Genetic diversity and phylogeography of the circumboreal peatland violets (Viola epipsila-V. palustris complex, Violaceae) as inferred from non-coding nuclear and chloroplast DNA regions

The Pleistocene glaciation cycles are a major factor shaping plant genetic diversity, speciation and radiations, and current distribution ranges in the Northern Hemisphere. Case studies are important for improving our understanding of postglacial migration patterns in the Northern Hemisphere taxa in general. Here, we studied the circumboreal Viola epipsila-V. palustris complex, whose taxonomy and patterns of variation are unclear, and phylogeography unknown, using non-coding nuclear (ISSRs; inter simple sequence repeats) and chloroplast (cpDNA; trnS-trnG and trnH-psbA spacers) DNA regions. Our ISSR results separated five groups corresponding to V. epipsila, V. palustris, V. pluviae, V. suecica and hybrids (V. epipsila x V. palustris). These groups were also confirmed by the genome size of selected samples from each group. The ISSR results showed low within-population genetic variation in V. epipsila, V. palustris, and V. pluviae as compared to V. suecica, which also has the widest distribution. The cpDNA data allowed us to infer postglacial migration routes and historical impact of gene flow between populations. A total of 13 haplotypes were identified across all species, with the largest number (eight) in European-eastern Asian-North American V. suecica. Five haplotypes were present in more than one taxon of the group. The low haplotype variation of European V. epipsila and V. palustris results from their status as glacial relicts. The cpDNA and ISSR phylogenies were congruent. The pattern of haplotype variation and distribution in the ranges of studied species indicates postglacial intercontinental migration of V. suecica across Beringia and V. palustris across the Atlantic.