Population genetics of the zebra mussel, Dreissena polymorpha (Pallas): local allozyme differentiation within midwestern lakes and streams

Several aquatic invertebrates with free-swimming larvae have paradoxically demonstrated fine-scale genetic heterogeneity. In this study, we tested for genetic heterogeneity in an exotic freshwater bivalve, the zebra mussel, D reissena polymorpha (Pallas), which like many marine molluscs has a free-sw imming larval stage. Zebra mussels were collected from 22 sites in the Grea t Lakes and from a small inland lake complex in southwestern Michigan and s cored for 13 allozyme loci. Sites were sampled in a hierarchical fashion to assess the spatial scale of genetic variation. Zebra mussel populations ex hibited significant genetic heterogeneity on a local scale within lakes, ev en though populations remained homogenous on a larger regional scale betwee n lakes or lake complexes. The allozyme loci that exhibited heterogeneity d iffered from lake to lake. Populations also displayed significant heterozyg ote deficiencies from Hardy-Weinberg expectations for a majority of loci, i mplying population subdivision and (or) inbreeding on a fine scale. Our res ults suggest that local genetic differentiation for zebra mussels is both s patially and temporally fluid and is the product of stochastic processes, s uch as spawning asynchrony and uneven mixing of larval cohorts, rather than natural selection.