A molecular approach to the study of a mussel hybrid zone on the west coast of Ireland
The shores of the northwestern Atlantic have two indigenous mussel species, the Blue mussel, Mytilus edulis and the Mediterranean mussel, Mytilus galloprovincialis. These two species once isolated by the last ice age, are now occurring sympatrically along 1500 miles of the northwestern Atlantic coast. They regularly interbreed and hybridise, producing a patchwork of mixed pure species and hybrid populations, their distribution mainly defined by environmental factors. In hybrid populations on exposed shores in Ireland mussels higher up the shore are more likely to be M. galloprovincialis. This study set out to test two hypotheses: is this pattern due to preferentially settlement of larvae at a given shore height, or is it due to blanket settlement of larvae followed by preferential secondary settlement or postsettlement selective mortality. The Mel 5/16 DNA marker was used to analyse the genetic composition of newlysettled spat onto artificial substrates, which were placed on the mid and low shore areas of two exposed shores in Galway Bay over the period May-October 2002. Samples of adults were collected at the same time, with additional samples collected from a nearby sheltered shore. This DNA marker can differentiate between M. edulis, M,galloprovincialis and hybrids. There was no evidence for preferential settlement by any of the three genotypes on exposed shores. Neither was there evidence of post settlement mortality, as adults from exposed shores were genetically similar to settling spat. Contrary to the results from published studies, there was no significant difference in the genetic structure of adult mussel populations with tidal height. However, in adult exposed shores samples there was a significantly higher frequency of M. galloprovincialis among larger individuals in the population, possibly indicating a selective advantage for the genotype in this environment. Adult mussels from a nearby sheltered site were found to be significantly different to exposed shore adults, due to higher frequencies of M. edulis and hybrid genotypes. There was no difference in the proportions of the three genotypes in small versus large mussels at this site. The difference between adults on exposed and sheltered shores may be due to the competitive advantage of M. galloprovincialis on exposed shore sites, e.g., resistance to wave action and faster growth rates.
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