Abstract
The locus-specific effects of heterozygosity upon individual growth rate were determined for 15 polymorphic enzymes among 1906 individuals from a single cohort sample of the marine bivalve Mulinia lateralis. Two measures of individual growth rate (total wet weight and shell length) were made at collection and after a period of growth in the laboratory. The correlation between heterozygosity and growth rate was independently determined for each locus using multiple linear regression, thereby providing a rank of individual locus effects; these differed significantly. The four estimated rankings of relative locus effects (initial length, initial weight, length added in the laboratory, and added weight) were not statistically different. That is, a locus with a large effect of heterozygosity on growth rate in nature had a similarly large effect on laboratory growth rate. The effect of a locus was not related to heterozygosity per se; some highly heterozygous loci had no detectable correlation with growth rate. The data contained two pairs of relatively tightly linked loci; in both cases one locus of a pair had significant effects on growth rate, while the other had no effect. Loci with large and significant correlations with growth rate synthesize enzymes which function in protein catabolism or glycolysis; heterozygosity in enzymes of the pentose shunt, redox balance, or other miscellaneous metabolic roles was not correlated with growth rate. Since the metabolic basis for the correlation is known to derive from individual differences in net energy status, particularly energetic costs of whole-body protein turnover, these data indicate that phenotypic effects (e.g., variation in growth rate) are determined by heterozygosity at the studied genes, not other linked loci.