Selection against blue mussels (Mytilus edulis L.) homozygotes under various stressful conditions.

Three mussel groups differing in mean multilocus heterozygosity (MLH) were used to examine the MLH-fitness relationship. Mussels were submitted to aerobic and anaerobic stressful conditions in the laboratory, and their LT(50) was measured. Mortality was not random in two of the three groups and affected the homozygous individuals more. This selective mortality caused a significant increase in the mean MLH of the survivors, but only for the two groups characterized by the lowest initial MLH and significant deficits in heterozygotes at the onset of the experiments. While these experiments were ongoing, the same two groups also suffered a 40% mortality rate in lantern nets under field conditions. This mortality also increased the mean MLH in survivors. All groups showed strong inverse relationships between MLH and standard metabolism. Our results suggest that the higher resistance of more heterozygous individuals is related to their lower metabolic needs.