MITOCHONDRIAL, DNA VARIATION IN THE FIELD VOLE (MICROTUS-AGRESTIS) - REGIONAL POPULATION-STRUCTURE AND COLONIZATION HISTORY

Restriction fragment length polymorphism analysis of mitochondrial DNA (mtDNA) was used to examine the genetic structure among field voles ( Microtus agrestis) from southern and central Sweden. A total of 57 hap lotypes was identified in 158 voles from 60 localities. Overall mtDNA diversity was high, but both haplotype and nucleotide diversity exhibi ted pronounced geographic heterogeneity. Phylogenetic analyses reveale d a shallow tree with seven primary mtDNA lineages separated by sequen ce divergences ranging from 0.6% to 1.0%. The geographic structure of mtDNA diversity and lineage distribution was complex but strongly stru ctured and deviated significantly from an equilibrium situation. The e xtensive mtDNA diversity observed and the recent biogeographic history of the region suggests that the shallow mtDNA structure in the held v ole cannot be explained solely by stochastic lineage sorting in situ o r isolation by distance. Instead, the data suggest that the genetic im prints of historical demographic conditions and vicariant geographic e vents have been preserved and to a large extent determine the contempo rary geographic distribution of mtDNA variation. A plausible historica l scenario involves differentiation of mtDNA lineages in local populat ions in glacial refugia, a moving postglacial population structure, an d bottlenecks and expansions of mtDNA lineages during the postglacial recolonization of Sweden. By combining the mtDNA data with an analysis of Y-chromosome variation, a specific population unit was identified in southwestern Sweden. This population, defined by a unique mtDNA lin eage and fixation of a Y-chromosome variant, probably originated in a population bottleneck in southern Sweden about 12,000 to 13,000 calend ar years ago.