Archive | Dilutions

Starting at the beginning – the process

FjordLight

So if we started at the beginning, with the horse before he was domesticated, we would be looking at an animal with a coloration much like the Fjord pictured above. The similarity between this particular type of bay dun and the coloring of wild equids is obvious, and was noted before the founding of the science of genetics. In his book The Variation of Animals and Plants Under Domestication, Darwin noted the primitive nature of this color.

[the similarity of color] in duns, of leg-stripes and of double or triple shoulder stripes… indicate the probability of the descent of all the existing races from a single, dun-coloured, more or less striped, primitive stock, to which our horses occasionally revert. — Charles Darwin (1896)

With the rediscovery of Mendel and his laws of inheritance, color variations came to be spoken of as “factors”, and then later as “genes”. Understanding physical reality behind these “factors”—the discovery of DNA—was still the better part of a half-century away. So a horse like this Fjord would have been said to have the “bay gene” and the “dun gene”. Much of how non-scientists talk about animal color still sounds like this. But within the scientific community, where advances in technology have given incredible insight into the mechanisms controlling development, color is spoken of as a process. A very complex and (at the moment) only partially understood process.

Let me try to give a simplified explanation of this developmental process and how it applies to bay dun.

Step 1 – Setting up for production

The precursors to pigment cells originate in the neural crest of the embryo. From there they migrate outward from the dorsal line, and multiply across the rest of the body. Once at home in the skin and hair follicles, these become the “factories” where pigment will be produced.

If you break something in this part of the system, you get spotted* phenotypes. The reason white patterns tend to trump all the other color traits—why, for instance, a dorsal stripe does not continue on through and bisect the white area of a tobiano—is that this is the start of the process. What happens within those cells after they start making pigment doesn’t matter if the cells never make it to a given location. Although horsemen, and certainly artists, tend to think of white as something added over the top of the basic color, understanding that color-producing cells have to migrate across the body makes it more obvious that white markings and patterns are actually the areas that those cells did not reach.

Markings
When pigment cells fail to migrate to the end of the limb, the result is a white marking on that leg.

Step 2 - Make the “factories”

Those pigment cells, once in place, are where melanin is produced. This process, which involves numerous steps, is known as melanogenesis. If you break something in this part of the process, the body may have viable pigment cells, but their ability to make pigment is compromised. Dilutions like cream, champagne and silver are mutations to (well-known) genes that control some aspect of melanogenesis, and the fact that different genes govern different parts of the process is why the phenotypes are different.

Step 3 - Regulate the product

Pigment cells also regulate the chemical composition of the melanin, which determines whether black (eumelanin) or red (pheomelanin) is produced. This is what was discussed in the series of posts on pigment-type switching, which is the proper term for this part of the process. If you break something in that part of the system, you change the proportion of red and black hair.

Step 4 – Maintain production

Because hair and skin are routinely replaced, pigment cells must continue to function to maintain coloration. If something in this part of the process is broken, there is a progressive loss of color. Greying is probably the most familiar example of an alteration that occurs in this step.

How this makes bay dun

So back to our domestic horse ancestor, who was bay dun. That first part, his bay “base” color, is determined in the process explained in Step 3. Back when colors were still “factors”, two major controls were theorized: Extension and Agouti. In modern terms, the theorized “Extension” is a gene known as MC1R and “Agouti” is a gene known as ASIP.  Together those two sites form the primary control of pigment-type switching in horses. In their original, unmutated form, they work together to give a pheomelanic (red) body and eumelanic (black) points. That is what is meant by the statement that “bay is the ancestral (base) color of horses”.

Because the pigment-type switches were formally identified first, it was possible to confirm that bay was the original wild color for horses. The assumption was that the ancestors of the domestic horse were also dun, but dun proved more elusive to identify at the molecular level. Part of the reason for that was that it was not in an expected place. In fact, it was not found in the expected part of the process. Because dun is thought of as a dilution—indeed the phenotype is diluted in appearance—the assumption was that the cause would be found in one of the sites known to be involved in melanogenesis (Step 2). In the paper I linked to in the previous post, the MLPH gene was examined and ruled out. That gene is involved in Dilute (d) coloration in dogs (often called “blue”), but had not yet been linked to any of the diluted phenotypes in equines, so it was a logical candidate.

But dun was found in a gene known as TBX3, which was not previously known to be involved in pigmentation at all, and the part of the process was not in the production of pigment but in the placement of pigment cells when hair follicles are formed. That’s back in Step 1. Hairs from the diluted areas of a Dun (D) horse have pigment asymmetrically distributed, so that the sides (viewed in cross-section) are unpigmented. It is that placement of the pigment on the hair shaft that gives the impression of paler hairs. In her dissertation on the topic, the lead author Freya Imsland describes the situation with Dun this way (emphasis is mine):

The precise nature of the reduced pigment intensity in Dun is highly unusual; it can in essence be described as a microscopic spotting pattern, whereby the pigment distribution within each hair is radially asymmetric.

She reiterates this explanation in the abstract for the dissertation:

This results in a microscopic spotting phenotype on the level of the individual hair, giving the impression of pigment dilution.

