Genetic isolation between Atlantic and Mediterranean albacore populations inferred from mitochondrial and nuclear DNA markers

Genetic population structure of Atlantic and Mediterranean albacore Thunnus alalunga was investigated using nucleotide sequence variations of the glucose-6-phosphate dehydrogenase gene intron (G6PD) and the mitochondrial DNA (mtDNA) D-loop region (Dloop). Restriction analysis using Ase I digestion detected two major restriction types (A and B) at the Dloop locus with strong frequency differences between Atlantic and Mediterranean samples. Thirty-six individuals of 100 Mediterranean albacore were of the B type whereas no B type individuals were found in the Atlantic samples (n=102). Phylogenetic analysis using nucleotide sequence data of the Dloop locus indicated that the B type lineage recently arose from the ancestral A lineage in the Mediterranean Sea and has not dispersed into the Atlantic Ocean. The frequencies of two alleles (L and S) at the G6PD locus were significantly different between the samples from the Atlantic (L=0.495) and the Mediterranean (L=0.725), but no significant heterogeneity was observed between mtDNA-A and -B types of the Mediterranean sample. These molecular data indicate that gene flow between the Atlantic and Mediterranean albacore populations have been considerably restricted and strongly suggest these populations should continue to be treated as two distinct management units.