Donkeys have been helping humans for millennia, but our knowledge of their origins has previously been limited to archaeology. Now, modern genomics paints a much more accurate picture of their domestic history.
A team led by Jifeng Zhong from the Shandong Academy of Agricultural Sciences, China, has found that modern domestic donkeys probably shared a single, African ancestral group around 6000 years ago.
In a paper in the journal Nature Communications, they report that donkeys appear to be the product of only a few male studs, revealing some well-defined breeding choices by farmers.
The team analysed the genomes of 126 domestic donkeys from nine countries and compared them with three species of wild ass. They found that all the domestic donkeys shared one group of wild asses as an ancestor – the critically endangered Equus africanus from Ethiopia and Somalia.
They then grouped these donkeys genetically to see how they migrated and to whom they were related, coming up with two groups: Tropical African and North African/Eurasian (including Australian).
They found that the Tropical African donkeys were the closest relatives of E. africanus, but all other donkeys fell into a kind of chain of relatedness – starting with these wild asses – that reflected genetics and migration.
Simply, when wild asses were domesticated, the result was Tropical African donkeys. These spread to North Africa, then up into Europe and Asia. Spanish donkeys were the ancestors of the donkeys taken to Australia, which then became a distinct group due to geographic isolation.
Zhong and colleagues also found there was a limited amount of paternal genetic material in domesticated donkeys, much like with horses. This means donkey breeders have been specifically selecting the best male donkeys to breed with multiple female donkeys for a very long time.
Based on these lineages, E. africanus diversified significantly between 5500 and 3600 years ago, which supports archaeological evidence that this was the time donkeys were domesticated.
Regardless of this complicated history of selective breeding and migration, differences in the colour of donkey coats have a relatively simple explanation.
The team found that a specific gene, TBX3, was responsible for grey coats in both horses and donkeys. This means the grey-coat gene is rather old and was inherited from the common ancestor of donkeys and horses.
Interestingly, other coloured donkey coats, such as chestnut or black, were caused by a mutation in only a single nucleotide pair in the TBX3 gene, which stopped the gene from working properly. While this is a simple mutation, it appears have been selected from in domestic donkeys in the same ways and coat colours were selected for in horses.
It is fairly common for domesticated animals to have a variety of coat colours due to artificial selection and lack of environmental pressure. This is the case with donkeys, too, as they were more colourful than their wild ancestors.
Despite all this, it is difficult to match this genomic evidence with archaeological evidence, because donkeys don’t appear in ancient depictions as much as some other domestic animals, such as horses and camels.
This means that more genomic data from ancient donkeys is needed to conclusively understand where donkeys came from.