Archive | November, 2014

Wild colors and the spectrum from all-red to all-black

Caroline

As I mentioned in an earlier post, the relatively small number of alleles in horses gives us a limited window into how pigment-type switches can work. This does make the basic coat colors much easier to explain, but without the bigger picture it is possible to misunderstand some fundamental aspects of animal coloration. The most common misunderstanding that I encounter is the idea that horses are “basically black” or “basically red”. Instead of Extension being one of a class of genes that control the two types of pigment (a pigment-type switch) it becomes the location where the “basic color” (red or black) is determined. This seems logical, I suspect, because there is a natural tendency to assume that anything “basic” should be reducible to one gene. So with this thinking, a horse with the dominant allele at Extension (E) becomes a “black horse”, while one with two copies of the recessive allele (ee) is a “red horse”. Any other color is the result of a “modifier”. 

But Extension is not a special site where some pure, basic color is ultimately determined. It is not the “red or black gene”—at least not in the sense that the black horse/red horse idea posits. It is part of a system where basic pigmentation (red and black) is controlled. Looking at a species that has a wider array of alleles at both Extension and Agouti can make this more obvious, which brings us to dogs. In this post I am going to outline some equivalent colors in dogs, and then follow it with a post that fills in the rest of the picture.

It is important to remember that the starting point for the color of an animal (as opposed to the color of pigment) is the wild color. In horses, that color is believed to be bay. In dogs the wild color is what is commonly called “wolf sable”. That term can be confusing, because in some breeds a genetically different color is also called sable. I have used a photo of my neighbor’s dog Caroline (above) to help highlight the difference between the two. Caroline is a German Shepherd-Rough Collie cross, and she looks quite a lot like a Collie except for her coloring. Obviously she lacks the white pattern associated with that breed, but the pattern of red (yellow) and black hair is also different. Here is my friend Melanie Miller’s Collie, Shelby, showing the other kind of sable. (Her Bully friend Barley probably also has that same gene as Shelby, but we’ll get to that later.)

BarleyNShelby

If you can imagine the difference between the typical “sable” German Shepherd and a sable Collie, you can get the idea of what the wild color in dogs is. Other breeds that are commonly wolf sable are the Keeshond and the Norwegian Elkhound, but I used a picture of a German Shepherd cross because that is a familiar breed that retains the pheomelanic (red/yellow) pigment while many of the other wolf sable breeds have other factors that bleach it out, giving the impression is of a grey and black dog. This coloring is quite similar to that of the domestic dogs’ direct ancestor, the Gray Wolf. This Gray Wolf has retained a good degree of pheomelanic pigment, and looks quite similar to Caroline in color. (Photo by Carlos Delgado, and provided courtesy of Wikipedia Commons.)

grayWolf

Like bay in horses, wolf sable is a two-part genetic recipe. The dog needs to have the production of black pigment enabled at Extension, as well as the allele(s) for wolf sable (aw) at Agouti. As you can see, the original coloring of a dog is not red or black, nor is it something that is controlled by one primary gene. It is the product of a system of pigmentation controlled by a number of genes. Changes to that system alter the placement of the two pigments. They can also alter the proportion of the two pigments, up to and including eliminating one or the other.

Turning the animal all-red 

Dogs also have a mutation to Extension that disables the production of black pigment. Like chestnut in horses, this mutation is recessive. Dogs with this form of Extension (e) do not have black pigment in their coat, so they are all-red (yellow). This is the familiar coloring of Golden Retrievers and yellow Labradors. A recessive all-pheomelanic coloring is common in many domestic animals.

REcRed

Turning the animal all-black 

Dogs also have a recessive black allele at Agouti, just like horses. The difference in dogs is that the gene for recessive black (a) is rare. It is, however, the only form of all-black possible in some breeds, like the Shetland Sheepdog. Most black dogs have a different gene, which I will discuss much later in this series. In dogs that have recessive black, though, it works like the color in horses. The dog must have black pigment enabled at Extension, and then must be homozygous for the recessive black gene (aa) at Agouti. Like recessive red, there are equivalents to this in a number of different species.

