Genetics of local adaptation of Atlantic Bluefin Tuna from the Mediterranean Sea.

The Atlantic Bluefin tuna (ABFT, Thunnus thynnus), one of the largest top-predator fish inhabiting the pelagic ecosystems of the North Atlantic Ocean and Mediterranean Sea, has been extensively overexploited in recent decades. However, in the Mediterranean Sea, the mixing rates between the eastern, central and western basins have not yet been resolved. Both electronic tagging, otolith and genetic markers cannot still depict a clear scenario of tuna movements and structuring, essential requirement for a proper management of ABFT fisheries. Here we used Expressed Sequence Tag-linked (EST-linked) microsatellites to explore the patterns of adaptive evolution of T. thynnus population and of its population dynamics in the Mediterranean Sea. For this purpose 16 EST-linked microsatellites were genotyped in 177 tuna individuals from the Mediterranean and several methods were used to explore population genetic structuring and estimate/detect signals of local adaptation. Bayesian clustering results indicated the presence of a single cluster, corroborated also by the Correspondence Analysis and pairwise FSTs. Similarly the two methods, used for the detection of FST outliers, did not reveal any pattern suggesting the presence of selective pressure. Our results advise that the low level of polymorphism detected in EST-SSR loci used in this study could be ascribed to the presence of relatively conserved regions flanking these microsatellites. These genomic regions are probably not involved in physiological responses to local adaptation and we were able to rule out action of divergent or balancing selection on EST-SSR polymorphism.