ALLOZYME AND RFLP HETEROZYGOSITIES AS CORRELATES OF GROWTH-RATE IN THE SCALLOP PLACOPECTEN-MAGELLANICUS - A TEST OF THE ASSOCIATIVE OVERDOMINANCE HYPOTHESIS

Several studies have reported positive correlations between the degree of enzyme heterozygosity and fitness-related traits. Notable among th ese are the correlations between heterozygosity and growth rate in mar ine bivalves. Whether the correlation is the result of intrinsic funct ional differences between enzyme variants at the electrophoretic loci scored or arises from non-random genotypic associations between these loci and others segregating for deleterious recessive genes (the assoc iative overdominance hypothesis) is a matter of continuing debate. A p rediction of the associative overdominance hypothesis, not shared by e xplanations that treat the enzyme loci as causative agents of the corr elation, is that the correlation is not specific to the type of geneti c marker used. We have tested this prediction by scoring heterozygosit y at single locus nuclear restriction fragment length polymorphisms (R FLPs) in a cohort of juvenile scallops (Placopecten magellanicus) in w hich growth rate was known to be positively correlated with an individ ual's degree of allozyme heterozygosity. A total of 222 individuals we re scored for their genotypes at seven allozyme loci, two nonspecific protein loci of unknown function and eight nuclear RFLPs detected by a nonymous cDNA probes. In contrast to the enzyme loci, no correlation w as observed between growth rate and the degree of heterozygosity at th e DNA markers. Furthermore, there was no relationship between the magn itude of heterozygote deficiency at a locus and its effect on the corr elation. The differences observed between the effects of allozyme and RFLP heterozygosity on growth rate provide evidence against the associ ative overdominance hypothesis, but a strong case against this explana tion must await corroboration from similar studies in different specie s.