Mosaicism in horses - part 2

The bleach spot on the hip of this horse is the type of marking that often raises questions about whether the individual is a chimera. It does look like portions of two horses, one chestnut and one grey, were combined into one. I do not know this horse’s name or parentage, but it is my understanding that he is a Belgian. In North American Belgians, the grey lines died out early in the 20th century. This makes chimerism unlikely. 

The marking is reminiscent of the Swedish Warmblood Lorando B. With chimerism, the animal starts as a pair of fraternal twins, so each ‘part’ must be a plausible descendant of the parents. In this case, the parents were both bays, so a grey twin would not have been possible. Since that is true, the marking is assumed to be the result of a somatic mutation. Given the visual similarity, the mark on the Belgian is probably the same. 

It is intriguing that the two patches behave similarly to grey even though genetic grey is impossible for Lorando B and unlikely for the Belgian. The mark on Lorando B’s forehand is white grey in the linked picture, but it was a progressive change. Here, he is at an earlier stage, looking similar to the Belgian, and where the mark begins to fade. 

The contrast between the color at the center of the marking and the rest of the body mimics the hyperpigmentation that happens to young greys. My guess is that when the Belgian was younger, that area looked like a dark or sooty patch. 

So, how could a portion of a horse be grey if the parents are not?  

The answer is that, early in fetal development, one of its cells mutated. In the case of the Belgian, the original cell was a melanocyte (pigment cell) destined for the left flank. Perhaps the somatic mutation involved duplicating a fragment of the STX17 gene—the same type of change responsible for grey (G). As the fetus developed, the mutated cell divided, increasing the number of mutated pigment cells. The result is a large bleach spot on the flank.

Of course, the specifics of the cause are a guess. Perhaps there are other changes to STX17, or different genes, that could give a similar effect to the action of the grey mutation. A mutation like the one involved in Grey (G) seems likely because duplication of an existing piece of genetic code is a type of change that could easily occur.

Mosaic patterning

The marking on the Belgian doesn’t show one of the mosaic patterns because the mutated cell divided after it migrated to the flank area. If it divided before that, the progression of its migration might be visible in the form of one of the mosaic patterns. 

For example, brindled duns like the Fjord mare Rachem Quintessa (above) likely had a cell with a somatic mutation that “silenced” the diluting aspect of their dun coloring. That mutated pigment cell would produce fully-colored (rather than dun-diluted) hairs. However, instead of migrating to one area and then dividing (or mutating in a cell that already migrated), this mutated dun cell underwent division first. That meant there were already a number of mutated (non-diluted) cells once pigment began migrating to the rest of the body. That is why the pathways (the Lines of Blaschko) are visible in the form of brindling.

Other colors that show a mosaic pattern are likely caused by somatic mutations prior to the migration of pigment cells. For example, greys with a brindle pattern – commonly called skewed greys – may have a cell that lost one of the duplications of their STX17 gene. Appaloosas with a checkerboard pattern of mismarks may have a cell that lost the inserted code that produces the Leopard Complex (Lp) coloring, effectively “turning off” the patterning process and reverting portions of the coat to solid. Meanwhile, horses with one odd patch, like a mismark on the face of a grey, might be more like the Belgian.

Understanding the process behind these patterns raises some interesting questions. For example, if duplication of fragments of STX17 occurs often enough in individual cells that we have at least two modern examples of bleach spots, why aren’t there multiple versions of the grey gene? (To date, all tested greys share a common ancestor.) The explanation for late greys would also explain why grey is prone to brindle skewing, but why roan? What does the fact that roan is the other color prone to brindle skewing tell us about roan? Roan has been the focus of several recent studies, so perhaps that question will have an answer in the future.