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

Although gene flow and population fragmentationwill often have opposed effects on geneticstructure, their actual effects on many elusiveanimal species are unknown. We assessed sucheffects in British populations of the Eurasianotter Lutra lutra by analysis ofgenotypes consisting of 12 microsatellites from618 carcasses representing the period 1982–1998. Spatial patterns of genetic subdivisionand levels of polymorphism in the continuouspopulation in Scotland were estimated. Theseresults were used to infer patterns of geneflow in Scottish otters and assess theinfluence of fragmentation on the geneticstructure of otters in Wales and SW England.The latter showed no higher genetic divergencethan expected given the degree of isolation bydistance found in the Scottish population, andtheir distributions of microsatellite allelesizes provided no evidence for populationbottlenecks. Nonetheless, otters in southernBritain contained significantly lower levels ofmicrosatellite polymorphism than otters inScotland, and the population in the westernpeninsula of SW England was geneticallydistinct. These results suggested that thegenetic structure of the Scottish population isdue more to restricted contemporary gene flowthan to historical fluctuations insubpopulation size, and that the geneticstructure of the southern British populationsis due more to small historical effective sizesthan to recent declines. If spatiallyrestricted gene flow is typical of all Eurasianotter populations then data on dispersal shouldbe taken into account when sitting protectedareas for this species.