Silver dilution and eye defects


The recent posts on eye defects in dogs reminds me that I meant to share a recent paper on eye defects in silver horses. The issue of eye defects first came to light within the Rocky Mountain Horse breed, where it was initially called Anterior Segment Dysgenesis (ASD). That name was recently discarded in favor of Multiple Congenital Ocular Anomalies (MCOA).

Since its discovery, questions remained about whether or not MCOA was directly linked to the silver dilution, or if it was a more recent mutation tied to one of the Rocky Mountain founders. The recent study, “Multiple congenital ocular anomalies in Icelandic horses“, tied the issue directly to the silver mutation.

In this study we have shown that the MCOA syndrome is segregating with the PMEL17 mutation in the Icelandic Horse population. This makes the hypothesis that the MCOA mutation has recently arisen unlikely.

The Icelandic population is significant because it has been isolated from other domestic horses since 982 AD. If silver Icelandics have the same problem as silver Rocky Mountain Horses, then it is far more likely that the silver mutation is involved.

One of the things that makes this interesting is that the silver dilution – which is linked to eye defects in horses – occurs in the same location as the merling gene in dogs. Both are  PMEL17 (SILV) mutations. Both also dilute black, but not red, pigment. Those are interesting parallels between two colors that have such a visually different appearance.

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10 Responses to Silver dilution and eye defects

  1. Kristin Berkery February 20, 2012 at 11:53 am #

    I tweeted about this 🙂

  2. Dorothy Robertson February 20, 2012 at 12:01 pm #

    What I know about color genetics in dogs could probably be written on the head of a pin but I have a question about the above statement that merle doesn’t dilute red pigment. I’ve had a couple of red merle Aussies….and there are RED patches and there are PINK patches (and the occasional odd sort of light brown patch…usually smaller and only one or two on a dog…and the blue merles have had these as well). What is making the PINK coloring if not dilution of the red?

    • The Equine Tapestry February 20, 2012 at 2:44 pm #

      I tend to forget how confusing the term “red” is when talking about dogs. Sorry about that!

      In genetics, we typically talk about two kinds of pigment – black (eumelanin) and red (pheomelanin). So when something affects black pigment, for instance, it only changes the eumelanin.

      In some dog breeds – like Aussies – the term red is used for the color other breeds call chocolate or liver. That coloring is actually a form of eumelanin, or black, pigment. That is why merling can be seen on those kinds of Aussies.

      Truly red dogs, from a genetic standpoint, are things lrish Setters and Dachshunds and Boxers. They don’t typically show merling, or the merling is only visible in the areas where the dog does have some black pigmented hair.

      • Dorothy Robertson February 20, 2012 at 4:52 pm #

        Totally confused (getting to be my normal state). I can understand the idea of merle effecting only black pigment (I think) and the use of sable dogs clearly shows that… but they also obviously HAVE black pigment. My red Aussie (a red tri with red body, bright red, not liver toned, with white markings and copper cheeks, legs above the white, eyebrows) had red nose leather, pads, eyerims and yellow eyes. His son, out of a blue merle bitch (sired by red merle, out of black tri), has not a hint of black pigment anywhere…red nose leather, pads etc, one yellow and blue eye and one blue. Are you saying that ALL Aussies are black pigmented even if they appear red including nose leather etc? I mean they aren’t sable to begin with and then merle added and effecting only the black areas or areas that are normally darker/black mixed hair. If red looking Aussies are actually black pigmented then what makes the difference between them and a black Aussie? Are we talking about an agouti that has an effect similar to the wild bay in horses that so restricts the black on the body that they don’t have the dark legs like “normal” bays (but even those still have black manes/tails usually). I’m TOLD that red is recessive in Aussies and that in order to get red one has to breed red x red, red x red carrier or red carrier x red carrier. So far this seems similar to chestnut in horses and seems to be accurate in the few litters of pups that I’ve had. Are there RED Aussies and if so how do they differ from the ones that appear to be red plus merle? Sorry to be so slow on this but I’m having some trouble grasping that my red merle male is actually a black pigmented dog (which I really should be able to get…after all I have a black pigmented perlino stallion and I GET that).

        • Lisa McDonald March 21, 2013 at 1:22 pm #

          What she means is that the red of a red Aussie is a mutation of the black pigment producing a red to reddish brown to liver pigment. Merle acts on that pigment. Sables are black or red dogs that have full intensity black (or dark red) tipping on their haris. The merle effect can be seen on the dark tips of the hairs but not on the diluted part of the shaft. If a sable happens to lack obvious tipping (in collies called “pure for sable”, the merle may not be visible at all.

  3. Kristina Pry February 20, 2012 at 2:25 pm #

    I was going to ask the same thing as Dorothy about sable merles. But my guess is that since sable is technically agouti, the diluted parts are the bands of black pigment on the hairs? Because if you look at a blue, the tan patches created by the tricolor genetics aren’t ever diluted – so my guess would be that the tan patches are truly red pigment while sable is a mixture of black and red bands. Is that correct?

    As for the silver horses, I thought the most current theory was that ASD/MCOA was linked to the silver gene, in that the loci were so closely placed as to almost always be inherited together. I thought it wasn’t actually the silver gene causing the eye defects. But seeing that it’s the same as the merle mutation in dogs… I’m curious now.

    • The Equine Tapestry February 20, 2012 at 2:53 pm #

      I have an older post about sable merles here:

      There is also a post with a sable merle where the patching can be seen in the red. It seems that some rare sables show it in their red areas.

      And I should have been more precise in my wording about the link to silver. I do not know that silver causes the problem, or if it is merely linked. I’d have to read the paper to see if there is some indication of that. To me the important thing was that MCOA was linked to the silver color, and not specifically to the Old Tobe bloodline as some had speculated.

    • The Equine Tapestry February 20, 2012 at 3:06 pm #

      I pulled up the paper to double-check, and this is what it said about the connection:
      “It is still unclear if the MCOA locus and Silver locus are two separate but closely linked loci, or if only one mutation is present with pleiotropic effects, influencing both coat color dilution and ocular development.”
      That was where I thought things stood, but I was paying closer attention to the fact that it was not just Rocky Mountain Horses. 🙂

      • Kristina Pry February 21, 2012 at 12:23 pm #

        Okay, so I read your latest entry and that does clear some things up a bit in regards to dogs.

  4. The Equine Tapestry February 21, 2012 at 9:08 am #

    I am going to put up a post with some pictures to better explain “red” in dogs. I think that might help a bit.