Genome‐wide insights into introgression and its consequences for genome‐wide heterozygosity in the <i>Mytilus</i> species complex across Europe

Abstract

The three mussel species comprising the <i>Mytilus</i> complex are widespread across Europe and readily hybridize when they occur in sympatry, resulting in a mosaic of populations with varying genomic backgrounds. Two of these species, <i>M. edulis</i> and <i>M. galloprovincialis</i>, are extensively cultivated across Europe, with annual production exceeding 230,000 tonnes. The third species, <i>M.</i> <i>trossulus</i>, is considered commercially damaging as hybridization with this species results in weaker shells and poor meat quality. We therefore used restriction site associated DNA sequencing to generate high-resolution insights into the structure of the <i>Mytilus</i> complex across Europe and to resolve patterns of introgression. Inferred species distributions were concordant with the results of previous studies based on smaller numbers of genetic markers, with <i>M. edulis</i> and <i>M. galloprovincialis</i> predominating in northern and southern Europe respectively, while introgression between these species was most pronounced in northern France and the Shetland Islands. We also detected traces of <i>M. trossulus</i> ancestry in several northern European populations, especially around the Baltic and in northern Scotland. Finally, genome-wide heterozygosity, whether quantified at the population or individual level, was lowest in <i>M. edulis</i>, intermediate in <i>M. galloprovincialis,</i> and highest in <i>M. trossulus</i>, while introgression was positively associated with heterozygosity in <i>M. edulis</i> but negatively associated with heterozygosity in <i>M. galloprovincialis</i>. Our study will help to inform mussel aquaculture by providing baseline information on the genomic backgrounds of different <i>Mytilus</i> populations across Europe and by elucidating the effects of introgression on genome-wide heterozygosity, which is known to influence commercially important traits such as growth, viability, and fecundity in mussels.