Distribution of Mytilus edulis, M. galloprovincialis, and their hybrids in open-coast populations of mussels in southwestern England

Abstract

The distribution of Mytilus edulis Linnaeus, M. galloprovincialis Lamarck, and their hybrids was examined in mussel populations in southwest England in 1996 and 1998. This is a region where both parental taxa and populations containing large numbers of hybrids co-occur yet a fine-scale mapping of the hybrid populations has not been conducted. In this study the geographic distribution of hybrid populations was determined in southwest England over 360 km of coast from Tintagel Castle in north Cornwall to Beer in south Devon. Sample localities were spaced from 5 to 20 km apart. Genotypes for individual mussels were determined using PCR to amplify a size polymorphism at the Glu-5′ locus that is completely differentiated between M. edulis and M. galloprovincialis. Hybrid populations, characterized by high frequencies of individuals with heterozygous genotypes and a pattern of decreasing frequency of M. edulis alleles with increasing shell length, were continuously distributed along 180 km of open coast in southwest England. This "hybrid patch" was bordered at one end by geographically extensive populations of pure M. edulis and at the other end by nearly pure populations of M. galloprovincialis. Strong natural selection in hybrid populations results in a decline in the frequency of M. edulis alleles with increasing size. Wave-exposure has previously been implicated as the agent producing this pattern of selection, but in the present study the relationship between allele frequency and body size was not correlated with variation in wave shock intensity among localities within the hybrid zone. The transition between hybrid populations and those containing pure populations of M. edulis or M. galloprovincialis is abrupt which suggests that coastal circulation patterns may provide strong barriers to larval dispersal which accounts for the position and maintenance of the hybrid zone.