The timing of the mechanism behind Dun explains why, unlike cream or silver dilutes, dun horses have diluted bodies regardless of their base color. It is the placement of the pigment cells in the hair follicle, and not impaired production of pigment, that make the coat lighter in those areas. And that happens before pigment is made or pigment-type determined. That may also explain why Dun characteristics (like the dorsal stripe) persist even when the color is paired with subsequent dilutions.

The discovery of a new role for an established gene is an important scientific discovery. As people interested in horse color, we tend to think of discoveries like Dun in terms of tests (or more accurate versions of existing tests). But the investigation into Dun is a good reminder that when it comes to the process by which animals acquire their color, there are is still much that is left to discover, and that some of it may be surprising. Another interesting aspect of this discovery is that the idea that dun hairs were different at the microscopic level is not entirely new; this idea was suggested in older books on horses color (Geurts, 1973 and Green, 1974). This also has intriguing implications for the type of color-shifting seen in some of the traditional (large-scale, as opposed to “microscopic”) spotting patterns. But that is all jumping ahead, and a topic for another day!

How we got to here from there

DunNotDun

So the original wild horses probably looked a bit like this Fjord, with pigment distribution causing a body color that was visually quite pale while the primitive markings were more intensely pigmented. But the color of the modern domestic horse is more typically pigmented at full intensity across the entire body, like the warmblood to the left. What changed? And why? The other big finding of the recent dun research was that this was a gradual process, and that it began long before—close to 40,000 years before—horses were domesticated. That will be the subject of the next post.

*Although it has been used by some horsemen for specific patterns, “spotted/spotting” is also the generic scientific term for any form of white marking or pattern.

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Frosted Buckskins

FrostMan

I am still sorting through some of the old photos I have, trying to remember what has been posted (as opposed to “meant to post but never did”). If I repeat something, please forgive me - though I would imagine that if after three years I cannot remember posting about a subject, maybe readers have forgotten it, too!

Recent conversations about flaxen-maned bays reminded me that I had meant to post these pictures of buckskins with frosted manes and tails. As the photo above shows, the hairs are pale flaxen or white. It is harder to tell because the pulled mane on this Paint gelding is so short, but most of the time the pale hairs are short, which gives the mane a frosted look. Pale hairs are also seen at the tailhead. This next picture shows the distinctive “V” shape that is typical of the frosting on a buckskin’s tail. This shot also shows more clearly how the white on the mane is concentrated at the base of the neck.

FrostTail5028

The frosting on the tail looks quite different from that kind often seen on duns. With a dun, the paler hairs are usually found on the sides of the tailhead, in part because the dark pigment of the dorsal usually runs down the core of the tail.

DunTail

Both frosting on buckskins and on duns looks a bit different from the white “coon tail” seen on some of the white ticking and sabino patterns. With this picture you can see both the paler hairs to the sides of the tail (relative to the deep red dorsal stripe) and the white hairs that are part of the patterning.

DunCoon6701

Frosting is more common in duns than in buckskins, but it is not always pronounced. This dun mare has very little contrast – just a few paler hairs – between the core of her tail and the sides.

DunTailhead

So what causes frosting on a buckskin? Most likely it is the Cream (Cr) gene turning what would be paler red guard hairs to a pale flaxen or white. This photo shows the similarity between the arrangement of the pale hairs on a light bay and those on a frosted buckskin.

CreamFrost

Here is the same bay Paint Horse mare that is pictured above. She has the reduced intensity at the points that is often seen on bays with paler hairs at the base of the mane and tail. If you look closely, you can see the lighter hairs at the base of her mane, too. (Unfortunately she was always on the wrong side for the sun, so none of the photos taken from her other side turned out.)

GuardHairs

I suspect that selecting for this kind of clear bay with reduced black points would increase the contrast on the frosting of both buckskins and duns. That is probably why frosting is so typical of the Fjord. That breed appears to carry almost every factor that might reduce black points.

The downside of frosting on buckskins is that is does not appear to be permanent. As the horses age, they seem to lose the contrast until their manes and tails are black. At least, that has been my observation based a limited number of individuals. Certainly if a reader has an older buckskin that still has pronounced frosting, I would love to hear from them!

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Cornaz albinos

Patty
Patty, an albino Pekingese from an early 20th century experimental breeding program

So the mutation responsible for white Dobermans has been identified, and is similar to the Cream and Pearl dilutions in horses. So what about Angel, the albino Shih Tzu that started this discussion? Is she also the canine equivalent of a cremello?

Probably not. An albino Lhasa Apso tested negative for the mutation found in Dobermans. What’s more, she did not have a mutation to that gene (SLC45a2). Whatever caused her pink skin and cream coat, it appears to be unrelated to the color in Dobermans. Because the Shih Tzu and Lhasa Apso shared a stud book up until 1934, it seems more likely that Angel would have the same mutation as the Lhasa. Both breeds also share a history with the Pekingese, which has one of the most throughly documented families of albino dogs. The influence of that family may explain why albinism in dogs is often seen in the smaller Asian breeds.

