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

INTRODUCTION

Mussel farming is one of the most important aquaculture sectors in Europe, with annual production exceeding 230,000 tons (FAO, 2015). The three mussel species present in Europe, collectively referred to as the Mytilus complex, readily hybridize when they occur in sympatry (Gosling, 1992). Two of them, M. edulis (hereafter referred to as ME) and M. galloprovincialis (hereafter referred to as MG), are extensively cultivated along the Atlantic coast of Europe as well as in the Mediterranean (Michalek, Ventura, & Sanders, 2016). By contrast, the third species, M. trossulus (hereafter referred to as MT) is undesirable for cultivation due to the possession of fragile shells and poor quality meat (Penney, Hart, & Templeman, 2007, 2008). Its hybridization with the other two species has been described as commercially damaging and has been linked to significant economic losses in regions of Scotland (Scott et al., 2010;Scottish Government, 2014).

Owing to the economic importance of mussels and because the Mytilus complex is ideally suited to exploring the processes that generate and maintain hybrid zones, several population genetic studies have sought to characterize the geographic distributions of these species across Europe and to identify areas in which introgression takes place. Studies using diverse genetic markers, from mitochondrial sequences through allozymes and microsatellites to single nucleotide polymorphisms (SNPs), have shown that MG is the dominant species in southern Europe (Daguin, Bonhomme, & Borsa, 2001;Sanjuan, Zapata, & Alvarez, 1994;Zbawicka, Drywa, Śmietanka, & Wenne, 2012), where it is found across the Mediterranean and along the coast of the Iberian Peninsula, whereas ME dominates the cooler northern European coastlines (Daguin et al., 2001;Zbawicka et al., 2012). By contrast, MT is better adapted to brackish conditions and occurs in the Baltic as well as in parts of Norway and Greenland (Kijewski, Zbawicka, Väinölä, & Wenne, 2006;Mathiesen et al., 2017;Stuckas et al., 2017;Väinölä & Strelkov, 2011;Wenne, Bach, Zbawicka, Strand, & McDonald, 2016;Zbawicka et al., 2012;Zbawicka, Sa´nko, Strand, & Wenne, 2014), while its hybrids have also been found in the Netherlands and Scotland (Beaumont, Hawkins, Doig, Davies, & Snow, 2008;Śmietanka, Zbawicka, Wołowicz, & Wenne, 2004;Zbawicka, Burzyński, Skibinski, & Wenne, 2010). Studies...