INVITED REVIEW: Local adaptation and species segregation in two mussel (Mytilus edulis × Mytilus trossulus) hybrid zones

Abstract

Few marine hybrid zones have been studied extensively, the major exception being the hybrid zone between the mussels Mytilus edulis and Mytilus galloprovincialis in southwestern Europe. Here, we focus on two less studied hybrid zones that also involve Mytilus spp.; Mytilus edulis and Mytilus trossulus are sympatric and hybridize on both western and eastern coasts of the Atlantic Ocean. We review the dynamics of hybridization in these two hybrid zones and evaluate the role of local adaptation for maintaining species boundaries. In Scandinavia, hybridization and gene introgression is so extensive that no individuals with pure M. trossulus genotypes have been found. However, M. trossulus alleles are maintained at high frequencies in the extremely low salinity Baltic Sea for some allozyme genes. A synthesis of reciprocal transplantation experiments between different salinity regimes shows that unlinked Gpi and Pgm alleles change frequency following transplantation, such that posttransplantation allelic composition resembles native populations found in the same salinity. These experiments provide strong evidence for salinity adaptation at Gpi and Pgm ( or genes linked to them). In the Canadian Maritimes, pure M. edulis and M. trossulus individuals are abundant, and limited data suggest that M. edulis predominates in low salinity and sheltered conditions, whereas M. trossulus are more abundant on the wave-exposed open coasts. We suggest that these conflicting patterns of species segregation are, in part, caused by local adaptation of Scandinavian M. trossulus to the extremely low salinity Baltic Sea environment.

The Future of Baltic Sea Populations: Local Extinction or Evolutionary Rescue?

“…The clines are explained either as a consequence of differential selection on individual gene loci, favoring one type of genes inside the Baltic Sea and another outside, or lack of gene exchange (migration) between Baltic and North Sea populations, or both. Evidence of selection for particular gene variants inside the Baltic Sea is particularly strong in blue mussels (Riginos and Cunningham 2005), Baltic clam (Luttikhuizen et al, unpublished data), herring (Larsson et al 2007;André et al 2010), and cod (Andersen et al 2009). …”

“…For some species, the Baltic-North Sea gradient unveil intriguing evolutionary interactions, such as between the two sibling or semi-species M. edulis and M. trossulus where a complex pattern of hybridization and introgression is evident at the entrance and in the southern parts of the Baltic Sea (Riginos and Cunningham 2005). A recent analysis of both nuclear and mitochondrial genetic markers shows strong clinal shape differences among markers but weak genome-related incompatibilities in reproductive isolation, high-lighting the role of genetic drift and hybrid zone movement in addition to selection in formation of hybrid zone genetic structures in blue mussels (Stuckas et al 2009).…”

“…Lower salinity will favor establishment and spread of more freshwater species than today, and we would also expect an increased evolution of tolerance to low salinities in some of the organisms already present. One such candidate is the blue mussel that has a large potential of evolutionary rescue in its enormous population size (10 13 ) and in having genes from two hybridizing evolutionary lineages already available in the Baltic Sea (M. edulis and M. trossulus, see Riginos and Cunningham 2005). Likewise, the Baltic clam (M. balthica) is a likely candidate as it has a very large population size, a distribution almost over the complete Baltic Sea (Väinölä and Varvio 1989) and a population that is a mix of two hybridizing lineages, resulting in a substantial increase in the standing genetic variation compared to populations outside the Baltic Sea (Nikula et al 2008).…”

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

“…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). 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).…”

“…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). 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.…”