Archive | August, 2014

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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Some new research into blue eyes and KIT mutations

BlueNAmber1
A tobiano pony with partially blue and partially gold eyes (cause unknown)

A year ago I posted expressing my doubts about the theory that blue-eyed horses with KIT mutations (tobiano, sabino and the white spotting patterns) must carry an additional mutation to account for their eye color. At the time, I did not think that there was enough evidence to merit the absolute terms that were often used regarding this theory. It is also true that, having tracked so many instances of blue eyes in the patterns in question, I thought the weight of probability favored the theory that the blue eyes were part of at least some of these patterns and not always a separate mutation. At the time I wrote:

The common theory in horses is that these W-series horses must have a splash mutation as well. And they may. Certainly there are far more mutations for white patterning than previous expected. I have long thought that the numbers of blue eyes on the dominant whites, particularly among the founder horses (ie., the horse that carried the initial mutation) were just too high for them all to happen to have a splash mutation as well. I did not have an exact number, though – just a sense that it was high. But the new sort gave me a number – six of twenty.

The argument that KIT mutations are “incapable” of producing blue eyes is based on information about how pigment is formed in the eyes of mice. I found the theory difficult to evaluate because the passages referenced by its supporters do not deal directly with eye color in horses, and I simply do not have the deep level of understanding of eye structure necessary to extrapolate beyond the specific subject, which was not the absence of blue eyes but the presence of dark eyes in a particular mouse color.

But perhaps more importantly, I just wasn’t sure that mice and horses were the same in this regard. In fact, I just wasn’t sure that eye color in various mammals might not be different in significant ways, just as other aspects of coloration vary between species. In that same post, I included a picture of what was then a newly-identified KIT mutation in dogs – the panda pattern in German Shepherds. I found the blue eyes on the founding dog particularly compelling because in dogs blue eyes are not generally associated with the common forms of white patterning, despite the fact that the mutations for most of those patterns have been found on MITF, which those of us more familiar with horses think of as the “splashed white” gene.

It seemed to me that these blue-eyed (KIT) German Shepherds, and the more common dark-eyed “irish marked” (MITF) dogs were a pretty good argument that when it came to eye color, there was probably some variation from the mouse model.

BlueNAmber4
The same pony, with one predominantly blue and one
predominantly gold eye (and an occluding spot over his blaze)

Some time after that post was made, I ran across a paper on Blue-Eyed White (BEW) alpacas. In that paper, the blue-eyed white phenotype was linked to the presence of two mutations to KIT. The alpacas, which were uniformly blue-eyed, were compound heterozygotes for two different white patterns (bew1 and bew2), both located at KIT. Since that time I have been able to confirm with one of the researchers that there were no mutations at the sites associated with splashed white in horses that could be correlated with the blue eyes on these alpacas.

Then just this month a paper was published linking the blue-eyed white phenotype in cats – called Dominant White (W) – to a mutation on KIT. In that study, the authors were quite clear about the connection between KIT and blue eyes.

In the population sample, we were also able to examine the correlation between genotype at the W locus and iris color. An individual that is homozygous W is much more likely to have blue iris, exhibiting odds 77.25 times larger than the odds of having blue irises of a genotype other than W/W (p < 0.0001).  An individual that is heterozygous (W/w+) also demonstrates increased odds of having blue iris (OR=4.667), four times larger than the odds of having blue irises of a genotype other than W/w+ (p=0.046) The odds of having blue irises of a wild type individual is 0.

With those two studies seeming to cast considerable doubt on the “Never From KIT” theory, I decided to contact one of the corresponding authors with some questions in hopes of getting a better understanding of this topic and of eye color in general. What I was told was that eye color is most likely a polygenic trait, and that it really does depend on the species, as well as the specifics of each particular mutation to KIT. That could explain why some KIT mutations are more prone blue eyes than others, as well as why there appears to be a higher incidence of blue eyes in homozygotes of some patterns.

BlueNAmber2
On this eye, the blue and gold portions are interspersed in such a way that the colors appear softly blended

BlueNAmber3
On this eye, there are fewer flecks of blue, as well as irregular patches of dark brown

So why does it matter if KIT mutations can produce blue eyes alone, or if they need a splashed white mutation? What purpose does this kind of information serve? The fact is that knowing the cause can help breeders more reliably get the outcome they desire, whether they wish to breed for or select against blue eyes. Likewise, breeders seeking to produce – or avoid – the splashed white phenotype need to know if blue eyes are always significant. If it is possible for some of the other patterns to produce blue eyes independent of a splashed white patterns, then assembling a herd of blue-eyed tobianos in hopes of developing a line of splashed whites is going to ultimately prove frustrating.

It is also true that quite a few blue-eyed horses have come back negative for the known splashed white patterns. It is likely that some of them have as-yet-unidentified splashed patterns. However, if some have blue eyes that are just a less common aspect of a pattern that is already identified, then knowing this could spare their owners time and money spent looking for something that is not really there. So while the subject is quite technical, it really does have a very practical aspect.

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