According to genomics researchers, foals that acquire two copies of a gene mutation responsible for white spotting aren’t likely to survive—if they’re born at all. That’s probably because that particular variant doesn’t just affect coloring but also other vital systems, said Rebecca Bellone, PhD, Genetics Laboratory Director at the University of California, Davis.
“We believe that this mutation (when inherited from both parents) is incompatible with life not simply because it causes lack of pigment (white phenotype) but because of the other cell types that are likely not properly developed,” Bellone said.
Genotyping Solid and White-Spotted Donkeys and an All-White Donkey Foal
Bellone and her fellow researchers collected photographs and hair samples of 55 donkeys, including 17 that had white spotting and one young foal that was entirely white, with pink skin and blue eyes. They ran genotyping on DNA they extracted from the donkeys’ hairs and looked for two variants of the KIT gene.
The KIT gene has 30 known mutations in horses, causing various kinds of white spotting. In donkeys, however, researchers have only found two variants: W and Ws. A single copy of the W variant, also known as Dominant White, leads to an all-white donkey with pink skin and dark-colored eyes, she said. However, it’s extremely rare, with only one known healthy donkey reported to have the mutation to date.
In the new study, Bellone and her team found that all 17 white-spotted donkeys had one copy (inherited from only one parent) of the Ws gene. Two of those donkeys were the parents of the all-white baby donkey, and he had acquired the Ws mutation from both parents, giving him two copies of the Ws gene. Sadly, the foal died at about two weeks of age, Bellone said. Although no postmortem studies were performed on the foal, the scientists suspect the untimely death might have been related to his two copies of Ws (homozygous Ws).
W/W and Ws/Ws: Two Copies of White Mutation Are Fatal
The researchers were surprised the all-white, blue-eyed donkey survived past the embryonic stage, said Bellone. A previous study on American and Australian white-spotted donkeys revealed they were all heterozygous for Ws, meaning they only had one copy of the mutation. The fact that those researchers didn’t find any homozygous donkeys led them to believe if both parents passed on a copy of the mutation, the embryo wouldn’t survive long enough to develop into a foal.
“Why the Ws/Ws animal in our study survived to term is still a bit of a mystery, and unfortunately the animal was not available for necropsy after he passed away,” she said.
The KIT gene encodes for a particular cell surface tyrosine kinase receptor protein, said Bellone. When that receptor is activated, it promotes the spread and division of pigment cells. But it doesn’t just affect pigment cells; it also affects white blood cells (mast cells) and certain cells in the gastrointestinal tract (interstitial cells of Cajal). So if the tyrosine kinase receptor isn’t working properly, it won’t just give the animal white hairs; it could also interrupt the functional development of major bodily systems necessary for life.
“The lethal nature is believed to be caused by deficient KIT signaling that impairs the development of these critical other cell types,” Bellone said.
The Ws/Ws donkey in their study might have lived long enough to be born because his body produced “just enough normal protein” to survive the embryonic stage but not enough mast cells and interstitial cells of Cajal to sustain life outside the womb, she added. “But because the animal was not available for further evaluation, this is just my hypothesis on why the mutation allowed for embryonic survival but was incompatible with life,” she said.
One White-Pigment Mutation Is Compatible With Life
It’s important to note that only when the offspring is homozygous for the mutation—he receives a copy of the white-spotting mutation from both parents—is Ws incompatible with life, said Bellone. In other words, white spotting alone isn’t lethal or even dangerous. But breeding two white-spotted donkeys could lead to embryonic loss, fetal abortion, or the death of a young foal.
“The easiest way to think about it is, with one normal copy of the gene, signaling is sufficient to produce enough normal protein to enable proper development of these other cell types,” she said. “But with two variant versions of this gene there is insufficient KIT signaling to promote the development of melanocytes, types of white blood cells, or interstitial cells (in the gastrointestinal tract).”
Smart Breeding of White-Spotted Donkeys and Mules
Thus far, scientists have found white spotting arising from the Ws variant in 52 donkeys from a variety of donkey breeds and the W variant in a single donkey of unknown breed, Bellone said. They’ve never found a donkey with two copies of W, nor one with one W and one Ws, so scientists suspect such combinations probably lead to embryonic loss.
To avoid embryonic loss, abortion, and newborn deaths, breeders should avoid putting two white-spotted donkeys together, said Bellone. Instead, if they want a healthy white-spotted baby, they should breed a white-spotted (W or Ws heterozygote) donkey with a solid-colored donkey (genotype N/N).
“White spotting is a favorable trait, but given that homozygosity is thought to be lethal, selectively breeding W/N individuals to individuals with the N/N should be considered by breeders to avoid producing nonviable offspring,” she said.
And considering that horses have 30 “W” variants of KIT genes that lead to white spotting, it might be advisable to avoid breeding white-spotted horses with white-spotted donkeys to create a white-spotted mule, she added. However, this depends largely on the kind of W variant (because at least one W variant, W20, doesn’t seem to be lethal in homozygous horses), and more research is needed to give concrete recommendations for white-pigmented mule production.