The ancestral roots of donkeys have been laid bare in fresh research, with genetic sequencing revealing that modern Tropical African donkeys and North African and Eurasian donkeys coalesced a little over 6000 years ago.
The study team led by Jifeng Zhong, with the Shandong Academy of Agricultural Sciences in China, based its findings on the analysis of the genomes of 126 domestic donkeys and seven wild asses from nine countries.
The researchers, writing in the journal Nature Communications, described how they grouped the donkeys in the study genetically to learn about the ancient migrations of their ancestors, as well as their relatedness.
Delving even further back, their work supported previous reports demonstrating that domestic donkeys originated from African wild asses and not from Asian wild asses.
Based on their work on the Y chromosome, the researchers found that the ancestral species of African wild ass and the Asian wild ass had separated between 1.55 and 1.66 million years ago.
The common African ancestral species of all domestic donkeys separated into two clades (groupings) between about 14,500 years ago and 18,000 years ago. Both clades then further split into diverse donkey breeds in a window of time between 5500 and 3600 years ago.
Their findings in relation to only domestic donkeys revealed three main modern-day clusters:
- The tropical Africa cluster (Kenya, Ethiopia, and Nigeria);
- The North Africa and Eurasia cluster (Egypt, Spain, Iran, Kyrgyzstan, and China); and
- The Australia cluster.
Like horses, limited diversity was found on the male Y chromosome, indicating the use of relatively few males in selective breeding practices.
The authors said the absence of text records and archeological remains, and a lack of pictorial and sculptural representations, makes it very difficult to infer the routes of spread of donkeys across Africa.
Scientists who conducted earlier research had proposed several potential migratory paths of diffusion, none of which connect West and East Africa, but this did not necessarily imply the absence of gene flow between these two geographic areas.
“We have also observed that Australian samples clustered separately from their African, Asian, and European counterparts,” the authors noted.
“The strong differentiation of Australian donkeys might be attributed to the limited number of founder individuals that were brought by the British colonizers to Australia 200 years ago, as well as to the effects of prolonged geographic isolation and high genetic drift.”
The researchers also looked into coat color. They found that the TBX3 gene is responsible for grey coats in both horses and donkeys, suggesting the grey-coat gene was inherited from the common ancestor of donkeys and horses.
While wild asses show diluted grey pigmentation (Dun coloring), domestic donkeys display non-diluted black or chestnut coat colors (non-Dun coloring) that were probably established during domestication.
The non-Dun coloring seen in donkeys arose from a mutation in the TBX3 gene, with artificial selection by breeders likely leading to its prevalence.
Wang, C., Li, H., Guo, Y. et al. Donkey genomes provide new insights into domestication and selection for coat color. Nat Commun 11, 6014 (2020). https://doi.org/10.1038/s41467-020-19813-7