August 2011


Hurricane Irene, which hit the Abaco Islands a few days ago, had many people wondering about the fate of the critically endangered Abaco Barbs. The good news is that the three mares that are kept on the preserve are apparently fine. To my knowledge nothing is known about the fate of the stallions, but that has been true for a long time. There are at most two, and they are not kept on the preserve with the mares. The last sighting was of the 22 year-old stallion Hadar, made in February of this year. The second stallion, the splash overo Capella (made famous by Breyer), has only been sighted twice in the last five years. A flyover of the island in 2010 did not turn him up.

Even without a hurricane, though, the future of this breeding group appears grim. No foals have been born since 1998, and the three remaining mares are believed to be infertile, possibly due to exposure to pesticides. But even if they were fertile, and the two stallions still alive and fertile, their small numbers would create dangerous levels of inbreeding. (Two of the mares are mother and daughter, so there are not even three distinct female lines now.)

From a color perspective, the splash overo pattern is still there by the slimmest of margins. If Capella, a homozygous splash, is still alive, then the prospect of preserving the pattern are much better. If not, probably the only source is the heterozygous splash mare Nunki. She looks very typical of splash carriers in breeds without sabino. The other two mares, Acamar and her daughter Alnitak, are plain bays. Acamar does have a star, and some splash carriers are pretty cryptic, but it would be slim hope were she the only one. Acamar and Alnitak do have frosty roaning in their coats, but it is not as pronounced as that on a dark-headed roan.

So the steps that must be taken now is to find one of the five horses still alive and still able to breed. If that can be done, out-crossing is going to have to take place. If experts are consulted, a preservation program can be created that would utilize animals from genetically similar backgrounds. That kind of research will be the subject of a future post, because it is a wonderful tool not only for preserving livestock diversity, but also for shedding light on the true relationships between the different breeds. The Barbs on Abaco may well be a cautionary tale – rather than a success story – about preserving genetically distinct populations, but there are many endangered groups where the outlook is more optimistic and there is still time to take action.

(Pictured is the lighthouse at Hope Town in the Abaco Islands of the Bahamas.)

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The conversation over the last few days regarding pintaloosas reminded me that it might be helpful to talk about epistasis.

Most people are familiar with the idea of dominance. Dominance describes the relationship between versions (alleles) of the same gene. The gene responsible for greying (G), for instance, is dominant to the one for not greying (g). Dominance is often misunderstood to mean prevalent, as in “the dominant color in Morgans is chestnut.” In those cases it is probably a lot less confusing to say a color is predominant in a breed, rather than dominant. All the chestnut Morgans in the world cannot make chestnut a dominant gene!

The term dominance is also misused to describe how genes interact with unrelated genes. Grey is again a good example, because it is probably the most common gene spoken of in this way. It is not unusual to hear “grey is dominant to all the other colors.” It is not; grey is only dominant to not-grey. The relationship grey has to the other colors is what is known as epistasis.

Epistasis describes the situation where the actions of one gene hide the actions of another unrelated gene. Grey is not dominant to the other colors, but it is epistatic. It eventually hides the colors and patterns the horse has. Chestnut is the very bottom in terms of dominance, because it is recessive to the black-based colors. But it is also epistatic, because the gene that controls where the black goes (ie., whether the horse is bay or black) cannot be seen on a chestnut horse. There is no black to show which version a chestnut horse has, so those instructions are hidden. They are, however, still there. That’s why the right chestnut horse, bred to a black horse (recessive to bay), can produce a bay foal. It was carrying the dominant bay gene, hidden by the actions of its recessive red gene.

The mare pictured above is another example of how epistasis can work. She is the chestnut Morgan, Amanda’s Suzie Q. As the link to her web page shows, she carries the silver dilution gene. She was one of the earliest identified silver carriers in Morgans. The color doesn’t show on her because silver dilutes black pigment. Since Suzie doesn’t have black pigment, the effects of the silver gene cannot be seen. Silver is a dominant gene, but it just doesn’t have anything to work with on Suzie. It was visible on her bay foals, though, which is how she came to be identified. (Suzie was also instrumental in disproving the idea that silver at least lightened the manes and tails of a chestnut, since she has a self-colored mane and tail.)

