Historical and size-dependent genetic variation in hybrid mussel populations

Abstract

Samples from the high and low shore were taken from mussel populations at two localities. Croyde Bay and Whitsand Bay in southwest England, within the zone of hybridisation of Mytilus edulis and Mytilus galloprovincialis, and analysed at five polymorphic allozyme loci. Strong correlations were observed between shell length and allele frequencies at both localities, with a higher frequency of alleles characterising M. galloprovincialis being found in larger mussels. Three hypotheses were considered as explanation of these results (1) differential mortality, (2) differential growth and (3) historical change within the hybrid zone. The last hypothesis was rejected because the pattern of length dependent allozyme variation was similar in samples taken in 1980–81 and 1986–87. The observation of small growth differences between M. edulis and M. galloprovincialis provided evidence against the second hypothesis. Thus, higher mortality of younger M. edulis was favoured as the cause of the length-dependent variation. Wave exposure and sand abrasion experienced by the mussels were thought to be the most likely selective factors. Strong correlations were also observed in these populations between shell length and allozyme heterozygosity. Analysis of genotype frequencies in small and large size classes of mussels provide evidence of a small added effect of heterozygosity, but no evidence of overdominance.