Archive | December, 2013

More on founders

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Fascination, a son of Trot by Trot’s grandson Chestnut

In the previous post I talked about how novel colors are the result of a mutation that occurs in a single, founding animal. I wanted to use a couple of different colors to illustrate this idea, and talk about what that means for anyone researching color in a given breed. In this post, I’ll talk about the search for the founder of the silver dilution. 

The silver dilution first came to the attention of researchers through the work of the early geneticist W. E. Castle. Castle had authored a number of articles that appeared in the American Shetland Pony Journal, and in 1953 he published the paper “Silver Dapple, A Unique Color Variety Among Shetland Ponies” in the Journal of Heredity. In that article, Castle concludes that silver originated with the American Shetland mare, Trot 31.

The first pony known to have shown this color (doubtless a mutation) was the mare Trot 31, born in 1886, the color being described as “fawn”.

It was reasonable for Dr. Castle to draw the conclusion that Trot was the founder for the mutation, because he had been told that her color had never been seen before, and that only her descendants possessed it. Since a study of her offspring and their production records already proved that the color was dominant, then the logical explanation for its sudden appearance was that she was the source for the mutation. This conclusion was flawed, in part because the interaction between the base colors (black, bay and chestnut) was not well understood. Castle and others had concluded that chestnut was the equivalent to Brown, a recessive version of black in mice and dogs. But there was another problem: the breeder was not entirely truthful about his ponies. In the same article, Dr. Castle quotes Mr. Bunn, the owner of Trot.

[Trot] was the only pony of this color in America and was also a beautiful color… However, every one of the ponies you now see of this color was the produce or descended from Chestnut. His granddam Trot was the only mare ever known to have that color of all the Shetlands ever bred or known.

Castle then adds that those who study genetics “call such abrupt origins mutations”. But Trot was not as unique as the previous quote might suggest. Her breeder had a full brother, Baron Keithsburg, who was also silver. When a horse has a sibling with the same color, that is proof that the color did not originate with them, and attention has to turn to one of the two parents. In the case of Trot and Baron Keithsburg, their sire Jeff was said to be of the same color. But perhaps Mr. Bunn did not lie outright, because Jeff was not actually a Shetland Pony. By multiple accounts, including Mr. Bunn himself, Jeff was an imported Welsh Pony.  (Jeff is discussed briefly in the chapter on Hackneys in Volume I of Equine Tapestry, and more of his story will come up in Volume II, which covers the pony breeds.)

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Chestnut, grandson of the silver “founder” Trot

The article about Trot did prompt at least one reader to write to Dr. Castle to say the color was not unique to Trot or even to Shetlands. She pointed to a half-tone photograph of Skerryvore, a Highland Pony stallion pictured in Lady Wentworth’s Thoroughbred Racing Stock. Images of Skerryvore are fairly easy to find, since he was often held up as a model of “improved” pony type. Those familiar with the dappling on silver dilutes will probably recognize right away, even in a half-tone, that Skerryvore was an ordinary dapple grey. (The image in his entry for allbreedpedigree.com is even more telling of his true color.)

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Skerryvore, Champion of the prestigious Highland and Agricultural Society Show in 1909 and 1911

Perhaps Dr. Castle had further information about other Highland Ponies, because the color was present, if not with Skerryvore, among the ponies of his time. Rhum Laddie, the stallion found in the pedigrees of most modern silver dilute Highlands, would have been a contemporary of Skerryvore. Or perhaps other Shetland breeders had alerted him to the discrepancies in the story he was given about Trot. Either way, Castle published an addendum shortly after the original article, stating that the color probably had older origins. It seems few read this, because the idea that this was a color unique to Shetlands – and to American Shetlands in particular – persisted for decades. In fact, when the first Welsh Pony tested positive for silver, there were vocal accusations that his American breeders must have slipped in American Shetland blood.

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Rhum Mary V, modern silver dapple Highland Pony

That kind of situation, where a color becomes so closely associated with a specific breed, tended to muddy the waters before there were genetic tests. Testing has allowed researchers to determine if a color in one breed is in fact the same color in another. When the same color is found in separate breeds, then barring dishonest pedigree records, that is proof that the mutation dates back to a time before the two groups separated. This fact is used as a way to guess the age of some mutations. If the same mutation is spread widely across a group of breeds with no documented connection, like yesterday’s example with grey, then it is pretty safe to assume the mutation is quite old. This can also be used to guess at the possible origins of a mutation, if it predates recorded pedigrees. When a mutation clusters with a group of related breeds, then it tends to suggest that it arose in the animals that were used to develop those breeds. The pearl dilution, for instance, is suspected to have Spanish origins because so many of the breeds were it is found have Spanish blood. These are still guesses, since it is quite possible for a mutation to occur in one population, spread to a second area as an outcross, while the color in the original area dies out. Still, this type of hypothesis can be useful when trying to guess how likely a horse may carry a given mutation – and how worthwhile it may be to send out hairs for a test.

Using the oldest breed with a positive test for silver, the Icelandic, it can be proven that this particular mutation dates back to at least 982 AD, which was when Iceland banned importation of new horses. There is another avenue to investigate questions about original mutations, however, which has been discussed in previous posts on this blog. It is possible to test ancient remains for mutations. In a study that did just that, the silver mutation was found in Siberian remains dating back to the Iron Age. That date might get pushed back further, if earlier remains with the mutation come to light, but we will likely never know much about the founding horse or pony. The long span of time since he or she lived does explain why the color has spread to so many diverse breeds. Because it often produces a very deep color (something generally preferred in the modern stud book era), and two traits that have wide appeal (dappling and flaxen manes and tails), in many cases it has been spared the kind of selective pressure that has reduced the prevalence of the some of the other dilute colors. In the next post, I’ll talk about splash white, what we can guess about its origins, and how it managed to survive in so many populations.