A very similar situation exists with the cream dilution. Just as silver only dilutes black pigment, cream only dilutes red pigment. Here the epistatic color is black, because there is no red pigment to dilute. This horse is a black Foxtrotter named Quick Trigger. He carries the cream gene hidden by his black coat. Cream is not hidden because it is recessive, but because the genes that made Trigger black set up a situation where the cream could not be seen.

Or perhaps a better statement would be “could not easily be seen”. In many cases epistatic relationships, while they hide the actions of a gene, don’t necessarily make it impossible to see the effects. Sometimes they just make it pretty difficult, or difficult to be sure. Some blacks with the cream gene look more faded than those without it, for instance. Unfortunately for people wanting to identify them visually, though, quite a few blacks without cream fade pretty badly.

That is what was happening with the pintaloosas and the grey appaloosas. Generally the more white the horse has, the harder the individual patterns are to identify. We can guess, based on what is found in a given breed, and what traits are most typical of this or that pattern, but without tests it can be hard to be sure.

Here are some shots of the horse used in the post to illustrate the difference between cremello and truly white skin.

Those were his colored areas, while the rest of him was white. (I was never in a position to get a good shot of his whole body, unfortunately.) He was also a rescue horse, so nothing much was known of his background.

You could overlay a typical tobiano pattern on a horse like this and not see it. Does that mean he is a tobiano? Not necessarily, since you can layer mutiple overo patterns and get that much white. There isn’t any way to know without testing. Whenever people breed a lot of different color genes together, things tend to get muddied like this. It often makes for very cool looking horses, like the ones in the last few posts, but it sure can make it hard to be sure what genes they carry.

Christine Sutcliffe shared this guy in the comments section of the previous post, and I wanted to post him here where he’ll be more likely to be seen. He carries the tobiano pattern, as did the others, along with varnish roan (leopard complex) and one or more of the overo patterning genes.

I say that because he has a broad blaze and two blue eyes.

Horses like this one often get misidentified as grey tobianos, especially if the version of their appaloosa pattern lacks spots, or if their tobiano pattern hides the area that would have shown the spots. It is an easy mistake to make, because varnish roans turn whiter over time much like a grey. (I’ve noticed that as my own black near-leopard mare has aged, more people call her a “grey appaloosa”.)

Some greys lose pigment on their faces, which can also confuse the issue. I suspect that is why varnish roan (leopard complex) has remained in some grey breeds (or strains in breeds) even when appaloosa patterns are not considered desirable or are outright banned. If true greys can develop mottled skin with age, then the facial mottling that develops from varnish pattern might not throw up warning flags.

What sets the progressive whitening of varnish roan apart from grey is that it leaves the spots. The dark spots on this fellow’s rump will remain, no matter how much paler his body becomes. It is thought that all appaloosas roan out, sooner or later. That only applies to the body color, though. When grey is added to the mix, it all lightens. The really loud appaloosa Friesian cross Mystic Warrior is a well-known example of this. This link shows his current appearance along with pictures of the loud black leopard he was as a foal.

Because grey eventually erases the spots in a way that varnish roan will not, it is often considered undesirable by appaloosa breeders. Contrast is often the name of the game in breeding for attractive appaloosa patterns, and grey removes it. That’s also why leopards have traditionally been so sought out by breeders; theirs is the pattern that keeps its contrast. Blanket patterns eventually look a lot more like varnish roans over time.

What is interesting about grey and appaloosa, though, is that before it takes the spots away, it tends to skew them. The angled spots on Mystic Warrior show that really well. On appaloosas with dark areas, like those with blanket patterns, it often adds dramatic white spotting that looks like a cross between marbling and dappling. For those that have the most recent edition of the Sponenberg book, there is a part-Arabian with this type of effect. (He is also pictured in the German book Pferde aus Licht und Schatten.)  It seems that not all grey appaloosas get altered in these ways, but it is common enough these are good clues that grey is there.

