Mitochondrial DNA evolution in the genus Equus

Employing mitochondrial DNA (mtDNA) restriction-endonuclease maps as thebasis of comparison, we have investigated the evolutionary affinities ofthe seven species generally recognized as the genus Equus. Individualspecies' cleavage maps contained an average of 60 cleavage sites for 16enzymes, of which 29 were invariant for all species. Based on an averagedivergence rate of 2%/Myr, the variation between species supports adivergence of extant lineages from a common ancestor approximately 3.9 Myrbefore the present. Comparisons of cleavage maps between Equus przewalskii(Mongolian wild horse) and E. caballus (domestic horse) yielded estimatesof nucleotide sequence divergence ranging from 0.27% to 0.41%. This rangewas due to intraspecific variation, which was noted only for E. caballus.For pairwise comparisons within this family, estimates of sequencedivergence ranged from 0% (E. hemionus onager vs. E. h. kulan) to 7.8% (E.przewalskii vs. E. h. onager). Trees constructed according to the parsimonyprinciple, on the basis of 31 phylogenetically informative restrictionsites, indicate that the three extant zebra species represent amonophyletic group with E. grevyi and E. burchelli antiquorum divergingmost recently. The phylogenetic relationships of E. africanus and E.hemionus remain enigmatic on the basis of the mtDNA analysis, although arecent divergence is unsupported.