Fmnerty and Block: Evolution of cytochrome b in the Scombroidei 



89 



This level of variability in amino acid sequence is 

 very similar to that reported in a study of placental 

 mammals, a group whose divergence times are prob- 

 ably comparable to scombroids (Irwin et al., 1991). 

 Much of the variation in scombroid cytochrome b 

 occurs in the transmembrane portion of the molecule 

 and represents substitutions between hydrophobic 

 residues (leucine, isoleucine, and valine). The larg- 

 est stretches of invariant residues (21 and 17) occur 

 in a region implicated as part of the Q o redox reac- 

 tion center (Howell and Gilbert, 1988; Howell, 1989; 

 Fig. 6). All of the functionally constrained sites iden- 

 tified by previous studies are conserved throughout 

 the fishes included in this study (see Fig. 5; Howell 

 and Gilbert, 1988; Esposti et al., 1993). 



Figure 7 presents a parsimony analysis based on 

 38 informative amino acid sites. The amino acid se- 



quences do not provide information about more re- 

 cent speciation events because they evolve very 

 slowly, but they contain important evidence about 

 the relationship of billfishes to other scombroids. The 

 amino acid analysis shares two important similari- 

 ties with the weighted nucleotide analysis: first, 

 Scombridae, Gempylidae, and Trichiuridae comprise 

 a clade, and second, Sphyraena and Coryphaena 

 share a common ancestry with this Scombridae- 

 Gempylidae-Trichiuridae assemblage to the exclusion 

 of the billfishes (Xiphiidae and Istiophoridae). The 

 node uniting Sphyraena with the scombrid-gempylid- 

 trichiurid clade is one of the more strongly supported 

 nodes according to the bootstrap analysis. Therefore, 

 cytochrome b amino acid substitutions support the non- 

 scombroid hypothesis and conflict with the scombrid 

 subgroup and scombrid sister group hypotheses. 