When it comes to how the different patterns interact, tobiano could be called the top dog. Pretty much no matter what else it gets paired with, the end result still looks pretty much like a tobiano. Sometimes the other patterns add new areas of white, like this tovero here with the bald face and white ear, but visually it is still pretty easy to identify the horse as carrying tobiano.

Here is an pony with both the appaloosa and tobiano patterns. Notice how the white areas from the tobiano just overlay the leopard pattern.

The lighting for that picture was just right to show the tobiano markings. In bright light, it would be possible to miss it. The outline is also lost as the pattern travels up his hindquarter, when it meets what would likely be the pink-skinned area on a leopard.

Here is tobiano overlapping dark-headed frosty roan.

Tobiano even stays intact when inherited by zebra hybrids. (Photo from Wikipedia Commons.)

It is tamped down and made more minimal in donkey crosses, but it is still quite obviously tobiano. (Photo by Amanda Slater.)

Which brings me back to the discussion about white Miniatures from a few days ago. I truly did not think the colt in question was a Dominant White, but rather a tobiano that was rapidly greying out. As a young foal, he looked like a chestnut tobiano. It did lead to the question, though, about what Dominant White might look like paired with tobiano. Would it overlap the pigmented areas (few though they might be in many cases), much like it did with the leopard above? Or would the instructions to make the horse white override the tobiano patterning altogether?

I suspect that the answer lies in the way the two patterns function at the molecular level. I enjoy reading papers about that aspect of genetics, but in many ways that is above my pay grade. As an artist, I am at heart someone who understands the nuances of phenotype (that is, how the horse looks) far more thoroughly than I understand the underlying mechanics. I will need to wait until someone crosses a Dominant White (particularly one of the families that tends towards the “leaky” variety rather than the all-white) with a tobiano to find out.

 

A short time after the posts about Dominant White went out, I was contacted by someone about a family of seemingly white Miniatures. The stallion was advertised as a maximum white sabino, which is often how Dominant White horses are described. The writer wanted to know if I thought the horse was a White or a Sabino. She thought they might be Whites because of this quote from the Wikipedia entry about the color.

Horses with the W3 allele often retain interspersed flecks or regions of pigmented skin and hair, which may fade with time.

That quote was in reference to the third identified family of Dominant Whites, which began with the mutation in the Arabian stallion R Khasper. A few of the other Dominant White families – but not all – have this same tendency. Photos of the Freiberger horses in the original white study (W1, the Cigale family) show this phenomenon really well.

Here is a Cigale descendant as a foal.

Here he is as a mature horse.

(Both photos are from the original journal article by Haas and Brooks.)

The writer wondered if the same thing was happening with the Miniatures. Looking at the pictures, it was clear that whatever else was going on with the stallion, he was a tobiano because he was throwing a lot of tobiano foals from unmarked mares. The fact that some of those tobiano foals turned white really quickly was why Dominant White was suspected. Because the dark areas of their tobiano patterns looked to have uniformly dark skin, my own suspicion was that the foals at least were not White, but early greys.

Andrea Caudill sent the picture at the top of this post, and it is perhaps helpful in this case. The colt is two years old, and almost entirely white grey. His legs are muddy in the photos, but Andrea says they were also white. As a thin-coated race horse in what is probably a wet environment, it’s easy to see his dark skin. In my experience, it can be much harder to tell white greys with extensive markings or facial depigmentation from truly white horses when they are dry and have denser coats.

Some horses, like this colt, do grey really early. Famous white grey breeds like the Lipizzans and Kladrubers have been bred specifically for early and thorough greying. Conversely, breeds like the Percheron have been bred for later greying, so it would appear that greying speed can be manipulated by selective breeding. Perhaps even more interesting, and relevant to the situation with the Miniatures, is that early studies on the silver gene mentioned that pairing silver (Z) with grey (G) produced really rapid greying. I do not believe this was studied in-depth, but it is true that a number of Shetland breeders in the mid-twentieth century were attempting to breed “white” ponies that were in fact early greys. That might be what was happening with the Miniatures in question.

