Abstract
The population genetic structure of sea bass (Dicentrarchus labrax) along a transect from the Atlantic Ocean (AO) to the Eastern Mediterranean (EM) Sea differs from that of most other marine taxa in this area. Three populations (AO, Western Mediterranean [WM], EM) are recognized today, which were originally two allopatric populations. How two ancestral genetic units have evolved into three distinct units has not been addressed yet. Therefore, to investigate mechanisms that lead to the emergence of the central WM population, its current status, and its connectivity with the two parental populations, we applied 20 nuclear loci that were either gene associated or gene independent. Results confirmed the existence of three distinct gene pools, with higher differentiation at two transitional areas, the Almeria-Oran Front (AOF) and of the Siculo-Tunisian Strait (STS), than within any population. Significant linkage disequilibrium and heterozygote excess indicated that the STS is probably another tension zone, as already described for the AOF. Neutrality tests fail to reveal marker loci that could be driven by selection within or among metapopulations, except for locus DLA0068. Collectively, results support that the central WM population arose by trapping two tensions zones at distinct geographic locations of limited connectivity. Population assignment further revealed that WM individuals were more introgressed than individuals from the other two metapopulations. This suggests that this population might result from hybrid swarming, and was or is still seeded by genes received through the filter of each tension zone.
Highlights
It is well established that, despite the apparent connectivity of marine habitats and the high dispersal capabilities of marine organisms, populations may be organized into well-defined genetic units (Hellberg et al 2002; Hauser and Carvalho 2008; Hellberg 2009; Sanford and Kelly 2011)
Genetic comparisons were performed with 20 loci (11 GAL, nine GIL), except those involving Eastern Mediterranean (EM) populations, which were only based on 15 loci
Genetic differentiation between Atlantic and Mediterranean populations of marine organisms is usually interpreted as the signature of secondary contact between two formerly allopatric lineages, occurring since the last glacial maximum (Borsa et al 1997; Patarnello et al 2007; but see, e.g., Grant 2005; Limborg et al 2012)
Summary
It is well established that, despite the apparent connectivity of marine habitats and the high dispersal capabilities of marine organisms, populations may be organized into well-defined genetic units (Hellberg et al 2002; Hauser and Carvalho 2008; Hellberg 2009; Sanford and Kelly 2011). If present-day processes can contribute to patterns of differentiation (Galindo et al 2006; Gerlach et al 2007; Selkoe et al 2008, 2010), recognized phylogeographic barriers shared by a wide range of species represent the main barriers to gene flow in the marine environment (e.g., Benzie 1999; Barber et al 2000; Patarnello et al 2007; Galarza et al 2009; Kelly and Palumbi 2010). Most marine organisms have sporadic opportunities to cross almost all physical barriers and to exchange genes between phylogeographically differentiated populations.
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