Heterozygosity and fitness: No strong association in Great Lakes populations of the zebra mussel, Dreissena polymorpha (Pallas)

A number of studies have found positive associations between allozyme heter ozygosity and fitness surrogates (e.g., body size and growth rate) for mari ne molluscs. We investigated whether similar relationships exist for freshw ater populations of the zebra mussel, Dreissena polymorpha. Only one signif icant correlation between multi-locus heterozygosity and shell length was o bserved for a total of 22 D. polymorpha populations surveyed from midwester n U.S.A. lakes and streams, and the result was not significant on a table-w ide basis. Meta-analysis revealed a significant common correlation coeffici ent (effect magnitude) between multi-locus heterozygosity and shell length across all 22 sites (r(c) = 0.052, P = 0.019, 1557 df). However, the varian ce in shell length explained by multi-locus heterozygosity was small (r(c)( 2) = 0.0027), implying a weak causal relationship ii any. Also, we saw no r elationship between heterozygosity and growth rate in a one-year field encl osure experiment. A significant heterozygosity-shell length correlation pre viously reported for a zebra mussel population at Put-in-Bay, Lake Erie, Oh io, may have been the product of unique population dynamics, rather than na tural selection. Similar demographic considerations may contribute to incon sistencies in heterozygosity-fitness correlations seen for other molluscs.