New SNP markers reveal largely concordant clinal variation across the hybrid zone between Mytilus spp. in the Baltic Sea

Abstract

Environmental conditions such as a pronounced salinity gradient and postglacial history make the Baltic Sea a suitable area for studying how selection and gene flow affect genetic differentiation in marine species. A cDNA library was used to identify new single nucleotide polymorphisms (SNPs) in Baltic populations of Mytilus spp. mussels. Sixty polymorphic SNPs were used to genotype 642 individual mussels from the inner Baltic, Danish Straits, northwest Denmark and a population of the northeast Pacific. We characterized 49 novel SNP markers that differentiate the populations of the North and Baltic Sea areas. Concordant narrow clines were observed at the entrance of the Baltic Sea for most of these markers. Considerable variance of hybrid index scores was observed in populations with intermediate allele frequencies within the hybrid zone, e.g. in Gedser and Hjelm. The presented results are in accordance with the existence of strong reproductive isolation, probably caused by a combination of exogenous (e.g. adaptation to brackish waters) and endogenous pre-and post-zygotic factors (e.g. selection against hybrids). The overwhelming majority of new SNPs markers showed a larger representation of M. trossulus than M. edulis genes in the nuclear DNA of Baltic Mytilus species. Finally, we identified a few markers with an elevated level of introgression of M. edulis alleles in the Baltic Sea M. trossulus populations in comparison to the reference M. trossulus population of the Pacific.

A first report on coexistence and hybridization of Mytilus trossulus and M. edulis mussels in Greenland

“…While until recently it has been believed that the distribution of M. trossulus was restricted to the North Pacific, eastern Canada and the Baltic Sea (Riginos and Cunningham 2005), more recent research has documented its occurrence on the coasts of Scotland, Iceland, the Barents Sea, the White Sea and Norway (Beaumont et al 2008;Zbawicka et al 2010;Kijewski et al 2011;Väinölä and Strelkov 2011). The presence of M. edulis has been reported in the northern part of the Atlantic and European seas from the White and Barents Sea to the Atlantic coast of southern France and M. galloprovincialis in the Mediterranean Sea, Black Sea and along the Atlantic coastline of Western Europe including the British northern islands (e.g., Väinölä and Hvilsom 1991;Borsa et al 1999;Bierne et al 2003;Riginos and Cunningham 2005;Riginos and Henzler 2008;Zbawicka et al 2012Zbawicka et al , 2014.…”

“…While M. edulis for example requires summer temperatures of a minimum of 4°C (Bennike and Wagner 2013), studies on larval development have shown that M. trossulus is less tolerant to temperatures at 10°C and above than M. edulis (Hayhurst and Rawson 2009). Further, M. trossulus has been found to inhabit lower salinity areas (Väinölä and Hvilsom 1991;Ridgway and Naevdal 2004;Zbawicka et al 2010Zbawicka et al , 2014Väinölä and Strelkov 2011). Experimental studies have shown that this taxon is less sensitive to lower salinities in the early ontogenetic stages than its congeners (Qiu et al 2002).…”

“…With a sufficient number of sequences, sequencing errors were disregarded by Staden software; other sequencing errors were identified during the validation. Additional SNPs were identified in Zbawicka et al (2012Zbawicka et al ( , 2014. Genotyping of SNPs was performed using the Sequenom MassARRAY iPLEX platform (Gabriel et al 2009).…”

“…SNPs were classified, based on manual inspection, as 'failed assays' (meaning that the majority of genotypes could not be scored and/or the samples did not cluster well according to genotype), 'monomorphic SNPs' (all studied individuals had the same genotype) or 'polymorphic SNPs.' Assays were designed for 79 candidate SNPs including 69 based on EST sequences and 10 identified by DNA sequences, selected from 385 putative SNPs (Zbawicka et al 2012(Zbawicka et al , 2014.…”

“…The M. edulis species complex consists of three morphologically similar congeners: M. edulis, M. trossulus and M. galloprovincialis. Recently research has been intensified on the distribution of the species complex (e.g., Moreau et al 2005;Shields et al 2008;Riginos and Henzler 2008;Smietanka et al 2009;Stuckas et al 2009;Zbawicka et al 2012;Smietanka et al 2013Smietanka et al , 2014Zbawicka et al 2014), which evidently reflects a history of inter-and transoceanic dispersal, secondary contact and hybridization, besides the effect brought about by human activities (Väi-nölä and Strelkov 2011). It is thought that the complex originated from M. trossulus in the Pacific followed by colonization in the Northern Atlantic giving rise to both M. edulis and M. galloprovincialis (Vermeij 1992;Riginos and Cunningham 2005).…”