ALTERNATIVE MODELS FOR ALLOZYME-ASSOCIATED HETEROSIS IN THE MARINE BIVALVE SPISULA-OVALIS

Correlations between allozyme heterozygosity and fitness-related trait s, especially growth, have been documented in natural populations of m arine bivalves. However, no consistent pattern has been exhibited, bec ause heterotic effects on size vary with age and individual growth par ameters are generally unknown. No consensus has emerged on the genetic basis of allozyme-associated heterosis. The species studied here, Spi sula ovalis, displays annual shell growth lines, which allows us to co mpute individual age and growth dynamics over the whole life span. Our morphological study was coupled to a protein electrophoresis study at seven polymorphic loci. while the maximum size gained is not related to heterozygosity, the age at half maximum size, t(1/2), is significan tly negatively correlated with heterozygosity, indicating an heterotic effect on initial growth. The correlation between heterozygosity and size is expected to vanish when age increases, due to the form of the growth function. This decreasing correlation is consistent with previo us studies. We compare the relative performances of five linear models to analyze the genetic basis of heterosis. Surprisingly, the largest part of variance in t(1/2) is due to additive effects, the overdominan t components being much weaker. Heterosis is therefore due to general genomic effects rather than to local overdominance restricted to alloz ymes or small neighboring chromosomal segments. A significant dependen ce of individual heterotic contributions of the enzyme loci upon expec ted heterozygosities, rather than metabolic function, further supports the hypothesis of enzymes acting as markers. General genomic effects can hold only if allozyme heterozygosity is positively correlated with heterozygosity at fitness-related genes scattered throughout the geno me. This hypothesis is supported here by heterozygosity correlations b etween enzymatic loci.