Genetic studies of laboratory reared mussels, Mytilus edulis: heterozygote deficiencies, heterozygosity and growth

Offspring derived from several single pair matings and a mass mating of Mytilus edulis were cultured up to the juvenile stage in the laboratory. Samples were collected from the cultures at an early post- settlement (‘spat’) stage and again at the juvenile stage and were scored at up to nine enzyme loci using cellulose acetate and starch gel electrophoresis. An overall significant deficiency of heterozygotes was evident at the juvenile stage but not at the spat stage. Selection against heterozygotes (underdominance) is proposed as the cause. For each mating, shell lengths of juvenile mussels were measured and individuals were classed according to the number of loci at which they were heterozygous. No significant correlations were evident between heterozygosity class and either growth (shell length), or coefficient of variation of shell length. It is argued that these results provide support for the ‘associative overdominance’ rather than the ‘overdominance’ hypothesis as an explanation for heterozygosity/growth correlations in marine bivalves. These data also show that although heterozygosity and growth co-occur in most data sets from natural populations, they may have distinctly differing causes.