Show Me The Science Month Day 16

Black wolves look like creatures out of frightening fairy tales, but their black color actually came from pet dogs. Today's evolution paper is about a potentially beneficial mutation for black coat color picked up by wolves as the result of interbreeding with dogs. This story got some press, so it may sound familiar, but here we're going to focus on just how scientists could know where the black color gene came from. This research is a great example of the genetic sleuthing that's now possible with easy, affordable DNA sequencing.

The basic story is this: while plenty of dog breeds have black hair, wolves generally don't. Black wolves are rare, and found almost exclusively in North America. Since wolves all around the world shared a recent common ancestor, the fact that black wolves are mostly limited to North America suggests that gene variant causing the black coat color was only recently introduced among the wolf population. What's more, the black coat color seems to be beneficial in some environments: black wolves are more common in forest populations of wolves, and unusual in tundra populations. It's possible that black provides better camouflage in the forest, and thus provides a survival advantage to forest wolves.

The black coat color is a genetic mystery: when was the mutation for black hair introduced into the wolf gene pool, and is that mutation providing an evolutionary advantage? A recent report published online in Science followed up on the clues.

How did scientists track down the source of black hair in wolves? The researchers already knew about a genetic variant in dogs that causes black hair (we'll call that variant capital K). Dogs with black hair have at least one copy of the K variant, inherited from at least one parent, while dogs without black hair have the lower case k variant of this gene. (To be technical, they have two copies of the k variant, one copy from each parent.)

What about wolves? Do black wolves have this K variant? The researchers checked dozens of wolves, and sure enough, black wolves had the same genetic variant causing black hair in dogs, while gray wolves didn't. They went so far as to check a family of wolves in Yellowstone Park, and found the same result. All black wolves in the family had the K variant, while the rest had the k variant, as you can see in this three-generation pedigree chart (in this chart, ∆G is the K variant, + is the k variant):

Figure 1C from Anderson, et al. Circles represent females, squares males.

These pedigree results (along with data collected from other North American wolf populations) are strong evidence that the K variant is responsible for black coat color.

So what is this K variant and where did it come from? This particular variant is a mutated copy of a gene called CBD103. The K version has a small, 3-letter deletion in the DNA that is not found in the k version. That deletion mucks up the molecular pathway controlling pigment production; the result is overproduction of black pigment.

This mutation, a unique molecular accident, occurred in a single individual dog (probably tens of thousands of years ago, the researchers estimate), and spread through the world as various dog breeds intermixed. The scientists suspected that this mutation then entered the North American wolf population as domesticated dogs occasionally bred with wolves. To test this idea, they examined the pattern of DNA changes around the CBD103 gene. In the k version of this gene, you could, by looking at the DNA sequence, see that k variants in dogs were more similar to each other than they were to k variants in wolves, as one would expect, because dogs are more closely related to each other than they are to wolves.

But the K variant is different. Wolf versions and dog versions are all equally (and closely) related. Wolves may be more closely related to each other than they are to dogs, but that doesn't hold true for this particular genetic variant - a clear sign that K recently hopped from dogs into wolves.

The molecular details also suggest that K isn't just an evolutionary accident; it looks like it has had survival advantage. We can know this because the K variant is extremely uniform in its DNA sequence, given how common it is in wolves. Generally, when a version of a gene becomes common in a population, it picks up some diversity along the way, small mutations here and there that are hard to keep out when there are so many copies of the gene floating around in the gene pool. When a particular version of a gene quickly spreads through a population, without picking up some diversity along the way, that's a strong sign that natural selection has been at work. In this case, the molecular evidence for natural selection is also nicely backed up by the geographic patterns of black coat color mentioned before - black coats are more common among forest wolves who live in an environment where black might be helpful.

So some clever molecular sleuthing solved the mystery of black coat genetics in North American wolves, a clear example of how domesticated organisms can influence evolution in the wild.

Join me tomorrow, here at Adaptive Complexity, for day 17 of 30 Days of Evolution Blogging Evolution as a science is alive and well. Each day I will blog about a paper related to evolution published in 2009.

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Front page image by Manfred Heyde, courtesy the Wikimedia Commons.