Tag Archives | sabino

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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Reminder – deadline for W20 samples

W20Test1

I wanted to post a quick reminder that the deadline for having your horse included in the upcoming W20 study is this Tuesday, April 15, 2014. As mentioned in the previous post, the W20 mutation is one of the newly identified White-spotting (W) patterns. The majority of these patterns have been new mutations that originated in specific families of horses within the last few decades. The W20 mutation is different in that it has been found in a number of unrelated breeds (a current list can be found here), which suggests that the pattern is old. That means it may be widespread in the equine population.

Unlike the other patterns in the W series, this one does not put a lot of white on the horse – at least not unless it is paired with another pattern. The exact range of expression is not known, however, nor is it clear how it interacts with some of the other forms of white patterning. To better understand that, one of the research groups in Germany is offering the W20 test for a brief time. The idea is to both collect a range of samples and raise the funds necessary to do the work. (Horses, unfortunately, do not tend to attract third party research grants, so the biggest impediment to progress is lack of funding.)

W20Test2

Dr. Rony Jude, one of the scientists involved in this project, has indicated that the horses submitted need not be pintos, or even have obviously flashy markings. Horses like the two included in this post would be helpful. I have submitted my own Appaloosa mare, who has only a moderate blaze and a white hind pastern. Groups of related horses are also helpful. Other good candidates are horses that have known, tested patterns but that have more white than might be expected.

The (English-language) form needed to have your horse included is located here. Owners in the United States will need to send their samples in now in order to meet the deadline. For those that have not done genetic testing before, collecting samples involves pulling 30 tail hairs (the root bulb must be visible). For this study, each horse needs two samples: two ziplock bags of 30 hairs. I would also recommend sending the materials in a padded envelope – not a package – to avoid delays in customs. The form indicates that you need to submit three photos (both sides and a face shot showing markings) and a pedigree. Pedigrees are useful for finding possible sources for the mutation, but they are not required for inclusion in the study; you can submit unregistered horses. Dr. Jude has also set up a PayPal account (rjude-consulting@hotmail.com) for participants in the United States, where foreign bank transfers tend to be costly. The necessary photos and the form can also be sent to that address, which is a good idea since we are so close to the deadline.

I hope that Dr. Jude and her team get a good turnout for the study. With luck, W20 could prove to be one of the bigger pieces of the puzzle when it comes to understanding how white markings and white patterns work in horses.

SprinklesTest

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The W20 Project

FlashyWhiteClyde

I mentioned in the last post that the known White-spotting (W) mutations tended to fall into three basic categories, at least in terms of appearance: white, sabino, and white markings. At the moment, there is only one mutation in that last category, known as W20. The paper that documented the pattern, published just last year, had this to say about it:

We previously discovered a missense variant… but did not immediately recognize its functional role… With sequence and phenotype data of more than 200 horses, we now realized that this variant actually appears to have a subtle effect on pigmentation and consequently termed it W20. This variant is common and segregates in many horse breeds.

The paper goes on to explain that of the 145 horses tested, 52 had the W20 allele. According to the supplementary material, the breeds where it was found included:

Appaloosa
Clydesdale
Franches-Montagne
German Riding Pony
German Warmblood
Gypsy Horse
Marwari
Morgan
Noriker
Old Tori
Oldenburg
Quarter Horse
Paint Horse
Paso Peruano
South German Draft
Thoroughbred
Welsh Pony

That is an impressively broad range of breeds. Along with the Thoroughbred and warmblood breeds, there is a mountain and moorland pony (Welsh), a number of drafts including one of the older ones (Noriker), a colonial Spanish breed (Paso Peruano), and even the Marwari. Taking that into account, and then looking at the known examples of W20, it is tempting to wonder if this might in fact be the mutation most often responsible for the common “flashy white” type of sabino. Those are the sabinos with blazes, stockings and perhaps some white on the belly. It is also the type of pattern thought to amplify the white in other patterns, producing what is sometimes called “sabino boost”.

FlashyWhitePaint

When the Sabino1 test became available, there was some disappointment that most horses thought of as sabino tested negative. Unlike the Splashed White patterns, which have one common and widespread mutation (SW1), there has not yet been a widespread testable form of sabino patterning. Looking at the list of breeds above, it does seem that if W20 is not the “flashy white” sabino gene, it is at least one of them. If that is the case, garden variety sabinos like the Shire at the top of this post, or the Paint Horse above and Arabian below, might carry it.

ArabSabino

There are still a lot of questions about W20. The original paper describes horses with the mutation as having “extended white markings”. From photographs of the known individuals with the mutation, it appears that homozygous W20 horses are more likely to have belly spots, but perhaps not much more than that. That does fit with what is known about breeding programs that center around horses with flashy white markings, and the relative rarity of loudly marked individuals in those lines. It is also clear that W20 interacts with at least one other White-spotting pattern (W5) to produce an all-white horse. What is not known is what it does with other patterns, both other White-spotting variants and unrelated white patterns. If the mutation is present in the Clydesdale, for instance, then what makes white-born foals so rare there? Does it play a role in horses with more extensive patterning or roaning? What is the maximum (and minimum) expression for W20, both when heterozygous and when homozygous?

ShewofGoldGF
Shew of Gold GF (W5) with her white foal Supernatural (W5 and W20)

These kinds of questions are important. For breeders, a better understanding of the way these patterns interact would make it easier to produce the “blaze and white socks” image that many find appealing, without getting excessive white on the body. Likewise a breeder of pintos might need to know how to ensure that enough white fell on the body to obtain regular papers.

That is why I am encouraging anyone interested in pinto patterning to participate in the W20 Project. The project is to raise funds as well as collect samples for a more comprehensive study of the W20 mutation. To participate, owners pay $38 – a fee lower than most commercial tests – and submit 2 samples (30 hairs each) for each horse they want tested. Participants will also need to send three pictures (both sides and a head shot showing any markings) and a copy of the horse’s pedigree. Horses with white patterning as well as those without are wanted. Results will be available in June of this year. More information, including information on where to send samples, can be found by clicking the link below, which takes you to the W20 Project page. There is, however, only a brief window for getting involved as all funds and samples must be submitted by April 15, 2014.

Instructions for Participating in the W20 Study

[Thank you, Jackie, for letting me know that the lovely heavy horse at the top of the post was actually a Shire, and not a Clydesdale!]

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