Postglacial Origin and Regional Differentiation of Microtus arvalis in the Baltic Region

Postglacial expansion dynamics strongly influence the genetic structure of temperate species; however, mitochondrial data from the Baltic region are limited. To assess diversity, phylogenetic origins, and regional structuring, we analyzed mitochondrial cytochrome b (726 bp) and control region (421–422 bp) sequences of the common vole (Microtus arvalis Pallas, 1779) from Lithuania. Of the 91 cytb sequences and 70 control region sequences analyzed, five and four haplotypes were identified, respectively. Markedly low haplotype and nucleotide diversity compared with most European populations were detected. Phylogenetic Maximum Likelihood and network analyses revealed that all Lithuanian haplotypes belong to the eastern European lineage and are most closely related to Polish and central European samples, which supports recolonization from a Carpathian refugium. Despite the overall low variation, we detected two distinct mitochondrial groups: a highly differentiated western group and a second group encompassing eastern, northern, and central–southern populations. This strong regional structuring suggests limited maternal gene flow on a small geographic scale. There was no evidence of introgression from related taxa, such as Microtus obscurus. Our findings refine the phylogeographic context of Baltic M. arvalis and highlight the region’s role in shaping postglacial diversity patterns.