Finnerty and Block: Evolution of cytochrome b in the Scombroldei 



95 



Thunnus thynnus, southern bluefin tuna, Thunnus 

 maccoyii, albacore, Thunnus alalunga, and bigeye 

 tuna, Thunnus obesus). According to this hypothesis, 

 the primitive condition for the genus Thunnus is a 

 tropical distribution, and the cold water tunas com- 

 pose a monophyletic group united by specializations 

 that allowed them to exploit cooler temperate or deep 

 waters. The nucleotide analyses presented in Fig- 

 ures 2 and 4 are not consistent with this hypothesis. 

 The cytochrome b phylogeny groups a tropical spe- 

 cies, the yellowfin tuna, Thunnus albacares, with two 

 species adapted for extremely cold water, the blue- 

 fin tuna and southern bluefin tuna (Thunnus thynnus 

 and Thunnus maccoyii). However, it is premature to 

 draw conclusions about relationships within the ge- 

 nus Thunnus until data are obtained from two tropi- 

 cal species not included in this study, Thunnus 

 atlanticus and Thunnus tonggol. 



Acknowledgments 



We thank A. Stewart, J. Kidd, A. Borisy, S. Eng, S. 

 Malik, D. Wang, F. Manu, and V. Master for techni- 

 cal assistance. We are indebted to C. Proctor, P. 

 Grewe, G. DeMetrio, P. Davie, J. Pepperell, K. 

 Dickson, B. Collette, and F. Carey for tissue samples. 

 B. Collette, D. Johnson, J. Graves, and M. Westneat 

 provided valuable assistance on the project and con- 

 structive comments on the manuscript. This research 

 was supported by NSF grant IBN8958225 to B. A. B. 

 and NEH molecular biology training grant IBN8958225 

 and a Sigma Xi Grant-in-aid of Research to J. R. F. 



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