I am often amused by horse books – often those about colored horses – that point to old art work as proof that this or that color has a long history. Perhaps because I am an artist working in a field where success is often measured by the accuracy with which color can be rendered, I am all too familiar with how this can go wrong. It is a lot easier to be sure that the Pech Merle cave paintings portray appaloosas when you haven’t looked at many artist’s first attempts at painting a dapple grey!

I thought of this again when I was in Germany and came upon these illustrations from a 13th century book of love poetry. Why such pictures were hanging in the Medieval Torture Museum, right along with the comfy chair, I cannot guess!

I suspect that the artists intended to depict dapple greys with most of these polka-dotted horses. Certainly that seems the most logical explanation for the blue ones at least. Just what the yellow ochre pony with the white dots is supposed to be is open to speculation. Is he a dappled palomino? Some kind of brownish horse turning grey? Or were dapples the crazy all those centuries ago, just as they once were with equine collectibles, so that they were added to just about any body color whenever the horse needed to be made more special? Were there thirteenth century equine art fans bemoaning the fact that the illuminators just couldn’t leave a plain color alone?

Maybe that could explain the white dots on the very red horse in the background of this illustration, as well as those on the gray (mostly covered with trappings) and the taupe pony.

These pictures illustrate the problem with using art to assess horse color in distant times. Our modern perspective makes it tempting to say that the taupe pony is evidence that silver dapple must have existed in Germany at the time. It is a color we associate with ponies, and he does look small relative to his rider. Unlike the other horse in the same series, he is taupe and not golden brown. Yet he matches the brown on the ground, just as the previous one matches the falcons. It might mean nothing. Likewise the size of the rider relative to the horse might not be intentional, or might be used to indicate something other than the size of his mount. We can only guess.

All artists take liberties, and most are influenced by the conventions adopted by their colleagues. This can skew an entire era of work in such a way that it distorts reality. (When it was still published, The Boat has a wonderful set of articles on painting conventions prevalent in equine collectibles. Excerpts can be seen here, here and here.) A good example of this in historical equine art is the seventeenth and eighteenth century depiction of horse form.

Cerbero by Johann Georg von Hamilton, 1725

Artists of that era tend to distort horses in a fairly predictable way. The accuracy and detail with which the color is rendered, however, is striking. (Note the cat tracks and the isolated roan patch on Cerbero, for instance.)

Yet even when horse color is rendered with such precision, it can be hard to know what liberties were taken – or what gaps in knowledge had to be filled in. That brings us back to yesterday’s post on the Royal Creams. Artists frequently painted the Creams. A print of this one hangs in the foyer of my home.

Adonis by James Ward, 1826

Adonis is usually referred to as a Hanover White, which were bred alongside the Creams in the early days of the Royal Stud. In the painting he has blue-green eyes. Was that accurate? Part of why the nature of the dilution carried by the Creams has remained a mystery is that there are so many conflicting stories about their eye color, so the eye color on a horse like Adonis is particularly interesting. Did the artist know the eye color? The painting was made six years after the death of his owner, King George III, so it is possible that he was only working from descriptions. Maybe the color wasn’t important to anyone, and the artist just liked how the blue picked up the tones in the background.

Beauty, another of King George III's horses

Is that why this Cream, another stallion owned by King George III, has golden eyes? Certainly the artist took liberties with the size of the eyes, as many artists did at the time. Could he have done the same with the color?

As much as I love historical artwork, and find myself searching it for clues about historical colors, it is not an absolutely reliable witness.

For the most part the upcoming book is about current breeds, but there were a few extinct ones that I couldn’t resist including because they were interesting in terms of color. One of those was the Royal Creams of Hanover. They are interesting because they were a remnant of the horses bred in the pre-stud book era, and also because their coloring remains a mystery. Were they double-diluted creams, champagnes or pearls – or something else entirely? I compiled as many accounts (and images) as I could and made some guesses in the book, but in fact no one really knows what they were. I suppose that is part of their allure for color researchers!

Recently I found this photograph of the horses in a Library of Congress archive. It was shot in London in June of 1911, so this would have been just shortly before the Crown Equerry determined that the remaining horses should be disbanded. The last of the Creams were dispersed in 1921, just ten years after the photo was taken. It was too late to include it in the section on the Creams, but I can share it here.

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