Archive | August, 2011

Fate of the Abaco Barbs

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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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What are you hiding?

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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.)

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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.

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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.

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Appaloosa roaning and pintaloosas

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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 – probably some kind of splash white.

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

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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.

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