The albino Pekingese were the focus of an experimental breeding program conducted around the turn of the last century. Extensive information on the foundation animals appears in A Monograph on Albinism in Man, published in 1913. Although the program was disrupted by World War I, it did continue for a time and a follow-up article was published in 1929. Because the information was so detailed, it is possible to know the founder for the color in the Pekingese.

AhCum

That is Ah Cum, the “grandfather” of the Pekingese breed. He was an ordinary red sable, but because he was the common ancestor in all the known albinos in that breed, the authors of the study believed that the albino gene came from him. There can be little doubt that his son, Ch. Goodwood Lo, carried the recessive gene for the color.

Lo

What is interesting about this particular family, and this experiment, is that many of the dogs were photographed. The written notes on the dogs can be less than helpful, because those studying the dogs did not yet understand something that those of us who study animal coloration take for granted now, which is the concept of base colors and modifiers. So instead of seeing these dogs as a basic color, like sable or black-and-tan, that had been diluted down to a nearly white color by a modifying gene, the researchers assumed they were dealing with separate colors. They called the near-white dogs “Dondo Albinos” and the somewhat darker dogs “Cornaz Albinos”. That latter term is still used for this color in many breeds where albinos are known to occur.

It should be noted that the authors knew these dogs did not have pink eyes, or even necessarily blue ones. They considered an eye to be albinotic if the pigment was reduced. In fact, their discussion of equine eyes touched on a question that has often been on my mind. We often hear that horses do not have “true albinism” because there has not yet been a documented case of pink eyes. What I have often wondered was whether a pink eye is actually possible in all animals given the varying structure of the eye. Is an eye without pigment always pink or red?

In regard to the colour of the iris as seen during life in the imperfectly albinotic eyes, the present observations confirm in an interesting manner our previous knowledge that when the mesoblastic pigment is absent the iris is either white (the so-called “wall” eye) or blue or slaty blue according to its thickness and texture, a thick and fibrous iris being white and opaque throughout or translucent only at its thinnest part. In the horse even the thinnest or pupillary zone is probably too thick to be translucent.

Here the authors – two of whom are ophthalmologists – seem to suggest that in some animals an eye without pigment might not necessarily appear pink or red. Yet they also mention the difficulty in finding a horse with perfectly unpigmented eyes.

We have hitherto not succeeded in meeting with a perfect albino horse; the epiblastic pigment of the iris seems peculiarly persistent.

The Pekingese family was considered an example of ‘imperfect’ albinism, which meant that there was some trace of pigment either in the eyes, skin or hair. That is still what this kind of coloring is called in the dog world: albino. If something like this turned up in the horse world, there is little doubt that it would be considered a dilution, just as champagne and pearl were when they were identified. But as I mentioned, when this breeding program was undertaken the concept of a diluting modifier was not understood. (To give some perspective on the understanding of inheritance at the time, James Cossar Ewart’s famed Penycuik Experiments disproving telegony – the idea that previous matings left a taint that could influence later offspring – had been published only a dozen years earlier. Crick and Watson’s discovery of the double-helix structure of DNA was still forty years away.)

To a modern student familiar with how diluting modifiers work, the underlying colors on some of these Pekingese is obvious. Hints of the dark ‘spectacle’ markings common in sable Pekingese can be seen in the photo of the dog at the top of this post. The dog below looks to be a dilution of the black-and-tan pattern, judging from the coloring on the face and forelegs.

Fo

It also appears that some of the darker dogs may have been carrying some combination of the Cornaz albino dilution and the more common dog dilution, Brown. The color of the darker Cornaz albinos was described as “scraped chocolate”. One of the ancestors of the foundation stock was described as “liver and white”, and there was at least one puppy from the experiment that was noted as having a brown, not pink, nose. When later generations were crossed on black Pomeranians – which the researchers, anticipating the “designer dog” trend by a hundred years, called Pompeks – one of the first generation litters resulted in two chocolate puppies. In this way, it seems possible that the Cornaz dilution combines with Brown to produce an intermediate shade, much like Cream combines with Pearl or Champagne in horses.

Some of the puppies were surprisingly dark at birth, but still had pink – not chocolate – noses. The authors noted that the color at birth tended to be darker than the mature color, which is also true for Champagne foals. This Japanese Chin shows the kind of deeper coloring that some of the adult dogs in the study were said to have. Although it is not (yet) possible to test for the Cornaz coloring, it would be interesting to test some of the darker Cornaz albinos for Brown.

Fe

One thing that I have found surprising, since the initial post about Angel, is the number of albino-like dogs, and the range of breeds where they have occurred. It is possible that some do share the same mutation as the Dobermans, either due to outcrossing or because the mutation predates the formation of those breeds. Others likely share whatever mutation is responsible for the albino Lhasa Apso. It is also possible that there are still more mutations unrelated to the one in Dobermans and the one in the Asian breeds. With the exception of Pearl, dilutions in horses have so far proven to be dominant, or at least incompletely dominant. Because the diluted colors in dogs are more often recessive, it is far easier for them to hide for generations, especially when they are rare in the population. If these are older dilutions, then it is possible that albinos may appear unexpectedly in different breeds, just as chocolates and blues do.

So what does this all have to do with horses? That’s the topic for the next post.

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