Genetic interchange of Dreissena polymorpha populations across a canal

The zebra mussel Dreissena polymorpha , has been invading western Central Europe since the early 19th century, coming from the Pontocaspian by two routes, each interconnected by large rivers and canals constructed at that time. By 1992 these two invasion populations had been given the opportunity to mix extensively across the newly built Main–Danube canal. Different hypotheses about the dynamics of genetic interchange have been postulated, ranging from a stable intergradation zone to complete mixing. We analysed the allozyme variation at nine loci in 14 or 17 populations, respectively, across the canal on two occasions: 2½ years and 4½ years after the opening. At the first sampling a strong genetic differentiation at three loci was found between the two invasion lines. This differentiation declined during the two‐year period. The main effect was an assimilation of the Main populations according to the gene frequencies of the Danube populations, which suggest a swamping of the Main populations by Danube‐type larvae. This inference is congruent with the overall water flow regime across the canal, i.e. active pumping from the Danube into the Main drainage system. The observed interchange processes allow some inferences to be made about the selective significance of the highly heterozygous allozyme loci in D. polymorpha .