Tageteae consist of approximately 216 New World species with a center of diversity in the Mexican highlands. The present study uses DNA sequences of the internal transcribed spacer region of nuclear ribosomal DNA and the 3′ end of the ndhF gene from the chloroplast to assess phylogenetic relationships within the tribe. Although trees from the two molecular makers are not completely congruent, both data sets provide support for the recognition of 21 genera of Tageteae. The combined data set generates a well-resolved tree with strong support for the monophyly of all but three genera. Tagetes is paraphyletic because Adenopappus and Vilobia are nested within the genus. Porophyllum is shown to be polyphyletic and P. tridentatum and P. crassifolium are recognized as belonging to a newly described genus Bajacalia. Two new combinations, B. tridentata and B. crassifolia, are proposed. Additionally, a new species Bajacalia moranii B. L. Turner, is described. The other species of Porophyllum examined are nested within or formed a sister group to Pectis. Dyssodia sensu lato is not monophyletic and its segregate genera should be recognized. The DNA phylogenies also support the transfer of Dyssodia jelskii into Schizotrichia, resulting in the new combination S. jelskii. The recognition of Pectis as a separate subtribe is also not supported. The lack of support for subtribal groupings and the small size of the Tageteae argue against formal recognition of subtribes. Chromosomal and biogeographic data correspond well with the combined molecular phylogeny. The phylogeny suggests that the Tageteae probably originated in Mexico and that there have been at least three independent introductions into South America. Chromosome numbers are quite variable within the tribe but are consistent within clades. The ancestral base chromosome number for the Tageteae is most likely x = 12 and there have been several episodes of aneuploid reduction in the evolution of the tribe.
Communicating Editor: John V. Freudenstein