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Pigmentation as a developmental process

June 4, 2021 0 Comments

This post first appeared on March 26, 2016
I talked in a previous post about changing the framework of understanding for horse color from the familiar “base + modifiers” to one that is based on the evolution. In describing the original wild horses, I explained that their color, that color in all animals, is the result of a developmental process.

Their coloration was the result of a pigmentary process, controlled by a number of genes. While still in the wild, mutations to those genes changed part of that process, which in turn changed the color of the horses.

Since more time has passed since I posted that than I intended, and because it is so central to thinking about horse color from an evolutionary standpoint, I want to expand on this idea of colors as a biological, developmental process. We think of colors as traits—as aspects we can see in the individual or in breeding outcomes—but there is an underlying process involved. In any given species, that process produces a specific coloration known as the wild-type. You can think of it as the default setting for the species. In horses that default color is yellow dun: buff body coloring, black points and primitive markings. Using the “base + modifier” framework, we would think of this as “bay modified by dun”, but in actual practice this coloration is the result of many genes (known and not-yet-known) that control pigmentation as the embryo develops.

Now if you disrupt that development, if you change one of the genes that controls the process, then the outcome is different. That is what color mutations do; they disrupt some aspect of the process which in turn changes the final color of the horse.

And although this is a different way of looking at horse color, it is not completely unrelated to the more familiar framework. The way we tend to group modifiers corresponds with ways the process can be disrupted. Dilutions, for instance, disrupt the process of pigment production within the cells. White markings and patterns, on the other hand, disrupt the migration of those cells to parts of the body. (The difference between those two situations—cells that don’t work quite right and cells that are not there at all—is key to understanding the difference between forms of albinism and forms of piebaldism.)

So, just keep that in mind as we go forward with the next few posts. Our starting point is the original color of the species, which is a yellow dun much like the kind pictured at the top of this post. In the next post I’ll talk a little about the processes involved that produce that color… and then we’ll start breaking things (or at least muck them up a bit) and see what that does to the final color!

[For folks that would like to jump ahead and get a sense of how the different modifiers fit into the process of pigmentation, “Coat colours and mitochondrial lineages of ancient horses to document domestication” by Michael Cieslak has a wonderful graphic on page 14.]

Photo of Przewalski Horses by DFoidl, courtesy of Wikimedia Creative Commons.

By lkathman

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