Effects of an Odh null allele and a GPI low-activity allozyme on shell length in laboratory-reared Mytilus edulis

Mytilus edulis L. heterozygotes with a null allele at the octopine dehydrogenase (Odh) locus or an allele coding for low activity at the glucose phosphate isomerase (Gpi) locus were found to grow significantly faster (p(0.05) than other juvenile mussels in the same laboratory cultures. Odh null homozygotes were not significantly different in growth from mussels with active Odh alleles. No additive effects were seen in individuals which had both the Odh null allele and the allele coding for low GPI activity. These results do not support the contention that null alleles are a significant cause of the observed correlation between multiple-locus heterozygosity and fitness in mussels. The apparent deficiencies of heterozygotes against Hardy-Weinberg expectations observed at the Odh locus in two Netherlands M. edulis populations can be more than accounted for by the null-allele frequency calculated, assuming Hardy-Weinberg equilibrium, from the observed frequency of null homozygotes in these populations.