Intraspecific Polymorphism of the Mitochondrial DNA Control Region and Phylogeography of Little Ground Squirrel (Spermophilus pygmaeus, Sciuridae, Rodentia)

Little ground squirrel (Spermophilus pygmaeus) is a polytypic species with a wide geographic range, divided by a number of large rivers (Don, Volga, and Ural rivers). Preliminary analysis of the mtDNA control region revealed substantial genetic differentiation (7%) between populations of little ground squirrel inhabiting the right and left banks of the Volga River, suggesting that large rivers act as geographic barriers for this species. Here, we present the results of the genetic diversity analysis of little ground squirrel (52 individuals from 35 localities) across the species range. The Don and Ural rivers, unlike the Volga River, had no considerable effect on the phylogeographic structure of little ground squirrel. Geographic subdivision found in ground squirrels living to the east of the Volga River could be explained by the influence of a large paleo-river flowing along the Turgai valley during glacial periods. The Volga River divides the species into two major groups (western and eastern), both with a high level of haplotype diversity. The genetic variability indices are almost twofold higher in the western clade. The results of neutrality and isolation by distance tests support a relatively recent range expansion of the eastern clade. In overall, the observed structure of genetic diversity supports the hypothesis on the European origin of S. pygmaeus with the center of species origin on the territory of Caucasian geographic region. The division of little ground squirrels into western and eastern phylogenetic lineages probably occurred under the influence of fluctuations of the Caspian Sea level, due to the formation of temporary migration corridors during the regression periods.