ANALYSIS OF DIRECTIONAL MUTATION PRESSURE AND NUCLEOTIDE CONTENT IN MITOCHONDRIAL CYTOCHROME-B GENES

We present a new approach for analyzing directional mutation pressure and nucleotide content in protein-coding genes. Directional mutation p ressure, the heterogeneity in the likelihood of different nucleotide s ubstitutions, is used to explain the increasing or decreasing guanine- cytosine content (GC%) in DNA and is represented by mu(D), in agreemen t with Sueoka (1962, Proc Natl Acad Sci USA 48:582-592). The new metho d uses simulation to facilitate identification of significant A + T or G + C pressure as well as the comparison of directional mutation pres sure among genes, even when they are translated by different genetic c odes. We use the method to analyze the evolution of directional mutati on pressure and nucleotide content of mitochondrial cytochrome b genes . Results from a survey of 110 taxa indicate that the cytochrome b gen es of most taxa are subjected to significant directional mutation pres sure and that the gene is subject to A + T pressure in most cases. Onl y in the anseriform bird Cairina moschata is the cytochrome b gene sub ject to significant G + C pressure. The GC% at nonsynonymous codon sit es decreases proportionately with increasing A + T pressure, and with a slope less than one, indicating a presence of selective constraints. The cytochrome b genes of insects, nematodes, and eumycotes are subje ct to extreme A + T pressures (mu(D) = 0.123, 0.224, and 0.130) and, i n parallel, the GC% of the nonsynonymous codon sites has decreased fro m about 0.44 in organisms that are not subjected to A + T or G + C pre ssure to about 0.332, 0.323, and 0.367, respectively. The distribution of taxa according to the GC% at nonsynonymous codon sites and directi onal mutation pressure supports the notion that variation in these par ameters is a phylogenetic component.