Geographic localization of a contact zone between bank voles Clethrionomys glareolus with distinctly different mitochondrial DNA

Fennoscandian bank voles Clethrionomys glareolus, Schreber 1780 show two dis­tinctly different mitochondrial DNA (mtDNA) lineages, one occurring in the southern and central, and another in the northern parts. The mtDNA of northern bank vole populations is very similar or identical to that of the northern red-backed vole C. rutilus, Pallas, 1779 and the presence of this mtDNA lineage among northern bank voles probably is a consequence of hybridisation between the two species, To localise the geographic position of the contact zone between bank voles with the two types of mtDNA, we have analysed a transect through the area were both types of mtDNA have been found. The contact zone coincides with two other intraspecific mammalian contact zones, and is adjacent to a third zone. These zones constitute "suture zones", reflecting secondary contact between populations derived from separate glacial refugia and with synchronous timing of secondary contact. The width of the bank vole contact zone, approximately 30-60 km, is consistent with the hypothesis that it is a neutral contact zone. However, because mtDNA may penetrate reproductive barriers easier than nuclear genes it is necessary to analyse the zone with nuclear markers before drawing any definite conclusion regarding the extent of introgression.