Abstract
Following our earlier reports on one-year-old oyster populations of Prince Edward Island (Singh and Zouros, 1978, 1981; Zouros et al., 1980), cohorts of three-year classes were studied from Cape Breton, Nova Scotia. Random samples of about 200 individuals from each year class were analyzed for four polymorphic enzymes. A number of generalizations emerged. There was a general deficiency of heterozygotes in the three age groups and this deficiency decreased with age (reflecting reduction in heterozygote deficiency). This suggests genotype-specific mortality during ontogeny. The deficiency of heterozygotes was more pronounced in slower growing, lighter individuals than in faster growing, heavier individuals. The number of heterozygous loci per individual was positively correlated with mean growth rate. The variance in weight was lower in heterozygotes; it decreased with increase in number of heterozygous loci in a given age group. Overdominance in growth rate appears to be the most plausible explanation for these observations.
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