Biblack

These all-red and all-black animals do not represent some essential essence of animal coloration. They are not the “basic” or “original” colors of their species. All-red (ee) and all-black (aa) are mutations that effectively turn off the production of one of the two original pigments. It can be helpful to think of them as the outermost limits for alterations to the regulation of the two pigments, with the other alleles sliding the scale to rearrange (and perhaps favor more or less) the two pigments. That arrangement can happen at either Extension or Agouti—or somewhere else entirely. I already mentioned the Melanistic Mask (EM) allele in a previous post, and how it points to the fact that Extension can direct the placement of black pigment as well as simply enable its production. I will expand on that with the next post about some of the other alleles, and hopefully give a more complete picture of what can happen with these two pigment-type switches.

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Translating horses to dogs

DogRed

Before I return to basic dog colors, here is a short guide for translating horse color and dog color. I have posted about this a few times before, but since there is going to be a lot of new material in the next few posts, I thought a refresher on this might eliminate at least some of the potential for confusion.

Yellow = Red

In horses, we talk about two types of melanin pigment: black and red. The latter type, known as pheomelanin, is sometimes referred to as red, yellow or even orange, depending on the species. In horses, we use red. In mice, which are the most common model for research on pigmentation, it is referred to as yellow. This is also true in dogs. Because pheomelanin in horses ranges into the deeper tones—and perhaps because “yellow” was once used in some regions to mean palomino or buckskin—red is the traditional term. These are just different names for the same thing.

Red ≠ Liver or Chocolate

Which brings us to the other area of potential confusion. When horsemen talk about red, they mean pheomelanic pigment. When some dog people speak of red, what they mean is a dog like the one at the top of this post, otherwise known as liver or chocolate. On this blog, this type of coloration in dogs, which is not thought to occur in horses, is referred to by its traditional name of brown while red is reserved for pheomelanin.

Seal brown ≠ Brown (b)

Brown is the other confusing term, because it means something very different in horses than in almost any other species. In other animals, brown is an alternate form of black (eumelanin). The dog at the top of this post is the brown version of tricolor (black, tan and white). In posts on this blog, brown usually means the seal brown coloring of the sort seen on the tobiano mare below.

Browntobi

To avoid confusion, when speaking of the non-equine brown I follow the term with the genetic notation “(b)”. When speaking about both, I either use the genetic notations (b and At) or the more specific equine term “seal brown”. (Dogs also have a different color called seal brown, but I will leave that for another day!)

Agouti vs. agouti

In horses, as in other mammals, Agouti is a locus (a specific location on a chromosome) that is involved in pigment-type switching. In other animals, agouti is also the name given to a specific coloration that involves black and red banded hairs. Horses do not appear to have banded (agouti) hairs, but dogs do. The general convention used by this blog is that when referring to a specific locus, the name is capitalized and italicized (or underlined when in a block quote). When used to mean a pattern of banded hairs, agouti appears in regular lowercase letters. This will be an important distinction later in this conversation, because in dogs some of the alleles at Extension and Agouti produce patterns that have both agouti and non-agouti areas. So Agouti is the locus where a number of the alleles for basic coloring reside, and agouti is a specific type of coloring involving banded hairs.

I am also going to post a quick comparison of the canine equivalents – from a genetic standpoint – to bay, black and chestnut. With luck using that as a common reference point, this will not be quite as confusing.

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The basic (horse) coat colors

BasicColors500
The three basic colors: bay, black and chestnut. Some include seal brown, making it four basic colors.

Since some time has passed since I last posted to the blog, I thought it might be worthwhile to do a quick recap of the basic colors before I ventured back into the more complicated realm of basic colors in dogs. This will also allow me to reframe the discussion of red and black hair in a way that might make the next set of posts a little less confusing.

