Abstract
We show that parasitism by the trematode Prosorhynchus squamatus in parental and introgressed Mytilus edulis/galloprovincialis (Bivalvia) mussels occurs in individuals with a predominantly M. edulis genome. This result suggests that the restricted specificity of P. squamatus is dependent on genetic factor(s) present in M. edulis. Because of its strong pathogenic effects (i.e. total castration and possible death), this parasite may be a source of intense selection against M. edulis genomes when they are present in a site. As a consequence, it may favour the geographic extension of the M. galloprovincialis genome. Previous studies have indicated that, in hybrid zones, recombinant genotypes are more susceptible to parasitic infections than either parental genotype. We demonstrate that this is not the case for the M. edulis/M. galloprovincialis system, and that the parental genotype alone determines susceptibility.