Tempo and mode of synonymous substitutions in mitochondrial DNA of primates

Nucleotide substitutions of the four-fold degenerate sites and the total third codon positions of mitochondrial DNA from human, common chimpanzee, bonobo, gorilla, and orangutan were examined in detail by three alternative Markov models; (1) Hasegawa, Kishino, and Yano's (1985) model, (2) Tamura and Nei's (1993) model, and (3) the general reversible Markov model. These sites are expected to be relatively free from constraint, and therefore their tempo and mode in evolution should reflect those of mutation. It turned out that, among the alternative models, the general reversible Markov model best approximates the nucleotide substitutions of the four-fold degenerate sites and the total third codon positions, while the maximum likelihood estimates of the numbers of nucleotide substitutions along each branch do not differ significantly among the three models. It was further shown that the transition rate of these sites during evolution, and therefore transitional mutation rate of mtDNA, are higher in humans than in chimpanzees and gorillas probably by about two times. However, transversional mutation rate and amino acid substitution rate do not differ significantly between humans and the African apes. These and additional observations suggest heterogeneity of the mutation rate as well as of the constraint operating on the mtDNA-encoded proteins among different lineages of Hominoidea.