ALLOZYME HETEROZYGOSITY, FOUNDER EFFECT AND FITNESS TRAITS IN A CULTIVATED POPULATION OF THE EUROPEAN OYSTER, OSTREA-EDULIS

The positive relationship between multilocus heterozygosity and growth rate has been extensively investigated in populations of bivalves, Co mparatively, the relationship between multilocus heterozygosity and vi ability is poorly known, We have studied the relationship of multilocu s heterozygosity (computed for ten allozyme genes) with viability betw een 6 and 12 months of age and growth rate in a population of the Euro pean oyster, Ostrea edulis, obtained in a hatchery by mass-spawning an d grown outdoors, No significant differences in viability among multil ocus heterozygotes were observed. The correlation between multilocus h eterozygosity and viability was also non-significant for the three rep licates into which the population was divided and when averages over r eplicates were considered, Similar results were obtained for the growt h 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 individua ls, and perhaps as low as 3.5. The absence of significant positive cor relations between multiple-locus heterozygosity and growth rate may be explained by this 'founder effect'. The absence of correlations betwe en multiple-locus heterozygosity and viability suggests that the relat ionship between these two variables may also be affected by founder ef fects, However, other explanations are possible, including: (a) the ag e transition considered was too short to detect significant difference s in viability; (b) selective mortality acting preferentially before 6 months of age. When looking at individual loci, we observed heterozyg ote advantage in viability at Ap-2 and Ark, two loci that have been sh own to exhibit high levels of geographical differentiation in natural populations of the species.