Microsatellite variation in Daphnia pulex from both sides of the Baltic Sea

Despite large genetic differentiation among neighbouring populations of man y freshwater zooplankton species, a macrogeographical homogeneity of allozy me variation is generally observed. A study on breeding systems in Scandina vian populations of Daphnia pulex suggested a latitudinally related dine in breeding system with both diploid cyclic parthenogens and diploid obligate parthenogens at the latitude of 60-61 degrees N. Variation at neutral mark ers may be more affected by selection at linked loci in such species than i n strictly sexual species. In this paper I present a study of variation at five microsatellite loci in a total of 34 populations from small ponds and rockpools on both sides of the Baltic Sea at 60-61 degrees N. Two major gro ups, which may represent different species of the D. pulex complex, are def ined with the microsatellites. Neighbouring populations show both similar a nd well differentiated genetic composition. Populations separated by larger geographical distances show only a large differentiation and a macrogeogra phic pattern. The large differentiation observed at small distances can be explained with small effective population size: variation at the microsatel lite loci has been shaped by population bottlenecks followed with expansion in size, and possibly by selection. No conclusive evidence is found for ob ligative parthenogenesis.