Introgression patterns in the mosaic hybrid zone betweenMytilus edulisandM. galloprovincialis

Abstract

Hybrid zones are fascinating systems to investigate the structure of genetic barriers. Marine hybrid zones deserve more investigation because of the generally high dispersion potential of planktonic larvae which allows migration on scales unrivalled by terrestrial species. Here we analyse the genetic structure of the mosaic hybrid zone between the marine mussels Mytilus edulis and M. galloprovincialis, using three length-polymorphic PCR loci as neutral and diagnostic markers on 32 samples along the Atlantic coast of Europe. Instead of a single genetic gradient from M. galloprovincialis on the Iberian Peninsula to M. edulis populations in the North Sea, three successive transitions were observed in France. From South to North, the frequency of alleles typical of M. galloprovincialis first decreases in the southern Bay of Biscay, remains low in Charente, then increases in South Brittany, remains high in most of Brittany, and finally decreases again in South Normandy. The two enclosed patches observed in the midst of the mosaic hybrid zone in Charente and Brittany, although predominantly M. edulis-like and M. galloprovincialis-like, respectively, are genetically original in two respects. First, considering only the various alleles typical of one species, the patches show differentiated frequencies compared to the reference external populations. Second, each patch is partly introgressed by alleles of the other species. When introgression is taken into account, linkage disequilibria appear close to their maximum possible values, indicating a strong genetic barrier within all transition zones. Some pre- or postzygotic isolation mechanisms (habitat specialization, spawning asynchrony, assortative fertilization and hybrid depression) have been documented in previous studies, although their relative importance remains to be evaluated. We also provided evidence for a recent migratory 'short-cut' connecting M. edulis-like populations of the Charente patch to an external M. edulis population in Normandy and thought to reflect artificial transfer of spat for aquaculture.

Unexpected mosaic distribution of two hybridizing sibling lineages in the teleplanically dispersing snail Stramonita haemastoma suggests unusual postglacial redistribution or cryptic invasion

“…The same grouping of alleles was obtained when using the allele coordinates in a correspondence analysis as explained in Bierne et al. (2003). The frequency of the compound A allele and compound B allele is reported in Figure 1.…”

Empirical study of hybrid zone movement

“…In a further 13 hybrid zones, patterns of introgression might be interpreted as evidence for movement: gulls Larus glaucescens and Larus occidentalis (Bell, 1996;Gay, 2006); pocket gophers Thomomys townsendii and Thomomys bottae (Patton, 1993;Patton and Smith, 1993); Thomomys bottae actuosus and Thomomys bottae ruidosae (Ruedi et al, 1997); fish Gambusia affinis and Gambusia holbrooki (Reznick, 1981;Scribner, 1993;Scribner and Avise, 1993;Scribner and Avise, 1994a, b); hares Lepus granatensis/Lepus europaeus and Lepus timidus (Thulin and Tegelströ m, 2002;Melo-Ferreira et al, 2005, 2007; salamander Chioglossa lusitanica North and South forms (Sequeira et al, 2005); mussels Mytilus galloprovincialis and Mytilus edulis (Gardner and Skibinski, 1988;Skibinski and Roderick, 1991;Willis and Skibinski, 1992;Gardner et al, 1993;Wilhelm and Hilbish, 1998;Bierne et al, 2003); beetles Carabus albrechti and Carabus lewisianus (Takami and Suzuki, 2005); lizard Sceloporus grammicus chromosomal races F5 and FM2 (Sites et al, 1996;Marshall and Sites, 2001) cottonwoods Populus angustifolia and Populus fremontii (Keim et al, 1989;Paige et al, 1991;Martinsen et al, 2001).…”

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.…”