Global genetic structure of the swordfish Xiphias gladius, as revealed by microsatellite markers

In the present study the potential of microsatellites to describe the global genetic structure of the swordfish (Xiphias gladius) and to allow assigning individuals to stocks was assessed. A total number of 2,026 individuals were genotyped for four highly polymorphic microsatellite markers from the Mediterranean, North and South Atlantic, southeast Indian and southwest Pacific. In previous studies analyses of mitochondrial DNA (mtDNA) variation have revealed strong population structuring of swordfish around the globe. Despite the great usefulness of mtDNA, this marker cannot address efficiently the complicated patterns of migration of the swordfish, which are encountered in mixed fisheries situations. A very strong population structure with highly significant divergence between the samples has been shown by microsatellites, in congruence with previous results. Significantly lower allelic diversity has been observed in the Mediterranean samples, again in agreement with mtDNA findings. A high potential of assigning individuals to stocks has been found in the case of Mediterranean versus Atlantic fish, and a small degree of diffusion of Mediterranean individuals to the Atlantic and vice versa has been detected. More information, by including additional markers will be needed to achieve individual assignment between the other pairs of neighbouring stocks that show a smaller level of genetic divergence.