The admixture zone of Betula humilis Schrk. phylogenetic lineages follows the eastern central European suture zone

Background: Contemporary genetic variation across species ranges reflects Pleistocene climatic changes. The highest genetic diversity is usually found within areas of known or presumed glacial refugia and within the admixture zones of phylogenetic lineages. Aims: The aim of our study was to evaluate the genetic diversity in the populations of Betula humilis, an endangered species, to distinguish between the refugial and suture zone hypotheses. Methods: We used the PCR-RFLP method to analyse the chloroplast DNA variation in populations of B. humilis, distributed across north-eastern Poland, western Belarus and Latvia. Rarity index (DW) was calculated for each population based on 'frequency-down-weighted marker values'. We tested phylogeographic structure by using the Permut software. Results: The area studied was phylogeographically structured; DW values were low. Conclusions: Based on the low DW values, we rejected the hypothesis of a periglacial refugium in north-eastern Poland and Belarus. Most likely, the substantial genetic diversity in the area under study is a consequence of the mixing of phylogenetic lineages derived from distinct glacial refugia. The admixture zone of B. humilis follows the eastern central European suture zone.