Larval viability and heterozygote deficiency in populations of marine bivalves: evidence from pair matings of mussels

Juveniles of the blue mussel M. edulis that had resulted from a large number of pair matings were examined at six polymorphic enzyme loci. In four loci, the overall number of heterozygotes was significantly less than expected from the parental genotypes. The degree of deviation from expectation varied considerably among families which shared the same male parent tended to show similar degrees of deviation. Inbreeding and population mixture, the two most cited explanations of heterozygote deficiency (a phenomenon commonly observed in populations of marine bivalves), do not apply in the case of pair-matings, and the null allele hypothesis is inconsistent with the data. These observations suggest that genotype-dependent larval mortality constitutes the most probable cause of this phenomenon.