Heterogeneity of tempo and mode of mitochondrial DNA evolution among mammalian orders.

A statistical analysis of complete DNA sequence data of mitochondrial genomes from human, bovine, and mouse (and partial sequence of rat) indicates that the transversion rate is lower in bovine than in human and murids, and that it is probably lower in human than in murids. However, it is unknown whether the transition rate is also lower in bovine. It is shown that the L-strand of human mitochondrial DNA has significantly higher C content and lower T and A contents than those of bovine and murids. This suggests that a directional mutation pressure which tends to change base composition has been operating in the human lineage.