Could Betula humilis have survived the last glaciation at a current margin of its distribution? Testing the hypothesis of a glacial refugium using nuclear microsatellites

The probability of population extinction seems to differ within the species range. Populations occupying former glacial refugia could harbor substantial genetic resources, hence they should be less prone to extirpation. It was hypothesized that the shrub birch Betula humilis could have survived the Last Glacial Maximum (LGM) at the current southwestern margin of its range. Using ten nuclear microsatellites, we studied genetic variation within and between 18 localities of B. humilis situated in marginal and subcentral areas. Six marginal populations were located in areas covered by an ice sheet during the LGM, and the remaining samples came from unglaciated areas. Analysis of private allele frequencies as well as hierarchical AMOVAs conducted for geographical regions, marginal versus central populations, and glaciated versus unglaciated areas did not confirm the hypothesis of glacial isolate of the shrub birch in southeastern Poland. On the other hand, very high genetic variation in some localities in northeastern Poland was found. Survival in periglacial areas followed by broad-fronted colonization or the existence of an admixture zone of phylogeographic lineages was proposed to explain this phenomenon.