A phylogenetic investigation of American members of the tribe Atripliceae (Chenopodiaceae) was conducted to examine evolutionary patterns and ecophysiological change in the North American lineages. Molecular phylogenetic analyses including representatives of all American genera of Atripliceae and data from 18S–26S nuclear ribosomal (ITS, ETS) and cpDNA (3′trnK intron) sequences suggest that neither Atriplex, as traditionally recognized, nor the North American members of Atriplex constitute a monophyletic group. Congruence between well-supported nuclear and cpDNA clades and results of expanded leaf anatomical and isotopic analyses indicate that American taxa belong to two distantly related lineages with different photosynthetic pathways; only one shift from C3 to C4 photosynthesis is required to explain photosynthetic pathway evolution in sampled lineages of Atripliceae. Conservatism in photosynthetic pathway evolution in the tribe is exemplified by the endemic, C3 North American Atriplex taxa, which are nested among other genera (e.g. Grayia, Holmbergia, and Zuckia) within a robust C3 clade. One North American C3 genus, Suckleya, often included within Atripliceae, should be excluded from the tribe. A previous hypothesis of variation in photosynthetic pathway within Atriplex phyllostegia sensu Hall and Clements is rejected. A revised taxonomy employing only monophyletic groups reflects this new understanding of relationships within the tribe. Extriplex includes E. californica (comb, nov.) and E. joaquinana (comb, nov.) and has not been recognized as a natural group by previous authors. Stutzia is proposed to replace the name Endolepis Torr. (1860), a later homonym of Endolepis Schleid. (1846), and includes S. covillei (comb, nov.) and S. dioica (comb. nov). Grayia (G. plummeri, comb. nov, and G. arizonica, comb. nov.) is expanded to include Zuckia. Recognition of Proatriplex as distinct from Atriplex is supported. Holmbergia is retained in Atripliceae. The improved phylogenetic understanding of Atripliceae should allow for more meaningful comparative studies of physiological and other functional adaptations of Atriplex, especially in North America.
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