Spatially restricted gene flow and reduced microsatellite polymorphism in the Eurasian otter Lutra lutra in Britain

Although gene flow and population fragmentation will often have opposed effects on genetic structure, their actual effects on many elusive animal species are unknown. We assessed such effects in British populations of the Eurasian otter Lutra lutra by analysis of genotypes consisting of 12 microsatellites from 618 carcasses representing the period 1982-1998. Spatial patterns of genetic subdivision and levels of polymorphism in the continuous population in Scotland were estimated. These results were used to infer patterns of gene flow in Scottish otters and assess the influence of fragmentation on the genetic structure of otters in Wales and SW England. The latter showed no higher genetic divergence than expected given the degree of isolation by distance found in the Scottish population, and their distributions of microsatellite allele sizes provided no evidence for population bottlenecks. Nonetheless, otters in southern Britain contained significantly lower levels of microsatellite polymorphism than otters in Scotland, and the population in the western peninsula of SW England was genetically distinct. These results suggested that the genetic structure of the Scottish population is due more to restricted contemporary gene flow than to historical fluctuations in subpopulation size, and that the genetic structure of the southern British populations is due more to small historical effective sizes than to recent declines. If spatially restricted gene flow is typical of all Eurasian otter populations then data on dispersal should be taken into account when sitting protected areas for this species.