Molecular Phylogeny of Pectis (Tageteae, Asteraceae), a C 4 Genus of the Neotropics, and its Sister Genus Porophyllum

Abstract

Pectis is a genus of +/- 90 xeric adapted New World species. Previous molecular phylogenetic studies showed Pectis closely related to Porophyllum, and one analysis resolved Porophyllum species nested within Pectis. Some Pectis species are known to use C-4 photosynthesis. Here we investigate the phylogeny of Pectis and Porophyllum, examine the ploidy levels and geographical distribution of Pectis species in light of its phylogeny, and infer the origin and extent of C-4 photosynthesis in both genera. Chloroplast and ITS data from 78 Pectis and 22 Porophyllum species were used to test the monophyly of Pectis and its previously described sections. Carbon isotope data were obtained to infer the photosynthetic pathway of 80 species, and the results mapped on the inferred phylogenies to determine the timing and pattern of evolution of the C-4 pathway. The ITS dataset supports a monophyletic Pectis sister to a monophyletic Porophyllum, while the chloroplast dataset places two Porophyllum species sister to a combined Pectis+Porophyllum dade. Five well-supported lineages are recovered in Pectis. All Pectis sampled have delta C-13 values consistent with C-4 photosynthesis, and all Porophyllum species sampled have delta C-13 values consistent with C-3 photosynthesis. We conclude that Pectis is monophyletic but only two of its recognized sections are monophyletic. Porophyllum is monophyletic but its sections are not. Porophyllum amplexicaule and Pr. scoparium should be treated as members of a new genus. The switch to the C-4 pathway in Pectis happened in the late Miocene, probably in north/central Mexico, at or after the divergence of Pectis and Porophyllum. This location and timing is consistent with the evolution of C-4 photosynthesis in other North American eudicot lineages, suggesting similar environmental conditions may underlie the switch to C-4 photosynthesis.