Genetic analysis of Eurasian otters (Lutra lutra) reveals high admixture in Finland and pronounced differentiation in Sweden

A number of mammal species in Europe, including the Eurasian otter (Lutra lutra), have experienced a decline in population size in the 20th century due to persecution, environmental pollution and ongoing habitat fragmentation. This has often led to a substantial loss of genetic diversity which may threaten population viability. While otters have been studied in some detail genetically, the northern part of the Fennoscandian range has not been covered well so far. By explicitly focussing on the genetics of otter populations from northern Sweden and Finland we aimed at closing that gap. To infer their genetic structure and diversity, we analysed sequences of the mitochondrial control region and 12 nuclear microsatellite markers in 197 Eurasian otters from Sweden and Finland. Variability of the mitochondrial control region was low overall but still revealed previously undetected haplotypes unique to the Finnish otter population. Expected heterozygosities in Fennoscandia were within the range previously reported.Bayesian cluster analysis of our microsatellite data revealed genetic structuring of the Swedish otter populations. In contrast, we observed a high degree of admixture among the Finnish populations that we also found at the geographic border of the two countries (Lapland).Admixed ancestry in Finnish otters suggests that gene flow from the Swedish to Central European populations is potentially facilitated via the Finnish otter populations connecting the Swedish animals with otter populations in mainland Europe.