Poa sect. Poa comprises three species and numerous infraspecific taxa in North America. Poa pratensis and P. arctica are widespread and morphologically highly polymorphic with numerous infraspecific taxa, while P. macrocalyx is restricted to coastal Alaska in North America. We used restriction site analysis of PCR amplified chloroplast DNA (cpDNA) to examine variation both among species and at the infraspecific level in P. pratensis and P. arctica. Based on our larger phylogenetic analysis of Poa the section was determined to be a strongly supported monophyletic group. Variation within the section was detected in only one of the five cpDNA regions examined (trnT-trnF, trnF-trnV, trnV-rbcL, rbcL-ORF106, trnH-trnK). All infraspecific taxa of P. arctica examined shared an identical restriction site haplotype, differing from the other two species in two or three restriction sites in the rbcL-ORF106 region. In contrast, P. pratensis exhibited infraspecific cpDNA variation in the same region, with two cpDNA groups: an indigenous arctic/alpine complex comprising P. pratensis subspp. alpigena and colpodea, and an introduced non-arctic/alpine complex. These two cpDNA groups differ in three restriction sites, a level of variation greater than is generally found within species complexes in Poa. They also differ considerably in length of the rbcL-ORF106 region, suggesting that these restriction site differences may be the result of an insertion-deletion event, specifically a deletion in arctic/alpine P. pratensis. While most P. pratensis subsp. pratensis had the non-arctic/alpine haplotype, Low Arctic plants were found to have the arctic/alpine haplotype and are intermediate in overall morphology, suggesting they may be the result of hybridization between subspecies. Poa macrocalyx was found to share an identical haplotype with arctic/alpine P. pratensis. We hypothesize that the two cpDNA groups in the P. pratensis/P. macrocalyx complex represent a fundamental division between an indigenous (sub)arctic/alpine complex and an introduced non-arctic/alpine complex.

Key words: chloroplast DNA, P. arctica, P. pratensis, P. sect. Poa, Poa, restriction site analysis