Phylogeographical patterns in the widespread arctic-alpine plant Bistorta vivipara (Polygonaceae) with emphasis on western North America

Aim We investigated genetic variation in Bistorta vivipara, a widespread Northern Hemisphere tundra species, to infer patterns of migration and where it may have survived during the Last Glacial Maximum (LGM). Location Samples came primarily from western North America, with a few from the Arctic and Eurasia. Methods We sequenced two chloroplast DNA spacer regions, trnHpsbA and trnSG, in individuals from 199 populations and mapped haplotype distributions and their relationships using a haplotype network. We calculated genetic and molecular diversity statistics for the seven geographical regions from which we obtained samples. Results Fifteen haplotypes were detected, with very little divergence among them. The haplotypes are separated into two main groups by the presence or absence of a 22bp tandem duplication. Four haplotypes are common, widespread and with substantial range overlap; 11 are rare and mostly unique to one region. Two rare haplotypes were found only in British Columbia (BC). Western North America and Asia have the highest levels of genetic and molecular diversity. Northern and southern BC have different haplotype complements. Main conclusions Bistorta vivipara has relatively low genetic diversity, with much less genetic structure than we expected for such a widespread species. We expected significant geographical structure due to the combined effects of genetic drift and geographical isolation. The asexual reproductive mode of B. vivipara may facilitate relatively rapid population establishment and spread compared with sexual reproduction by seed. Bistorta vivipara probably originated in Asia and spread to North America and Europe prior to the LGM. In western North America it spread to its modern distribution from Beringia and the western USA following the LGM. Populations in northern and southern BC may have different histories, possibly related to the timing and extent of glaciation. The occurrence of two unique haplotypes within BC suggests that some individuals may have survived in full glacial refugia within BC.