Context-dependent survival differences among electrophoretic genotypes in natural populations of the marine bivalve Spisula ovalis.

We investigated the relationships between allozyme genotypes at nine polymorphic loci and survival in a natural population of the bivalve Spisula ovalis sampled on three occasions (1993, 1994, and 1995) in three different sites (2855 individuals analyzed). This species displays annual growth lines allowing identification of annual cohorts. Therefore we could avoid cohort mixing, a frequent bias in such studies, and evaluate the consistency of the observed effects across cohorts and sites. Significant viability differences were observed both among alleles and between heterozygotes and homozygotes at some loci. Multiple-locus heterozygosity was positively correlated with viability in the 1993-1994 period, but not in the 1994-1995 interval. The observed selective effects were significantly dependent on the cohort and the site considered. A bibliographic survey suggests that such variability is a common feature of studies analyzing heterozygosity-survival relationships. Two explanations are consistent with our results. First, allozyme genotypes may have direct effects on viability that interact with subtle environmental variation in a complex and unpredictable way. Second, allozyme genotypes may be transiently associated with other viability genes responsible for heterotic effects. In any case, the results militate against allozyme loci being themselves consistently overdominant for viability in natural populations.