GROWTH RATE IN OYSTERS: AN OVERDOMINANT PHENOTYPE AND ITS POSSIBLE EXPLANATIONS

It is a well established phenomenon that homozygosity induced either by inbreeding or by especially designed experiments (such as, e.g., the technique of Sved and Ayala [1970] for comparing overall performance of chromosomal heterozygotes) generally results in low fitness. Because in all such cases a large part of the genome is made homozygous, the reduction in fitness can be equally well attributed to either reduced heterozygosity at heterotic loci, or to homozygosity for detrimental recessive genes. The problem of distinguishing between the two explanations is one of major importance in population genetics and is thoroughly discussed by Lewontin (1974). Another approach to studying the effect of heterozygosity is to correlate heterozygosity with a quantifiable character in individuals drawn randomly from a population. This approach does not rely on inbreeding, and although some inbreeding may occur in the population, if reduced homozygote performance is observed, it may well be attributed to other factors. Chromosomal inversions and electrophoretic variants provide suitable means for such an approach, the latter having the additional advantage of allowing one to study the effect of homozygosity or heterozygosity at a rather small segment of the genome. It appears, however, that most researchers have preferred to search for balancing selection in natural populations by comparing the observed levels of heterozygosity in the populations with that expected under the assumption of no selection (e.g., Marshall and Allard, 1970; Richmond and Powell, 1970; Allard et al., 1972). Only rarely has the individual's state in regard to homozygosity or heterozygosity been compared with its score for a quantitative character. Most of these studies have shown that heterozygotes perform differently than homozygotes (Koehn et al., 1973; Chaisson et al., 1976; Schaal and Levin, 1976; Stalker, 1976; Mitton, 1978). In a previous paper (Singh and Zouros, 1978) we reported the main findings from a study of a cohort of oysters. The cohort consisted of individuals of same age, drawn as spat (settling larvae) from a natural population, and grown under uniform conditions. We observed that the number of heterozygous individuals in the population was much lower than predicted from Hardy-Weinberg equilibrium. We also observed that there was a correlation between an individual's weight and the number of loci at which the individual was heterozygous. The first observation is common for populations of marine molluscs (for references see Singh and Zouros, 1978, and Skibinski et al., 1978). The second observation is less common in the literature and has an obvious bearing on the question of adaptive significance of enzyme heterozygosity. Here we report the findings of a second study. Our main objective was to see whether the patterns observed in the first study are reproducible. We have employed more loci and analyzed a much larger sample. This allowed us to firmly establish the correlation between heterozygosity and growth rate. It also provided us with a sizeable body of informati6n on which to evaluate certain hypotheses about the nature of the correlation, and ' Present address: Department of Zoology, University of Western Ontario, London, Ontario, Canada N6A 5B7.