MITOCHONDRIAL-DNA ANALYSIS OF NORTH PACIFIC SWORDFISH XIPHIAS-GLADIUSPOPULATION-STRUCTURE

Restriction fragment length polymorphism (RFLP) analysis of mitochondr ial DNA (mtDNA) was used to investigate population structure of the No rth Pacific swordfish Xiphias gladius. Samples of X. gladius from the western (n = 47), central (n = 42), and eastern (n = 59) Pacific were examined for mtDNA variation using 17 six-base pair restriction endonu cleases. A total of 27 composite mtDNA genotypes were revealed, and 50 % or more of individuals from each area shared the most common composi te genotype. Pooled nucleon diversity and nucleotide diversity were 0. 637 and 0.0019, respectively, and the mean nucleotide sequence diverge nce among genotypes was 0.88% (rank 0.11 to 2.80%). Both nucleotide di fferences between populations and Gst analysis revealed low values. No significant geographic variation in genotypic frequencies was reveale d by G-test and Monte Carlo randomization analysis with 1000 replicate s. This suggests that there is sufficient gene flow across the North P acific to prevent genetic differentiation, and is consistent with the interpretation that North Pacific swordfish represent a single genetic stock.