Poa (at ca. 500 species perhaps the largest grass genus) occurs throughout temperate and boreal regions of the globe. A new phylogenetic study of Poa, based on data from restriction site analysis of PCR amplified regions of chloroplast DNA (cpDNA) (trnT-trnF, trnF-trnV, trnV-rbcL, rbcL-ORF106, trnH-trnK), expands sampling to where about 1/6 of the species have been characterized for cpDNA types. A broad phylogenetic structure detected in an earlier study (Soreng, AJB 77: 1383-1400. 1990) gained additional and robust support. Accounting for wide extrageneric sampling, Poa still remains monophyletic if Austrofestuca is included within it. The new analysis suggests that divergence in morphology and ecology is often decoupled from the frequency of fixation of phylogenetically informative cpDNA variation. Some species groups are widely distributed with strikingly disparate morphology and ecology, yet have diverged little in their cpDNA profiles, whereas the opposite pattern occurs in other groups. One of the largest and most derived clades with morphologically and ecologically diverse species that show little cpDNA divergence includes: Austrofestuca, several P. subg. Poa sections (Dasypoa, Dioicopoa, Homalopoa, Madropoa) and other informal groups, with species endemic to Europe, China, Australia, North America, and South America. One small, early diverging cpDNA lineage includes two morphologically and ecologically unrelated subgroups (Poa subg. Arctopoa of alkaline seeps of the high Tibetan Plateau to sub-arctic salt marshes; and P. subg. Poa sect. Sylvestres of forests of the Appalachian Mts.), but few cpDNA characters distinguish them. In contrast, in another large, derived clade (P. sects. Abbreviatae, Oreinos, Pandemos, Secundae, and Stenopoa) cpDNA variation is extensive and phylogenetically informative variation has been detected even within species. Morphological and ecological divergence, however, is not as great as in the previous two clades.

Key words: Austrofestuca, chloroplast DNA, Poa, restriction site analysis