Genetic structure of Siberian lemmings (Lemmus sibiricus) in a continuous habitat: large patches rather than isolation by distance

In a continuous habitat, restricted dispersal and local genetic drift are l ikely to create a pattern of increasing genetic differentiation with distan ce. Here, we describe the genetic structure of Siberian lemming (Lemmus sib iricus) populations in a continuous tundra habitat on the western coast of the Taimyr Peninsula, in order to determine the spatial scale at which gene tic differentiation and isolation by distance occur. Sampling was carried o ut at three different geographical scales: (1) a continuous 11 km transect; (2) localities 10-30 km apart; and (3) two localities at 300 and 600 km fr om the main study area. Two types of genetic markers were used: partial seq uences of the mitochondrial DNA control region and four microsatellite loci . On this basis the study populations were genetically quite homogeneous wi thin patches extending over 8 km or more. Contrary to theoretical predictio ns, no pattern of isolation by distance among patches could be identified. This observation was interpreted as representing populations in migration-d rift disequilibrium after a recent major mixing event. The lack of concorda nce between mtDNA haplotype phylogeny and the geographical distribution of haplotypes supported this interpretation. Spatial autocorrelation among ind ividual genotypes on a local scale was weak and observed only in females, i ndicating a considerable amount of mostly male-mediated gene flow. Average gene flow per generation was estimated to be in the range of several hundre d metres.