Transfer of mitochondrial DNA from the northern red-backed vole (Clethrionomys rutilus) to the bank vole (C. glareolus).

Using a silver staining method to detect DNA fragments produced by restriction enzymes, it was possible to compare mitochondrial DNAs (mtDNAs) from 85 individuals of the bank vole (Clethrionomys glareolus) trapped at 25 localities in Fennoscandia. There are two distinctly different mtDNA lineages, one occurring in southern and central Fennoscandia and the other in the northern parts. A fragment comparison method shows about 12.7% nucleotide sequence divergence between these two lineages. This major difference between animals of the same species could theoretically be explained by intraspecific lineage survivorship independent of species hybridization, or by introduction of an atypical mtDNA via hybridization with a closely related species. Analysis of mtDNAs from the two other Clethrionomys species present in Fennoscandia (C. rutilus and C. rufocanus) shows that the mtDNA of northern C. glareolus is very similar to that of C. rutilus and that the mtDNA lineages of these two species cluster together in a phenogram, with small genetic distances among them. By contrast, electrophoresis of proteins encoded by 17 nuclear loci reveals fixed allelic differences between these two species at 8 loci. Hence the presence of two distinctly different mtDNA lineages within C. glareolus may be a consequence of a limited episode of hybridization between C. glareolus and C. rutilus, probably during the postglacial recolonization of Fennoscandia 8000-13,000 years ago.