No abstract

SummaryMany plant species comprising the present-day Arctic flora are thought to have originated in the high mountains of North America and Eurasia, migrated northwards as global temperatures fell during the late Tertiary period, and thereafter attained a circumarctic distribution. However, supporting evidence for this hypothesis that provides a temporal framework for the origin, spread and initial attainment of a circumarctic distribution by an arctic plant is currently lacking.Here we examined the origin and initial formation of a circumarctic distribution of the arctic mountain sorrel (Oxyria digyna) by conducting a phylogeographic analysis of plastid and nuclear gene DNA variation.We provide evidence for an origin of this species in the Qinghai-Tibet Plateau of southwestern China, followed by migration into Russia c. 11 million yr ago (Ma), eastwards into North America by c. 4 Ma, and westwards into Western Europe by c. 1.96 Ma. Thereafter, the species attained a circumarctic distribution by colonizing Greenland from both sides of the Atlantic Ocean.Following the arrival of the species in North America and Europe, population sizes appear to have increased and then stabilized there over the last 1 million yr. However, in Greenland a marked reduction followed by an expansion in population size is indicated to have occurred during the Pleistocene.

Aim  Late Pleistocene ice sheets are thought to have covered most of western Canada, including all of British Columbia (BC). We examine patterns of genetic variation in an Arctic–alpine plant to evaluate the possibility of full glacial refugia within the area covered by the Cordilleran ice sheet (CIS) and to uncover post‐glacial migration routes.Location  Western North America.Methods  We sampled 1030 individuals of the Arctic–alpine plant Oxyria digyna from 117 populations distributed over much of its range in western and northern North America. DNA haplotypes were identified using restriction site analysis of two chloroplast DNA intergene spacer regions, psbA‐trnH and trnT‐L. We examined the geographical distribution of haplotype diversity in relation to latitude, and also compared various indices of diversity in putatively glaciated and unglaciated regions. Patterns of migration were inferred using nested clade analysis.Results  We detected a total of 20 haplotypes. High haplotype diversity was found in Beringia, in unglaciated western USA, and in northern BC at 57–59° N, well within the accepted limits of the CIS. Ancestral haplotypes were also centred in northern BC.Main conclusions  High genetic diversity of Oxyria digyna is expected in unglaciated regions, but unexpected in northern BC if British Columbia was entirely covered by ice during the late Pleistocene. Our observations suggest the presence of unglaciated areas providing late Pleistocene refugia in northern BC. Such refugia would have important implications for the origins and migrations of many plant and animal species in north‐western North America.

The ranges of arctic–alpine species have shifted extensively with Pleistocene climate changes and glaciations. Using sequence data from the trnH-psbA and trnT-trnL chloroplast DNA spacer regions, we investigated the phylogeography of the widespread, ancient (>3 million years) arctic–alpine plant Oxyria digyna (Polygonaceae). We identified 45 haplotypes and six highly divergent major lineages; estimated ages of these lineages (time to most recent common ancestor, TMRCA) ranged from ∼0.5 to 2.5 million years. One lineage is widespread in the arctic, a second is restricted to the southern Rocky Mountains of the western United States, and a third was found only in the Himalayan and Altai regions of Asia. Three other lineages are widespread in western North America, where they overlap extensively. The high genetic diversity and the presence of divergent major cpDNA lineages within Oxyria digyna reflect its age and suggest that it was widespread during much of its history. The distributions of individual lineages indicate repeated spread of Oxyria digyna through North America over multiple glacial cycles. During the Last Glacial Maximum it persisted in multiple refugia in western North America, including Beringia, south of the continental ice, and within the northern limits of the Cordilleran ice sheet. Our data contribute to a growing body of evidence that arctic–alpine species have migrated from different source regions over multiple glacial cycles and that cryptic refugia contributed to persistence through the Last Glacial Maximum.

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, trnH–psbA and trnS–G, 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 22 bp 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.