As I mentioned in the previous post, there are some caveats with the possibility that the EDN3 gene may control the mealy pattern. One of them comes from a 2019 study on regions of homozygosity (ROH) in European horse breeds. Because the goal of selective breeding is more predictable outcomes – animals that “breed true” – over time, the population becomes more homozygous. The genes in those regions give clues about the traits being selected. For example, a group of Suffolk Punches would be expected to have an ROH where the control for chestnut is located.
In the European breed study, a small group of non-mealy Hungarian Lipizzans had a homozygous region that included the EDN3 gene. Although the homozygous region distinguished the Hungarian group from the other Lipizzans in the study, because it is only briefly mentioned in the article, it is unclear if they had the same allele previous researchers found in the Exmoors. The authors recommend a future study that focuses more directly on EDN3 and color.
Picture provided by Diane Blanzy. To see more bays (and roans!) from this breeding program, please check out Pipe Dream Percherons.
This also raises questions about horses identified as “non-mealy.” The mealy bay colt above is Lucifer PDP. He is from the Pipe Dream Percheron program. His breeder, Diane Blanzy, has been working to preserve the remaining bay lines in the breed. Many do not realize that there are bay Percherons, but even those who do, probably did not expect one with such a pronounced mealy pattern. Until I saw Lucifer, I did not! However, in a primarily black (or black-turning-grey) breed, a gene that alters the intensity of red areas of the coat could pass unnoticed.
Mealy as a factor that dilutes red pigment
There was a time when it was thought that seal brown horses like Billy (above) might be mealy black. Visually, that makes sense. The placement of the palest areas of the coat – the muzzle, eyes and undersides – are similar. However, seal brown horses consistently test as bay (A) and not black (a). Seal brown also looks different in breeds where mealy is present.
Exmoor Ponies on Traprain Law. Photo by Paradasos.
As these seal brown Exmoors show, the areas that would normally be red have been altered to cream. Even though the mealy effect is strong on the pony in the foreground, he lacks the pale belly visible on Lucifer. That is likely because, as a dark seal brown, he had less red in that area. The pony behind him has a little more red in his brown coat, so it shows just a bit more mealiness.
If mealy patterning is specific to red pigment, that would explain why black dun Fjords do not have pale muzzles. It would also allow for mealy bays with dramatic sooty contrast like the Exmoor in last week’s post, because only the red areas would change. It is also consistent with recent findings that a recessive mutation at Agouti in donkeys removes their mealy points because like a black horse, a black donkey would not show mealy points.
Not “mealy black,” and probably not really bay, either…
Seal brown was also proposed as an allele at Agouti (At). There was a commercially available test for brown using a marker on Agouti, but it was pulled when inconsistent results were found in several Miniature Horses. It seems likely that the control for this pattern will be found at another locus. If that is the case, then in the same way that dominant Extension (E) “allows” for either bay or black depending on the allele at Agouti, bay (A) at Agouti allows for seal brown at its locus. And just as a recessive red (ee) horse does not show its alleles at Agouti, a recessive black (aa) horse would not show its alleles at the brown locus.
This proposed explanation would add two additional loci to the existing two controls for basic color – one for seal brown and one for mealy. The dark Exmoor Pony would then have dominant Extension (E) and Agouti (A), plus the allele for seal brown (location unknown) and the allele for mealy (location currently proposed at EDN3).
Other controls for red and black pigment
Genes that control the distribution of red and black pigment are known as pigment-type switches. Once it seemed that for horses those controls were limited to the two oldest described controls for pigment type, Extension and Agouti. The idea that there could be three, four or even more genes involved in “base” colors is something that many find difficult. Base colors in horses was supposed to be simple!
However, one thing that seems to have proven true for animal color is that few things stay simple on close examination. With horses, white patterning genes have been examined closely. What looked like a half-dozen patterns is now closer to forty, and there are still more left to find. Pigment-type controls have not gotten the same scrutiny, but in species where they have – like dogs – more sites have been found. Horses have some variations in their basic colors that raise questions about pigment-type switching. In future posts, I’ll look at another potential pigment-type gene (RALY) and talk about some of the outstanding questions.
Feature photo: Exmoor Ponies at Greenham Common in Berkshire by Marie Hale.