Scoles and Graves Genetic analysis of the population structure of Thunnus albacares 



693 



Njn = (VG sl - l)/2 (Birky et al., 1983; Nei, 1987). Chi- 

 square analysis was conducted by using the Monte- 

 Carlo method of Roff and Bentzen (1989) with 1000 

 randomizations of the data to evaluate heterogeneity 

 of genotype frequencies among samples without com- 

 bining rare genotypes. 



Results 



Analysis of mtDNA from 120 yellowfin tuna with 12 

 restriction enzymes revealed a total of 34 genotypes, 

 comprising 83 unique fragments. The most common 

 genotype consisted of 52 fragments, representing a sur- 

 vey of 304 bp, or about 1.8% of the mtDNA genome. 

 The mean size of the yellowfin tuna mtDNA genome. 



determined from the most common restriction frag- 

 ment profiles for each of the 12 restriction enzymes, 

 was 16,549 ± 309 bp(SD). 



RFLP analysis of yellowfin tuna mtDNA demon- 

 strated considerable variation. While four restriction 

 enzymes, Aval, Dral, Hindlll, and Xhol, showed no 

 variation, the remaining eight revealed two to seven 

 restriction morphs each. Of the 34 genotypes, two were 

 represented by 20 or more individuals, five were rep- 

 resented by four or more individuals, and 20 geno- 

 types occurred only once (Table 1). Within-sample 

 nucleon diversities were high and showed little varia- 

 tion among samples, ranging from 0.82 to 0.86, with a 

 value of 0.84 for the pooled samples (Table 2). Mean 

 nucleotide sequence diversities were also high, ranging 



