When I’m in the US, I live in the Wild West, and that means rabbits. Where there are rabbits, there are probably man-eating rabbits, and I hate them. So, the chart explaining rabbit coat coloration that you see above intrigued me–to survive the man-eating rabbits, you must be able to spot them, and you can’t always rely on seeing their long, sinister ears protruding from the grass, so you need to know their coat colors. But, how do those particular genes explain the devilishly sly diversity of color and pattern that you see in the illustration?
For context, let me give you the rundown (as I understand it–bear in mind that I’m a linguist, not a geneticist) on Labrador retrievers:
- Labs come in three colors: black, “chocolate,” and yellow.
- Which color they are is determined by two genes.
- One gene determines whether your hair is black or “chocolate.”
- The other gene determines whether or not your hair has any pigment (think of pigment as the molecule that actually has the color) at all.
- If you have the form of the gene (the “allele”) that allows your hair to have a color, then you will be either black or “chocolate” (assuming that you are a Labrador retriever).
- If you have the form of the gene (the “allele”) that keeps your hair from having any pigment at all, then regardless of which form of the black-versus-chocolate gene you have, you will be yellow–yellow being what a Labrador retriever hair looks like if it doesn’t have any pigment deposited therein.
My point being: you don’t actually need to have a large amount of genetic variability to get a large amount of “phenotypic” variability (in this case, variability in appearance)–actually, very few things are affected by a single gene. Rather, most traits are affected by a combination of a number of different genes.
OK, so: how do those rabbits come about? They differ not just in their colors, but in the pattern of those colors. Here’s a reasonable guess.
The odd data point in that graphic is the Himalayan. Everybody else is monochrome, but the Himalayan has a color difference between his (I’m pretty sure that rabbits are generically male, probably due to the known viciousness of the man-eating variety–le lapin anthropophage in French, el conejo antropófago in Spanish, Lepus anthropophagos in Latin, I think, but I couldn’t swear to it) extremities and his…well, everything else.
You’ve seen that pattern before–in Siamese cats, for instance. My understanding is that the distribution–lighter towards the center, darker at the extremities–is related to reduced blood flow in said extremities. The reduced blood flow gives you a reduced temperature, and that has some effect or another on the deposition of pigment. (As I said, don’t quote me on this–I’m a linguist, not a Siamese cat expert.) Looking at the rabbit that way, you wonder: OK, dark on the extremities and light on the rest, but which dark? Which light? Why doesn’t the rabbit have the same colors as a Siamese cat, for instance? (Think of the evolutionary advantage for a rabbit who looked like a cat–it would be soooo much easier to get humans to take you in, in which case if you were the man-eating variety of rabbit, you could just gobble those overly-trusting humans right down.)
I went digging around for evidence for this explanation for the coloration patterns in Siamese cats. I found a few papers on a group of related temperature-sensitive tyrosinase mutations that are associated with eye color differences in a range of Siamese cats and Himalayan mice and a rare mink discovered on a ranch in Nova Scotia–and with albinism in humans. (As an albino, your likelihood of going blind due to a lack of protective pigment in the iris and the retina is high–and that’s why we spend your tax dollars on studies of Himalayan mice.) I found a paper on a temperature-sensitive tyrosinase mutation in a human with the following: white hair in the warmer areas (scalp and axilla) and progressively darker hair in the cooler areas (extremities) of her body. I haven’t tracked it down to the fur color question in Siamese cats, though. Still think I just make this shit up? Here’s the paper on the mink found on the ranch in Nova Scotia. I mean, yeah, I make up the zombies and the man-eating rabbits–but, the rest of the stuff is “for reals,” as the kids say.
Look to the left, look to the right: if the colors in the figure are true to life, the Himalayan rabbit extremities are the color of the rabbit to the left, while the center is the color of the rabbit to the right. (I am cursed to always remember a scene from an autobiography that I read when I was a kid. The author has been arrested by the NKVD and finds himself in their notorious Lubyanka prison. Whenever a prisoner is taken from one room to another, the machine-gun-toting guards intone step to the left, step to the right: attempt to escape. The NKVD were murderous fuckers, and the threat was entirely believable. Hence: look to the left, look to the right.) Likely cause of the pattern of the Himalayan: temperature-dependent pigment deposition gradient of whatever pigment the chinchilla and albino rabbits have or do not have.
Yes, I have been known to spend my Saturday mornings looking for scientific literature on the topic of pigmentation deposition in Siamese cats when I could have been taking a walk in the beautiful fall weather. This is probably related to why I get divorced so often. French notes below–no English notes today.
le dépôt: deposition, in the sense of deposition of a substance. This seems to be what would be used to talk about pigment deposition. For example: La synthèse et le dépôt de mélanines continuent jusqu’à ce que la structure interne ne soit plus visible, on parle alors de mélanosome de stade IV. (biologiedelapeau.fr)
le gisement: deposit, in the sense of a deposit of minerals, of archeological finds, and the like. I haven’t been able to find any examples of it being used in a medical or biological context to refer to deposition of pigments in the skin.