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 the Mississippi River. We scored 6 allozyme loci for all individuals using cellulose acetate gel electrophoresis and estimated heterozygosity of, and genetic variation among, populations. There were relatively large genetic differences among populations at the leucine aminopeptidease (Lap) locus (θ = 0.0673, 95% CI: 0.0005–0.037). Allele frequencies differed significantly among sampling sites at 2 of 6 loci (Lap p = 0.001, manose phosphate isomerase [Mpi] p = 0.004), and the same results were observed for genotype frequencies (Lap p = 0.003, Mpi p = 0.001). A Mantel’s test indicated a significant correlation between geographic distance and genetic distance at the Lap locus (Mantel’s r = 0.649, p = 0.03). We interpret these results to indicate that nonrandom factors such as selection may be acting on the Lap locus or on linked genes. In the Mississippi River, gene flow would tend to balance allele frequencies among populations. Therefore, selection would have to be strong enough to overcome recruitment from upriver populations.