Apparent overdominance of enzyme specific activity in two marine bivalves

Electrophoretic examination of a natural population sample of 332 mussels (Mytilus trossulus) revealed ten active allozyme alleles for the octopine dehydrogenase (Odh) locus and a statistically significant (P<0.005) departure from expected genotypic proportions caused by a deficiency of heterozygous genotypes. In vitro specific activity for octopine dehydrogenase (E.C. 1.5.1.11) was determined for 207 mussels representing 17 different Odh genotypes. Odh heterozygotes had an average specific activity that was 19% greater than that of apparently homozygous genotypes, a significant (P<0.05) difference. Electrophoretic examination of a natural population sample of 209 oysters (Crassostrea virginica) revealed 23 active allozyme alleles for the leucine aminopeptidase-2 (Lap-2) locus and a non-significant (P>0.05) deficiency of heterozygous genotypes. In vitro specific activity for leucine aminopeptidase (E.C. 3.4.-.-) was determined for 89 oysters representing 19 different Lap-2 genotypes. Lap-2 heterozygotes had an average specific activity that was 56% greater than that of homozygous genotypes, a significant (P<0.0001) difference. Possible explanations for the apparent overdominance in enzyme specific activity and the deficiency of heterozygotes include null alleles, molecular imprinting and aneuploidy.