The silver dilution was mistakenly believed to have originated in the American Shetland Pony, but the mutation is much older.
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.)
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.
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.
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.