Boundary between the north and south Atlantic populations of the swordfish (Xiphias gladius) inferred by a single nucleotide polymorphism at calmodulin gene intron

Genetic differentiation of the Atlantic swordfish (Xiphias gladius) was investigated by a single nucleotide polymorphism (SNP) at the calmodulin gene (CAM) intron locus. Clearly distinct allele and genotype frequencies were observed between the north (20-41 degrees N) and mid-south (10 degrees N-33 degrees S) Atlantic samples. Much lower frequency of A allele (37.5-57.1%) was observed in the north samples (n = 160 in total) than in the mid-south samples (83.3-92.6%; n=354), and homozygote BB was common in the north samples (23.4-31.3%) but very rare or absent (0-3.9%) in the mid-south samples. Very strong population subdivision was observed between the two groups (F ST = 0.34, P < 0.001), while the allele and genotype frequencies within each ocean basin persisted over time (1990-2002 in the north, and 1994-2002 in the mid-south). Of two samples from the presumed boundary zone, one (n = 18) (14 degrees N, 48 degrees W) presented intermediate frequencies of the A allele (66.7%) and BB homozygote (11.1%), while the other (n = 23) (10-17 degrees N, 28-37 degrees W) shared similar frequencies of the A allele (89.1%) and BB homozygote (4.3%) with those of the mid-south Atlantic samples. These results indicate that the gene flow and individual migration between the north and mid-south Atlantic populations are considerably restricted and that the current management boundary between the north and south Atlantic swordfish stocks of 5 degrees N should be reconsidered. [PUBLICATION ABSTRACT]