Glacial perturbations shaped the genetic population structure of the endangered thick-shelled river mussel (Unio crassus, Philipsson 1788) in Central and Northern Europe

Abstract

The thick-shelled river mussel, Unio crassus Philipsson 1788, was formerly one of the most abundant and widely spread freshwater bivalves among the order of Unionida in Europe, but there is still a lack of information on its genetic diversity and spatial genetic structure. We characterised the genetic constitution of 18 U. crassus populations in Germany and Sweden originating from six major drainage systems using a set of nine microsatellite markers. U. crassus populations from Northern Germany and Sweden revealed similar genetic constitution with high genetic diversity, whereas populations from Southern Germany showed higher proportions of common ancestors and a stronger host-dependent diversification. The structured spatial genetic patterns suggest two glacial refugia and a strong north–south differentiation. The Elbe river basin was likely an important source for recolonisation northwards to Sweden, and populations from the low mountain range in the Danube–Rhine systems acted as a source for recolonisation southwards to the Alps after retreating glaciations. We identified three major conservation units for the analysed U. crassus populations which should be considered in future conservation efforts.