PROTEIN EVOLUTION IN DIFFERENT CELLULAR ENVIRONMENTS - CYTOCHROME-B IN SHARKS AND MAMMALS

DNA sequences for the mitochondrial cytochrome b gene were determined for 13 species of sharks. Rates and patterns of amino acid replacement are compared for sharks and mammals. Absolute rates of cytochrome b e volution are six times slower in sharks than in mammals. Bivariate plo ts of the number of nonsynonymous and silent transversions are indisti nguishable in the two groups, however, suggesting that the differences in amino acid replacement rates are due primarily to differences in D NA substitution rates. Patterns of amino acid replacement are also sim ilar in the two groups. Conserved and variable regions occur in the sa me parts of the cytochrome b gene, and there is little evidence that t he types of amino acid changes are significantly different between the groups. Similarity in the relative rates and patterns of protein chan ge between the two groups prevails despite dramatic differences in the cellular environments of sharks and mammals. Poor penetrance of physi ological differences through to rates of protein evolution provides su pport for the neutral theory and suggests that, for cytochrome b, patt erns of evolution have been relatively constant throughout much of ver tebrate history.