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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.
We present a microgeographic analysis of 34 allozyme loci and the control region of mitochondrial DNA (mtDNA) in the common voleMicrotus arvalis (Pallas, 1779), performed to assess the effects of environmental heterogeneity on the distribution of genetic variation among populations in the Biebrza river valley, NE Poland. The common vole occurs there in two types of habitat: open grassland and pastures around the valley (GP populations); and abandoned fields on small hills isolated by wetlands (SH populations). No significant genetic differences were found between SH and GP populations with respect to allelic richness, nor average observed and expected heterozygosities. The average genetic differentiation at allozyme loci among the SH populations was significantly lower (F ST =0.066) than among the GP populations located around the Biebrza valley (F ST =0.112), and an isolation by distance pattern was detected (r=0.26,pr<0.05). Mitochondrial DNA differentiation among the GP populations was great (F ST =0.357,p<0.01), indicating that female dispersal was 4.4–6.5 times lower than for males. Our results and reviewed published data onM. arvalis dispersal suggest that common vole dispersal in patchy natural and semi-natural habitats is male-biased and could generate moderate population divergence, with relatively high levels of genetic variation retained within populations.
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