Multilocus heterozygosity at protein loci and fitness in the European oyster, Ostrea edulis L.

Abstract

The relationship between multilocus heterozygosity at protein loci and two fitness traits, growth rate and viability, has been investigated in the European oyster, Ostrea edulis L. A cohort of individuals of the same age was sampled at two times when they were 18 and 30 months old. Weight and genotype for five polymorphic enzyme loci were determined for individuals of these two samples. A positive correlation between growth rate (measured as weight at a specific age) and multilocus heterozygosity is detected in the two age classes. The correlation is statistically significant for the 30-month-old oysters, in which heterozygosity explains about 2 per cent of the variability in growth rate among individuals. This value is very similar to those reported in several marine molluscs, as the American oyster, Crassostrea virginica (Zouros et al., 1980) and the mussel Mytilus edulis (Koehn and Gaffney, 1984), and other species. The relationship between multilocus allozyme heterozygosity and viability has usually been described in statistical terms, but the viability effects associated with heterozygosity have not been quantified. In this paper, we attempt to measure viability for individuals differing in their degree of multilocus allozyme heterozygosity by using appropriate fitness estimators. In the O. edulis cohort, a decrease of the postsettlement viability estimates is observed when individual heterozygosity increases, and this unexpected pattern might be related to the high mortalities observed. On the other hand, published data of C. virginica and M. edulis, from which a heterozygosity-viability positive correlation was reported (Zouros et al., 1983; Diehl and Koehn, 1985), are used for estimating viability fitnesses for heterozygosity classes. In all three species large differences in estimates of viability and selection coefficients appear to be associated with multilocus allozyme heterozygosity (mean postsettlement viabilities for heterozygous individuals at one or more loci are 0·51 ± 0·12 in O. edulis, 2·68 ± 0·67 and 2·35 ± 0·38 in two populations of C. virginica, and 2·19 ± 0·47 in M. edulis). The extent of the fitness estimates suggests that the magnitude of the association between allozyme heterozygosity and viability is stronger than that corresponding with growth rate in the three species of marine molluscs. The implications of these findings in relation to the genetic mechanism underlying the heterozygosity-fitness correlation are discussed. © 1989, The Genetical Society of Great Britain.