The positive relationship between multilocus heterozygosity and growth rate has been extensively investigated in populations of bivalves. Comparatively, the relationship between multilocus heterozygosity and viability is poorly known. We have studied the relationship of multilocus heterozygosity (computed for ten allozyme genes) with viability between 6 and 12 months of age and growth rate in a population of the European oyster, Ostrea edulis, obtained in a hatchery by mass-spawning and grown outdoors. No significant differences in viability among multilocus heterozygotes were observed. The correlation between multilocus heterozygosity and viability was also non-significant for the three replicates into which the population was divided and when averages over replicates were considered. Similar results were obtained for the growth rate. An analysis of the genetic structure of the population showed that the actual number of parents contributing to the progeny was one order of magnitude lower than the total number of broodstock individuals, and perhaps as low as 3.5. The absence of significant positive correlations between multiple-locus heterozygosity and growth rate may be explained by this 'founder effect'. The absence of correlations between multiple-locus heterozygosity and viability suggests that the relationship between these two variables may also be affected by founder effects. However, other explanations are possible, including: (a) the age transition considered was too short to detect significant differences in viability; (b) selective mortality acting preferentially before 6 months of age. When looking at individual loci, we observed heterozygote advantage in viability at Ap-2 and Ark, two loci that have been shown to exhibit high levels of geographical differentiation in natural populations of the species.
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