EVOLUTIONARY ANALYSIS OF CYTOCHROME-B SEQUENCES IN SOME PERCIFORMES -EVIDENCE FOR A SLOWER RATE OF EVOLUTION THAN IN MAMMALS

To obtain information relative to the phylogenesis and microevolutiona ry rate of fish mitochondrial DNA, the nucleotide sequence of cytochro me b gene in seven fish species belonging to the order of Perciformes was determined. Sequence analysis showed that fish mitochondrial DNA h as a nucleotide compositional bias similar to that of sharks but lower compared to mammals and birds. Quantitative evolutionary analysis, ca rried out by using a markovian stochastic model, clarifies some phylog enetic relationships within the Perciformes order, particularly in the Scombridae family, and between Perciformes, Gadiformes, Cypriniformes , and Acipenseriformes. The molecular clock of mitochondrial DNA was c alibrated with the nucleotide substitution rate of cytochrome b gene i n five shark species having divergence times inferred from paleontolog ical estimates. The results of such analysis showed that Acipenserifor mes diverged from Perciformes by about 200 MY, that the Perciformes co mmon ancestor dates back to 150 MY, and that fish mitochondrial DNA ha s a nucleotide substitution rate three to five times lower than that o f mammals.