Complex Evolutionary Patterns Revealed by Mitochondrial Genomes of the Domestic Horse

The domestic horse is the most widely used and important stock and recreational animal, valued for its strength and endurance. The energy required by the domestic horse is mainly supplied by mitochondria via oxidative phosphorylation. Thus, selection may have played an essential role in the evolution of the horse mitochondria. Besides, demographic events also affect the DNA polymorphic pattern on mitochondria. To understand the evolutionary patterns of the mitochondria of the domestic horse, we used a deep sequencing approach to obtain the complete sequences of 15 mitochondrial genomes, and four mitochondrial gene sequences, ND6, ATP8, ATP6 and CYTB, collected from 509, 363, 363 and 409 domestic horses, respectively. Evidence of strong substitution rate heterogeneity was found at nonsynonymous sites across the genomes. Signatures of recent positive selection on mtDNA of domestic horse were detected. Specifically, five amino acids in the four mitochondrial genes were identified as the targets of positive selection. Coalescent-based simulations imply that recent population expansion is the most probable explanation for the matrilineal population history for domestic horse. Our findings reveal a complex pattern of non-neutral evolution of the mitochondrial genome in the domestic horses.