Genetic divergence between Atlantic and Indo‐Pacific stocks of bigeye tuna (Thunnus obesus) and admixture around South Africa

Abstract

Two mitochondrial DNA segments of the bigeye tuna (Thunnus obesus) were amplified by polymerase chain reaction (PCR), and restriction fragment length polymorphism (RFLP) analyses of these segments were used for the genetic stock study. The variation in a segment flanking the ATPase and COIII genes was low; only two genotypes (alpha and beta) were detected by RsaI digestion. Yet a large difference in the genotype distribution was observed between ocean basin samples. The alpha type predominated in four Atlantic samples, where 178 of 244 individuals were the alpha type. In contrast, only one of 195 individuals collected in the Indo-Pacific was the alpha type? The frequency of the alpha type varied considerably from 0 to 80% among seven samples collected off the Cape of Good Hope. The variation found in the other segment, containing the D-loop region, was much higher; two endonucleases (DpnII and RsaI) detected five genotypes each and 15 composite genotypes. A highly significant difference in genotype frequencies was observed between the Atlantic and Indo-Pacific samples, but no heterogeneity was observed among the four Atlantic or among four Indo-Pacific samples. These results clearly indicate that not only gene flow, but also fish migration, between the Atlantic and Indian Oceans are severely restricted, and that fishes from these distinct stocks are intermingling around South Africa. The simple and diagnostic genetic marker found in this study can be used to estimate mixing ratios between Atlantic and Indian stocks around South Africa.

Population structure of albacore Thunnus alalunga inferred from blood groups and tag-recapture analyses

“…This differentiation is supported by the highly significant heterogeneity found between the Atlantic and Pacific populations in a recent RFLP analysis on the mitochondrial ATPase gene (Chow & Ushiama 1995), and a similar pattern is also found in cogeneric bigeye tuna (Chow et al 2000) and other large pelagic fishes such as the swordfish (AlvaradoBremer et al 1999). Blood group analyses are consistent with this differentiation.…”

Interspecific and Intraspecific Genetic Diversity of Thunnus Species

“…In other tuna species DNA studies have revealed little intra-specific divergence within, but significant divergence between ocean basins. In bigeye tuna (T. obesus) DNA markers showed no divergence among population samples from the western Pacific Ocean (Chiang et al, 2006), but high divergence among Atlantic and Indo-Pacific populations (Chow et al, 2000;Chiang et al, 2006;Martinez et al, 2006). Similarly, in albacore tuna (T. alalunga) no genetic differences were detected among samples from the Northwest Pacific Ocean (Wu et al, 2009), but significant differences were reported among Atlantic and Indo-Pacific populations (Vinas et al, 2004).…”

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“…In particular, genetic differentiation is low between tuna populations within and between oceans (Alvarado Bremer et al, 1998;Gerwe and Hampton, 1998;Chow et al, 2000;Appleyard et al, 2002;Durand et al, 2005) because of the occurrence of continuous, circumtropical pelagic environment and a wide range of suitable spawning grounds. Kawakawa is widely distributed in the tropical and subtropical waters of the Indo-Pacific region and was observed to spawn in waters of Hong Kong, India, Thailand, and Philippines (Yesaki, 1994).…”