A molecular phylogeny of the grass subfamily Panicoideae (Poaceae) shows multiple origins of C-4 photosynthesis

DNA sequence data from the chloroplast gene ndhF were analyzed to estimate the phylogeny of the subfamily Panicoideae, with emphasis on the tribe Pani ceae. Our data suggest that the subfamily is divided into three strongly su pported clades, corresponding to groups with largely identical base chromos ome numbers. Relationships among the three clades are unclean In unweighted parsimony analyses, the two major clades with x = 10 (Andropogoneae and x = 10 Paniceae) are weakly supported as sister taxa. The third large clade c orresponds to x = 9 Paniceae. In analyses under implied weight, the two cla des of Paniceae are sisters, making the tribe monophyletic. Neither resolut ion is strongly supported. Our molecular phylogenies are not congruent with previous classifications o f tribes or subtribes. Based on this sample of species, we infer that C, ph otosynthesis has evolved independently several times, although a single ori gin with multiple reversals and several reacquisitions is only slightly les s parsimonious. The phosphoenol pyruvate carboxykinase (PCK) subtype of C. photosynthesis has evolved only once, as has the NAD-malic enzyme (ME) subt ype; all other origins are NADP-ME. Inflorescence bristles are apparently h omologous in the genera Setaria and Pennisetum, contrary to opinions of mos t previous authors. Some genera, such as Digitaria, Echinochloa, and Homole pis are supported as monophyletic. The large genus Paspalum is shown to be paraphyletic, with Thrasya derived from within it. As expected, Panicum is polyphyletic, with lineages derived from multiple ancestors across the tree . Panicum subg, Panicum is monophyletic. Panicum subg. Dichanthelium, subg. Agrostoides, and subg. Phanopyrum are unrelated to each other, and none is monophyletic. Only Panicum subg, Dichanthelium sect. Dichanthelium, repres ented by P. sabulorum and P. koolauense, is monophyletic. Panicum subg. Meg athyrsus, a monotypic subgenus including only the species P. maximum, is be tter placed in Urochloa, as suggested by other authors.