This series of posts began in response to a common simplification used when explaining horse color, and the misunderstandings that arise from presenting color in this way. That problematic explanation usually goes something like this:

Horses are basically red or black. The genes at Extension determine if the horse is black (“E”) or red (“e”). If a horse is black (“EE” or “Ee”) and ‘has agouti’, then black is modified to bay. If ‘agouti’ is absent, then the horse is just black.

I do understand that the simplicity of this is appealing, but it is not correct. It paints an inaccurate picture of what goes on with color and encourages a number of unnecessary misconceptions. Countering it with an accurate explanation that is still simple is a challenge. The latest series of posts was an attempt to do that, and here is a brief summary. (Note that in these two paragraphs I have boldfaced the distinction between talking about horses and talking about pigments.)

In mammals, there are two types of pigment: red and black. The original coat color of a species, known as the wild color, is typically some combination of both red and black pigment. In horses, the wild color is bay. Cells have the ability to make either of the two pigment types (red or black), and the genes that control them are called “pigment-type switches”.

In horses, like many other mammals, the primary pigment-type switches are Extension and Agouti. The un-mutated (“wild-type”) form of those two genes (“E” and “A”) combine to produce the color we know as bay. The other two basic horse colors, chestnut and black, are the result of mutations to one of those two genes. At Extension, there is a recessive mutation (“e”) that restricts the black pigment to the skin, which means the hair is all-over red. This results in the color we call chestnut. At Agouti, there is a recessive mutation (“a”) that removes all restriction from black pigment, so if the horse can have black hair—that is, if he is not chestnut—he will be all-over black. This is the color we call black. Additionally, it is believed that there may be other variants (alleles) at Agouti that restrict black pigment more or less strongly than either bay (“A”) or black (“a”). These are proposed to be responsible for colors like seal brown and wild bay.

In previous posts in this series I have explained that Extension is the locus that “permits the expression of black pigment” and Agouti is the locus that “determines the location of black pigment, if present.” That is true. All I have done in the paragraph above is turn it around so that the three basic colors are seen, not through what is happening to black pigment, but to what is happening to the normal production of pigment in horses. In this case, that is a mutation (e) that has taken away the ability to make black pigment, and another mutation (a) that has allowed black to express throughout the coat. That is actually what is being identified when you submit a horse for the standard Extension and Agouti tests. The test for Extension is looking for the mutation for chestnut (e) and the test for Agouti is looking for the mutation for black (a). Explaining it in this way makes the tests easier to understand, but it is even more important when we begin to talk about basic dog coloring. That’s because more common black-centric way of explaining the relationship between Extension and Agouti, and the implication that Extension sits at the top of a hierarchy of basic color controls (and that Agouti “modifies” it), does not work as well once you start adding more alleles, or more pigment-type switches.

As the most recent post explained, in dogs (and other animals), the distribution pattern of black and red hair is not solely controlled by the Agouti locus. That is, Agouti is not necessarily “modifying” the outcome at Extension. The Extension locus itself can direct the placement of black pigment, as it does in the Melanistic Mask (EM) allele in dogs. In fact, the relationship between the two loci is even more complex, so that alleles at either (or both) locations can influence the distribution of red and black hair. What’s more, looking at dog color it becomes obvious that a specific “turn on the black” allele is not required in order to have a coat with some portion of black hair. It would be more helpful to think of black hair as being part of the normal coloration that is there unless some mutation occurs that prevents it. For that matter, the same can be said for the presence of red hair. That is because ultimately these variations in basic color are really just mutations to the original wild color—to the original production and distribution of color in the species—which typically has some portion of both colors.

If the construct of “basic color” is understood to be “different distributions of black and red pigment on the animal”, with all-red and all-black simply occupying the extreme ends of that spectrum, and the pigment-type switches as being nothing more than the loci that control the options in that category, that should make some of these dog colors less confusing. That’s important, because after I talk about those, I hope to make a post about the third major pigment-type switch in dogs and what that might tell us about horse color.

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