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Founders and mutants

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It is known that the two horses above share a common ancestor. Although most people would not think of a Shetland Pony and an Arabian as being related in any meaningful way, if we could trace their pedigrees back far enough, there would be at least one common name on both sets of papers. That same name would appear on these Percherons’ pedigrees.

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They are relatives, too. We know that because all four of these horses are grey. That means that somewhere in the pedigree is the horse – one single horse – that carried the original mutation for greying. That is the founder for the trait, and all grey horses trace back at least once to him or her.

This discovery, made by a research team in Sweden in 2008, made headlines. Around the same time, a similar story about a common ancestor for blue-eyed people also attracted attention. I have heard more than one person express surprise about this, and I think that speaks to a common misunderstanding about horse color. There is an assumption that while most horses get their color from a parent, from time to time color mutations repeat themselves. This leads to the idea that there might be an “Arabian grey” which is different from, say, a “Shetland grey”. Yet in most cases, the mutation is a one time event. What’s more, mutations involve so many variables that even when the same gene is mutated, the results are not necessarily the same.

That is why a patterns like Dominant White and Splash White, which do have multiple versions, require different tests for each. Rather than just scanning across a gene to see if something is different, the test is looking for a specific mutation – the exact change to the genetic code. And that exact change began with the founding animal where the mutation first occurred. Unfortunately, the terms mutation and mutant have a negative connotation. They are often used in a derogatory way, particularly among animal breeders who are talking about what they view as a negative trait. But mutants are simply animals that carry some kind of alteration (mutation), whether that change is good, bad or a mix of the two.

The concept of a founder – the original ‘mutant’ animal – is really helpful for breeders who want to determine what colors or patterns their animals might have. With more recent mutations, often the founder is known either by name or at least by breed. If it is known that the second Splash White mutation (SW2) occurred in one American Quarter Horse family, then it can be ruled out for those breeds that have no conceivable connection to the Quarter Horse, and considered highly unlikely for those Quarter Horses not known to be related to the founder. Although the three splash white tests are currently offered as a set, for the other colors knowing something about the founder can prevent the purchase of unnecessary tests.

Founders are also a fascinating subject for people like me who are interested in breed histories. The older mutations, where the founder lived long before the advent of recorded pedigrees, offer a way to look at how different populations spread and how breeds were developed. In those situations, color is sometimes the most visible marker of the connection between what are now separate groups. In the future, I hope to do a series of posts on what is currently known (or not known!) about the founders of the different colors and patterns. But I wanted to put this concept up for the moment, since it will tie in with a number of upcoming posts.

I’d also like to add a quick administrative note. I want to thank the many people who offer stock images through sites like Flickr, Picassa, and DeviantArt. Today’s images came from CitronVert and ransu.kuvat.fi. The ability to include breeds not typically found in the southeastern United States is invaluable to me, so I am always grateful to those who share their photos either directly (as with the Splash White Project) or through stock photo sites. Your generosity with your intellectual property makes this blog better!

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Another minimal classic splash

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Hairicane Lil Bit Rowdy, Miniature Horse mare

Here is another interesting splash white that is homozygous for SW1, sent in by Robin Cole. Unlike the Gotland in the previous post, this mare does have blue eyes, but the white on her face is quite minimal. Like the Gotland, she has four white feet, but all but one leg are very minimally marked. Her pattern looks even more minimal when seen from her other side, probably because the skew in her blaze keeps most of the white away from this side of her face. 

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Although she comes from a line that carries frame (Oh Cisco), she tested negative for that pattern. And while she might look tobiano, especially to readers familiar with how tobiano often skews in Miniatures and Shetlands, neither parent is a tobiano. They do look like heterozygous splashes, and the family is known to produce splash patterns. In fact, this is her full sister.

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So while homozygous SW1 horses are pretty consistent in appearance, minimized patterns can be found. That seems especially true among the breeds – like the Gotlands, Icelandics, Shetlands and Miniatures – that are already known to minimize the tobiano pattern. Here are links to a few interesting examples of minimal homozygous SW1 from those breeds:

Bládís vom Moarschusterhof, an Icelandic mare with what appears to be one dark front leg
Hnísa vom Römerberg, an Icelandic mare, also with a seeming dark foreleg
Unun frá Efri-Úlfsstöðum, an Icelandic mare with an irregular blaze and minimal white
Glæta frá Ártúnum, an Icelandic mare with a large star, large snip and what may be a dark eye
(with each of these links, you can click on the first image to pull up additional images)

Opp, a Gotland mare with a face marking much like the one on the Miniature in this post
She does have a more extensive body pattern, though.

And finally, while it is not a minimized pattern, this Gotland mare Nanna does have an unusual skew to her pattern. Along with minimizing the white, skewing patterns seems to be another (relatively) common aspect in this group of breeds.

I had intended to do a post on blue eyes and negative splash tests, but I think it makes more sense to shift that post to after the ones about white markings since those two things are closely related. I am also going to take a little detour into blue eyes of a different kind, though, before tackling the markings. Watch for that in the next day or two. And if you have not done so already, you can get notifications of new posts to the blog via email by filling out the subscription form in the right hand sidebar. I apologize to long-time readers that had a subscription before and now have to re-subscribe, but that was the one thing that did not transfer with the blog when it was moved to a self-hosted site.

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