Mosaicism in horses - part 1

If not all brindle horses are chimeras, what are they?

When unusual horses are posted on social media, a typical response is for someone to suggest the horse is a chimera. Although chimeras have been documented in horses, when a horse looks like two different-colored animals were fused together, it is far more likely a mosaic.

Chimerism vs. mosaicism

Just like the mythical creatures they are named for, chimeras are a fusion of two separate organisms. Horses like Dunbars Gold and Sharp One (described in the linked article) started life as fraternal twins. Early in development, the two embryos fused. Their brindle coloring comes from the difference in color between the twins. 

In contrast, a mosaic starts as a single embryo. At some point during development, a cell within the embryo mutates. As the embryo develops, that altered cell continues to divide. The result is a single organism with two different sets of genetic instructions. 

What chimeras and mosaics have in common is that their condition is not something they can pass on to their offspring. Because these events – either the fusion of two individuals or a mutation to a particular cell – occur after conception, they don’t involve the DNA found in the reproductive cells. 

What about somatic mutations?

When someone misidentifies an unusual horse as a chimera, they are often greeted with a chorus of “That’s a somatic mutation!” How are somatic mutations related to mosaicism? A mosaic has a mutation that occurred in one of its cells after fertilization but before it was fully formed. That mutation is somatic, which means “of the body,” rather than germline, which means “in the reproductive cells.” In this context, somatic describes the mutation by indicating that it occurred somewhere in the body (but not the reproductive cells.) Mosaic describes the organism by indicating that while it has differing DNA in some portion of its body, it started as one individual.  

Pigmentary mosaicism

Mosaicism is not exclusive to pigment cells; any cell can mutate and pass that mutation down as it divides. Mosaics involving pigment are visible, though, which made it possible to study them even before modern molecular genetics. As early as 1901, scientists understood that during embryonic development, pigment cells migrated from the midline of the body to the extremities in predictable paths. These lines were first described by Alfred Blaschko, a dermatologist studying skin disease. Although Blaschko was treating human patients, scientists studying pigment in mammals recognized some of the same patterns in fur coloration. 

This drawing appeared in Alfred Blaschko's original monograph. Brindle animals show a similar pattern.

The lines of Blaschko produce a handful of different patterns when two visibly different sets of pigment cells are present. I’ve adapted the diagrams of the four most common types of pigmentary mosaicism using horses from Herman Dittrich’s anatomy studies. It’s easier to visualize how these patterns translate to four-legged animals this way. 

The patterns in this illustration were adapted from a diagram in "The Puzzle of Skin Patterns (2016)."

It’s important to remember that the Lines of Blaschko are the pathways—not the patterns themselves. Some patterns, like the narrow linear, follow the lines more closely. Others just hint at the direction of those paths when viewed dorsally.

The original drawings are simplified, and it’s important to note that in animals, the pattern’s scale often changes. For example, the checkerboard type of pattern is seen in tortoiseshell cats and merle dogs, but the patches are usually smaller than in most human diagrams. The checkerboard pattern is why split faces are frequently seen in those colors. (Examples 1, 2, 3, 4, 5, 6)

Photo provided by Wikimedia through Creative Commons

Note: Tortie cats with split faces are misidentified as chimeras even more often than odd-colored horses. For a detailed look at the topic, I highly recommend Sarah Hartwell’s “Cat Chimeras” on the Messy Beast blog.)

Mosaic patterns in horses

In horses, the narrow linear pattern can be seen in skewed roans and greys, brindled duns, and horses with irregular white striping. Not all brindled horses are mosaics, however. Several documented chimeric horses are brindled, and at least two forms of genetic brindle exist. There, the lines of Blaschko are still responsible for the pattern, but other factors cause the difference in the colors of the stripes.

Rachem Quintessa, a brindled grulla Fjord owned by Rachel Young. Photo used with permission.

The checkerboard pattern is less common in horses. Some of the more extensive forms of blood-marking have this look. 

It is also visible in some horses with random black and red body spotting. Probably the best-known example of this is the Icelandic mare Miljon fra Grund. The broader linear and phylloid patterns do not seem to have equine equivalents. There are also odd patterns that do occur in horses that behave like other somatic mutations but do not have a human counterpart, so they are not documented among the classic forms of pigmentary mosaicism. The oblong chubari spotting comes to mind as a good example of this. 

Although there are recurring patterns, not all mosaics have a pattern. If the mutation occurs later in development, after the cell has migrated, the change may be isolated to one portion of the body. Mismarks, bleach spots, and revertant patches are good examples of this type of mosaicism.

In the next post, I’ll look at some examples of mosaics and the somatic mutations that caused the change in their color. 

I would like to extend a special thank you to Rachel Young for allowing me to share pictures of her brindled dun Fjord, Rachem Quintessa. Thank you also to Heather Forrest for providing the picture of the blood-marked Highland Pony and Maria Hjerppe for providing the picture of the blood-marked Arabian.