, and (2) a character suite related to carpel fusion, including postgenital fusion between carpels and the presence of septal nectaries. It is likely that the trimerous-pentacyclic flower represents a major synapomorphy of monocots; this flower groundplan is virtually absent from the closest relatives of monocots. Such close correlation of a particular groundplan with a phylogenetic group is analogous with the absence of the typical eudicot flower groundplan in basal eudicots, though in both instances the underlying constraints are obscure. In monocots, morphogenetic studies and analysis of character correlations lead us to favor a hypothesis that the ancestral monocot conditions were postgenital fusion between carpels and presence of septal (gynopleural) nectaries. This character-suite optimization contrasts with optimizations of individual morphological characters, which suggest that the ancestral monocot flower possessed congenitally united carpels (with no contribution of postgenital fusion) and lacked septal nectaries. Among extant early divergent monocots, flowers of Japonolirion Nakai (Petrosaviaceae s.l. or Japonoliriaceae, Petrosaviales) appear to most closely resemble those of the ancestral monocots. A gynoecium with free carpels represents a derived condition in monocots; it evolved independently in three unrelated groups (Triuridaceae, Arecaceae, Alismatales), with several gains of apocarpy in Alismatales and palms. All three monocot groups that include free-carpellate species show significant variation in their individual floral groundplans.
Symbioses of grasses (Poaceae) with fungi of family Clavicipitaceae vary widely in relative benefits and detriments to the plant, and include mutualisms characterized by vertical transmission and protective effects of the fungus and its metabolites against vertebrate and invertebrate herbivores. This review focuses on the epichloae, a group of fungi within the Clavicipitaceae that are symbiotic with members of the grass subfamily Poöideae and comprise sexual fungi of genus Epichloë (Fr.) Tul. & C. Tul. and their asexual derivatives (form genus Neotyphodium A. E. Glenn, C. W. Bacon & R. T. Hanlin). Most epichloid fungi are vertically transmissible, and many produce antiherbivore alkaloids belonging to any of four distinct chemical classes: lolines, peramine, ergot alkaloids, and indole-diterpenes. Like many plant-associated Clavicipitaceae, fruiting of Epichloë species chokes host inflorescences, preventing seed production on the affected tillers. However, most Epichloë-infected grasses also produce asymptomatic inflorescences, which produce normal seeds that vertically transmit the symbiont. Vertical transmission provides the main or sole means of dissemination for asexual epichloae. Molecular phylogenetic evidence suggests that the origin of Poöideae-Epichloë symbioses was approximately coincident with the origin of this grass subfamily, with considerable cophylogenetic evolution since then. Most asexual epichloae arose from a more complex process of interspecific hybridizations, which can provide evolutionary diversification and counteract accumulated deleterious mutations in otherwise clonal symbionts. Evolution of the Poöideae-epichloae symbioses required a break in the link between virulence and transmission, by establishing highly vertical transmission without negative effects on the colonized seeds and seedlings.