Testing hypotheses on the dispersal and evolutionary history of freshwatermussels (Mollusca : Bivalvia : Unionidae)

The relationship between dispersal and differentiation of the European fres hwater mussel Unio pictorum (Linnaeus, 1758) was studied with molecular gen etic methods. Forty-two populations from France, Italy and central Europe w ere analysed. Genetic relationships were assessed from the geographical dis tribution of allele frequencies at 17 enzyme loci. Neighbouring groups of p opulations show small to moderate mean genetic distances (0.020 < D-mean < 0.263). With a few exceptions the genetic affinities of the populations are the closest within the same drainage basin. In central Europe and Northern Italy genetic differences between drainage systems are relatively large. P opulations from north-eastern Italy are genetically similar to Danubian pop ulations. Mussels from the islands of Corsica and Sardinia are more closely related to populations from the Italian peninsula than to French populatio ns from the Rhone drainage system. Genetic relationships within U. pictorum from central Europe reflect palaeogeographical relationships between river systems during the Pliocene and Pleistocene. Literature data on two North American unionid species and one European fish species show the same relati onship between genetic diversity and the history of drainage systems, altho ugh the correlations are less strong. In France and Italy this corresponden ce is much less evident. Population dynamic processes and human activities leading to populational bottlenecks might have obscured it.