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



695 



per ranges reported for marine fishes (Avise et al., 

 1989; Ovenden, 1990; Gold and Richardson, 1991) in- 

 cluding other large, pelagic fishes (Table 4). 



Genetic variability in tuna species has also been 

 demonstrated by sequence analysis of a 307-base-pair 

 region of the mitochondrial cytochrome 6 gene of blue- 

 fin tuna, Thunnus thynnus; bigeye tuna, T. obesus; 

 albacore, T. alalunga; and yellowfin tuna (Bartlett and 

 Davidson, 1991). The sequence data presented for 33 

 yellowfin tuna result in a nucleoli diversity of h = 0.28, 

 a value that is considerably lower than the nucleon 

 diversity of 0.84 obtained in our study. Although 

 nucleon diversity values are greatly influenced by the 

 number of base pairs surveyed (Nei, 1987), the num- 

 ber examined in this study and in Bartlett and 

 Davidson (1991) were very close (304 and 307, respec- 

 tively). This difference in nucleotide sequence diver- 

 sity may reflect a slower evolutionary rate for the mi- 

 tochondrial cytochrome b gene relative to the entire 

 mtDNA genome. 



Significant genetic differentiation among yellowfin 

 tuna from geographically distant locations was not 

 found. Corrected nucleotide sequence divergences av- 

 eraged only 0.049r, indicating that the mean differ- 

 ence between two genotypes randomly chosen from any 

 two samples was essentially the same as the differ- 

 ence between two genotypes randomly drawn from the 

 same sample. The frequencies of the two most com- 

 mon genotypes were similar among all locations, and 

 an overall chi-square test for heterogeneity was non- 



Table 4 



Genetic variation within selected pelagic species determined 

 by RFLP analysis of mtDNA employing 11-13 informative 

 enzymes. Variation is expressed as nucleon diversity Oi) and 

 percent nucleotide sequence diversity (p). 



'This study. 



-Graves and Dizon (1989) and Graves, unpublished data. 



'Graves and McDowell, in press. 



significant. Furthermore, nucleon diversities and nucle- 

 otide sequence diversities were similar among loca- 

 tions, although uniformity in the latter estimates was 

 less pronounced. 



The apparent genetic homogeneity of yellowfin tuna 

 is consistent with the hypothesis that there is genetic 

 exchange among locations. Rare mtDNA genotypes had 

 low frequencies of occurrence (Table 1), characteristic 

 of high gene flow (Slatkin, 1985), a situation similar to 

 that found in other species for which high gene flow is 

 suggested: American eel, Anguilla rostrata (Avise et 

 al., 1986); marine catfishes, Ariidae (Avise et al., 1987); 

 weakfish, Cynoscion regalis (Graves et al., 1992a); and 

 bluefish, Pomatomus saltatrix (Graves et al., 1992b). 

 Low pairwise estimates of G s , (0.011-0.025) also indi- 

 cated homogeneity in yellowfin tuna and gave rise to 

 high N,m values (>19) which are consistent with high 

 rates of gene flow among regions (Birky et al., 1983). 



The absence of genetic differentiation between Pa- 

 cific and Atlantic samples of yellowfin tuna is similar 

 to that reported for other vagile pelagic species. RFLP 

 analysis of mtDNA of skipjack tuna, Katsuwonus 

 pelamis, and of albacore from the Atlantic and Pacific 

 oceans revealed modest amounts of within-sample 

 variation, but no significant differentiation was found 

 between Atlantic and Pacific conspecifics (Graves et 

 al, 1984; Graves and Dizon, 1989). Similarly, RFLP 

 analysis of mtDNA of the pelagic dolphin, Coryphaena 

 hippurus, revealed no significant differentiation be- 

 tween samples from the Atlantic and Pacific oceans 4 . 

 However, spatial partitioning of genetic variation oc- 

 curs in at least some pelagic fishes. Significant genetic 

 differentiation was shown between Atlantic and Pa- 

 cific blue marlin, Makaira nigricans, by direct sequence 

 analysis of the mitochondrial cytochrome b gene 

 (Finnerty and Block, 1992) and RFLP analysis of 

 mtDNA (Graves and McDowell, in press). Similarly, 

 differentiation was found among striped marlin 

 samples, Tetrapturus audax, and sailfish, Istiophorus 

 plalypterus, from the Pacific Ocean by RFLP analysis 

 of mtDNA (Graves and McDowell, in press). 



There are several characteristics of yellowfin tuna 

 which could promote gene flow among locations. Yel- 

 lowfin tuna are distributed circumtropically (Collette 

 and Nauen, 1983) and occur around the Cape of Good 

 Hope in the southern summer (Talbot and Penrith, 

 1962). Tagging studies demonstrated that adults are 

 capable of traveling large distances between Pacific 

 regions (Fink and Bayliff, 1970; Bayliff, 1984; Itano 

 and Williams, 1992), and are capable of undergoing 

 trans-Atlantic crossings (Bard and Scott, 1991). The 

 existence of suitable spawning areas throughout the 



J Carol Reeb, University of Hawaii, pers. commun. April 1992. 



