Genomic versus chromosomal polytypy in studies of mitochondrial and nuclear DNA markers in the Microtus arvalis group

Common voles of the Microtus arvalis group distributed over the territory of European Russia are represented by three karyotypic categories, i.e., sympatric sibling species with 2n = 46 and 54, and two allopatric karyoforms in one of them, 2n = 46. For each category, molecular markers were found. For two 46-chromosome forms (arvalis and obscurus), iDNA was for the first time studied in karyotyped and non-karyotyped specimens for a parapatric hybrid zone, where high diversity of intermediate karyotypes was recorded. Preferential migration of the mitochondrial markers in arvalis and significant differences in the cline width for chromosomal and nuclear markers in obscurus were shown. The hybrid zone examined exhibited unusual combination of such features as the practically complete absence of “pure” representatives of the original parental forms and a clear deficiency of the first generation hybrids. The mtDNA divergence for the arvalis and obscurus karyoforms (4.6%) is comparable to the lowest limit for interspecies differences within the large and complex genus Microtus.