We are using sequences of phytochrome B, a low-copy nuclear gene, to reconstruct phylogenetic relationships within Poaceae subfamily Panicoideae. Results corroborate relationships found by two earlier studies based on chloroplast markers (ndhF; combined sequences of the trnL intron and the trnL-trnF intergenic spacer regions). The Old World genus Danthoniopsis is excluded from the Panicoideae. Tribe Andropogoneae, including Arundinella, is monophyletic. Tribe Paniceae is also monophyletic, but resolution within the tribe differs from that of the chloroplast phylogenies. The chloroplast trees recovered two major clades corresponding to chromosome number, with one clade comprising the x = 10 taxa and a second clade composed of taxa with a base number of x = 9. The nuclear tree also recovers a monophyletic x = 9 clade but it is nested within a paraphyletic x = 10 group. This apparent paraphyly may be a sampling artifact, since our sampling to date within this group is minimal. Relationships within the x = 9 clade correspond to groups sharing the same C4 photosynthetic subtype, with one group using NAD-malic enzyme, one using PCK, and a third using NADP-malic enzyme (the "bristle" clade). All studies have demonstrated multiple origins of the C4 photosynthetic pathway within the Panicoideae, leading to the unexpected revelation that chromosome number is a better predictor of phylogenetic relationships than is photosynthetic pathway.

Key words: Panicoideae, phytochrome B, Poaceae