Nuclear and mitochondrial DNA analyses reveal four genetically separated breeding units of the swordfish

Abstract

Variants in the calmodulin gene intron 4 (CaM) and the mtDNA D‐loop region (D‐loop) detected by RFLP and nucleotide sequence analyses were used to investigate the global stock structure of the swordfish Xiphias gladius. Two alleles (A and B) were observed at the CaM locus among 8 samples (Mediterranean, Tarifa, four Atlantic areas, Indian and Pacific oceans) comprising 567 individuals. Genotype distributions at this locus within samples were in accordance with Hardy‐Weinberg equilibrium. Significant differences in allele frequencies were observed between Mediterranean‐Northwest Atlantic samples (A=0·347–0·493) and those from tropical Atlantic and Indo‐Pacific (A=0·81–1). In the D‐loop region, Alu I and Rsa I digestions detected 4 (A‐D) and 8 (A‐H) genotypes, respectively. The frequency of type C for Alu I was lower in Indo‐Pacific samples (0·2) than Atlantic and Mediterranean samples (0·365–0·643). Frequency of type C for Rsa I increased from the Pacific (0·398) to the Northwest Atlantic (0·698) and Tarifa (0·949) samples, and the Mediterranean sample was fixed for type C. These samples were classified into three groups (Mediterranean‐Tarifa, Atlantic and Indo‐Pacific) using a heterogeneity test of mtDNA genotype distributions. Combined analysis of the CaM and D‐loop loci indicated that there are at least four breeding units: Mediterranean, northwestern Atlantic, tropical to South Atlantic, and Indo‐Pacific.