MITOCHONDRIAL-DNA CONTROL REGION SEQUENCES INDICATE EXTENSIVE MIXING OF SWORDFISH (XIPHIAS-GLADIUS) POPULATIONS IN THE ATLANTIC-OCEAN

Pacific, Atlantic, and Mediterranean populations of the swordfish (Xip hias gladins) are currently considered as separate fisheries managemen t units, and populations in different regions of the Atlantic are thou ght to constitute different stocks on the basis of recapture data. To test these hypotheses we sequenced hypervariable segments of the contr ol region of mitochondrial DNA in 35 swordfish from three regions of t he Atlantic, as well as in 8 and 7 individuals from the Pacific and Me diterranean regions, respectively. Sixty of the 81 variable sites were confined to a 280 base pair stretch in the left domain of the control region, indicating that this segment is a rich source of genetic mark ers. Thirty-three haplotypes were found that could be assigned to two clades differing by 3.8% on average, and that diverged approximately 5 50 000 years ago. Clade I haplotypes were ubiquitous, but haplotypes f rom clade II predominated in the Mediterranean, and thus likely origin ated there during Pleistocene marine regressions. Overall, we conclude that there is extensive mixing of swordfish within the Atlantic Ocean . Sister-group relationships of haplotypes from different oceans indic ate historical gene flow between these populations, but the co-occurre nce of the same haplotypes in different oceans must reflect recent or ongoing dispersal.