Lineage-specific evolutionary rate in mammalian mtDNA

The existence of a Lineage-specific nucleotide substitution rate in mammali an mtDNA has been investigated by analyzing the mtDNA of all available spec ies, that is, 35 complete mitochondrial genomes from 14 mammalian orders. A detailed study of their evolutionary dynamics has been carried out on both ribosomal RNA and first and second codon positions (P12) of H-strand prote in-coding genes by using two different types of relative-rate tests. Result s are quite congruent between ribosomal and P12 sites. Significant rate var iations have been observed among orders and among species of the same order . However, rate variation does not exceed 1.8-fold between the fastest (Pro boscidea and Primates) and the slowest (Perissodactyla) evolving orders. Th us, the observed mitochondrial rate variations among taxa do not invalidate the suitability of mtDNA for drawing mammalian phylogeny. Dependence of ev olutionary rare differences on variations in mutation and/or fixation rates was examined. Body size, generation time, and metabolic rate were tested, and no significant correlation was observed between them and the taxon-spec ific evolutionary rates, most likely because the latter might be influenced by multiple overlapping variable constraints.