Aspects of the population genetics of Mytilus (Mytilidae; Mollusca) in the British Isles

Starch gel electrophoresis was used to study variation at 11 loci in mussels sampled mainly from British coastal sites. Two types of mussel were identified, Mytilus edulis, the common mussel and its southern relative Mytilus galloprovincialis. Several partially diagnostic loci were used to map the distribution of the two forms. Mytilus edulis was present at all sites sampled in Britain and Ireland but was at low frequency in SW England; M. galloprovincialis was detected in SW England, the south and west of Ireland. Scotland and NE England, but was absent from south Wales, the Irish sea coasts of Wales and Ireland, and SE England. Apart from the occurrence of M. galloprovincialis in NE England, this distribution conforms with the results of studies using morphological characters and parallels the distribution of many other southern species in Britain. At the microgeographical level, M. edulis was found to prefer more sheltered and estuarine conditions than M. galloprovincialis. Analysis using the best diagnostic loci showed that hybridization is occurring between M. edulis and M. galloprovincialis at all localities where they occur sympatrically but that the extent of hybridization varies considerably between localities. The distribution of localities having high proportions of hybrid individuals is best interpreted by assuming that hybrids have higher fitness than parental types at these localities. A study was made of variation within and between those localities where only M. edulis individuals were observed. Little significant geographic variation in allele frequency was detected, but significant deficits of heterozygotes compared with Hardy-Weinberg expectations were seen for most loci. Analysis suggests that the Wahlund effect is not involved and that the most likely cause of the deficit is low frequencies of null alleles. In M. edulis no differences in phenotypic variance in shell height and width were observed between samples of multiply heterozygous and multiply homozygous individuals and no genetic differences were found between juveniles and adults. Overall little evidence was found that balancing selection is responsible for maintenance of the polymorphisms studied in M. edulis. The pattern of geographic variation in gene frequencies in Mytilus in the British Isles is discussed in relation to variation in the south and north of Europe and North America. It is concluded that steep clines in gene frequencies in M. edulis observed by other workers in the Baltic and in Long Island Sound cannot be attributed to the presence of M. galloprovincialis.