THE COMPLETE MITOCHONDRIAL-DNA (MTDNA) OF THE DONKEY AND MTDNA COMPARISONS AMONG 4 CLOSELY-RELATED MAMMALIAN SPECIES-PAIRS

The nucleotide sequence of the complete mitochondrial genome of the do nkey, Equus asinus, was determined. The length of the molecule is 16,6 70 bp. The length, however, is not absolute due to pronounced heteropl asmy caused by variable numbers of two types of repetitive motifs in t he control region. The sequence of the repeats is (a) 5'-CACACCCA and (b) 5'-TGCGCGCA, respectively. The order of (a) and (b) can be express ed as {n[2(a)+(b)]+m(a)}. In 32 different clones analyzed the number o f n and m ranged from 0 to 9 and 1 to 7. The two rRNA genes, the 13 pe ptide-coding genes, and the 22 tRNA genes of the donkey and the horse, Equus caballus, were compared in detail. Total nucleotide difference outside the control region was 6.9%. Nucleotide difference between pep tide-coding genes ranged from 6.4% to 9.4% with a mean of 8.0%. In the inferred protein sequences of the 13 peptide-coding genes the amino a cid difference was 0.2-8.8%, and the mean for the 13 concatenated amin o acid sequences was 1.9%. In the 22 tRNA genes, the mean difference w as 3.5%, and that in the two rRNA genes was 4.1%. The mtDNA difference s between the donkey and the horse suggest that the evolutionary separ ation of the two species occurred approximate to 9 million years ago. Analyses of differences among the mtDNAs of three other species-pairs, harbor seal/grey seal, fin whale/blue whale, and Homo/common chimpanz ee, showed that the relative evolutionary rate of individual peptide-c oding genes varies among different species-pairs and modes of comparis on. The findings show that the superimposition of sequence data of one lineage for resolving and dating evolutionary divergences of other li neages should be performed with caution unless based on comprehensive data.