HYBRIDIZATION AND THE ORIGIN OF THE ARCTIC GRASS POA HARTZII (POACEAE) - EVIDENCE FROM MORPHOLOGY AND CHLOROPLAST DNA RESTRICTION SITE DATA

The hypothesized hybrid origin of Poa hartzii Gand. (Poaceae) was inve stigated by analysis of morphological and molecular data. This endemic nearctic caespitose grass has been considered to be a hybrid between two of the three sympatric arctic species, Poa glauca M. Vahl, Poa arc tica R. Br., and Poa abbreviata R. Br. Field observations and morpholo gical studies indicate that Poa hartzii is a morphologically distinct apomictic species that reproduces and disperses by seed. Restriction e nzyme analysis of polymerase chain reaction amplified chloroplast DNA revealed the presence in Poa hartzii of two very different haplotypes. One haplotype is identical to the dominant type found in Pou glauca, while the second is identical to the haplotype of Poa secunda J. Presl ., located south of the arctic region. These results are consistent wi th an hypothesis of ancient hybrid origin involving Poa glauca and Poa secunda, but not Poa arctica nor Poa abbreviata. They are also consis tent with an hypothesis of cytoplasmic transfer via hybridization and introgression from Poa glauca to an ancestral Poa hartzii in the Poa s ecunda complex. Direction of transfer is suggested by the widespread o ccurrence of the Poa secunda haplotype in Pna hartzii and by closer mo rphological similarity with Poa secunda than Poa glauca. The origin of Poa hartzii provides an excellent example of reticulate evolution and the importance of hybridization in the speciation of arctic grasses.