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 deh ydrogenase (Odh) locus or an allele coding for low activity at the glu cose phosphate isomerase (Gpi) locus were found to grow significantly faster (p < 0.05) than other juvenile mussels in the same laboratory c ultures. Odh null homozygotes were not significantly different in grow th from mussels with active Odh alleles. No additive effects were seen in individuals which had both the Odh null allele and the allele codi ng for low GPI activity. These results do not support the contention t hat null alleles are a significant cause of the observed correlation b etween multiple-locus heterozygosity and fitness in mussels. The appar ent 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, a ssuming Hardy-Weinberg equilibrium, from the observed frequency of nul l homozygotes in these populations.