COLONIZATION HISTORY OF NORTH EUROPEAN FIELD VOLES (MICROTUS-AGRESTIS) REVEALED BY MITOCHONDRIAL-DNA

The genetic structure of field vole (Microtus agrestis) populations fr om northern Europe was examined by restriction fragment length polymor phisms of mitochondrial DNA (mtDNA) in 150 individuals from 67 localit ies. A total of 83 haplotypes was observed, most of which were rare an d highly localized geographically. Overall nucleotide diversity was hi gh (1.34%),but showed a tendency to decrease with higher latitude. Two major mtDNA lineages differing by 2% in nucleotide sequence were iden tified. A southern mtDNA lineage was observed in field voles from Brit ain, Denmark and southern and central Sweden, whereas voles from Finla nd and northern Sweden belonged to a northern lineage. The strict phyl ogeographic pattern suggests that the present population genetic struc ture in field voles reflects glacial history: the two groups are deriv ed from different glacial refugia, and recolonized Fennoscandia from t wo directions. A 150-200-km-wide secondary contact zone between the tw o mtDNA groups was found in northern Sweden. Distinct phylogeographic substructuring was observed within both major mtDNA groups.