Shifts in allele and genotype frequencies in zebra mussels, Dreissena polymorpha, along the latitudinal gradient formed by the Mississippi River

We sampled 7 populations of zebra mussels along a latitudinal gradient in t he Mississippi River. We scored 6 allozyme loci for all individuals using c ellulose acetate gel electrophoresis and estimated heterozygosity of, and g enetic variation among, populations. There were relatively large genetic di fferences among populations at the leucine aminopeptidease (Lap) locus (the ta = 0.0673, 95% CI: 0.0005-0.037). Allele frequencies differed significant ly among sampling sites at 2 of 6 loci (Lap p = 0.001, manose phosphate iso merase [Mpi] p = 0.004), and the same results were observed for genotype fr equencies (Lap p = 0.003, Mpi p = 0.001). A Mantel's test indicated a signi ficant correlation between geographic distance and genetic distance at the Lap locus (Mantel's r = 0.649, p = 0.03). We interpret these results to ind icate that nonrandom factors such as selection may be acting on the LW locu s or on linked genes. In the Mississippi River, gene flow would tend to bal ance allele frequencies among populations. Therefore, selection would have to be strong enough to overcome recruitment from upriver populations.