4 | DISCUSSION
The mtDNA genome of donkeys is 16 670 bp, of which the D-loop region is 1 207 bp (Xu et al., 1996). As reported in previous genetic studies on donkeys (Lopez et al., 2005; Ma et al., 2020; Ozkan Unal et al., 2020), the D-loop region of ancestral mtDNA supplies adequate information to assess genetic variation, evolutionary relationships, and matrilineal genetic origins.
We detected a total of 23 nucleotide polymorphic sites in the D-loop region sequences of HGDs. For the D-loop 418 bp sequence, nucleotide polymorphic sites account for 9.21%, among which 94.44% are transformed or transposed, with two insertion sites accounting for 5.56% (Ivankovic et al., 2002). Therefore, the frequency of transposition was much higher than transversion, which is consistent with the results of our study.
At present, there is a traditional view on the origin and evolution of the Chinese donkey, which is that the Chinese donkey originated from the African wild ass after domestication. Lei et al. (Lei et al., 2007) found two mitochondrial origins of African wild ass, lineage Somali (Clade Ⅱ) and lineage Nubian (Clade Ⅰ) in Chinese domestic donkeys and that Clade Ⅱ was prevalent in Chinese domestic donkey breeds. These findings are consistent with those of our study.