As long-time readers of this blog know, the black coloration seen in North America and Italian wolves and in coyotes originated in domestic dogs. Indeed, the black coloration in North American wolves originated from a single introgression of a black domestic dog in the Northwest Territories or the Yukon between 1,598 and 7,248 years ago. Of course, we now know that there is significant gene flow between dogs and certain populations of gray wolf and that this gene flow has been going on for some time.
I have often wondered about color genetics and gene flow between species. One species that is particularly beguiling for speculation for me was always the origin of melanism in Eastern gray squirrels. Melanistic Eastern gray squirrels are more common in Ontario, Quebec, and Michigan, but there are isolated populations south of these locations.
A new paper just published in BMC Biology revealed that melanism in Eastern gray squirrels most likely had its origins from hybridization with the fox squirrel.
Melanism has evolved twice in fox squirrels. The melanistic ones in the Southeast have a mutation called ASIP A3. Melanistic Western fox squirrels have a mutation that causes a deletion in the MC1R. This allele is called MC1R∆24.
What is interesting is that melanistic Eastern gray squirrel have the same mutation.
The authors contend that the most likely explanation for this shared mutation is hybridization between fox and Eastern gray squirrels, although ancestral polymorphisms and earlier hybridization between gray squirrels and fox squirrels cannot be ruled out as possible origins either. However, The authors think it originated in fox squirrels because it resembles other fox squirrel MC1R haplotypes.
This finding is pretty interesting because I live where both species are common, and I use to live where there were lots of black gray squirrels. I had read accounts of fox squirrels mating with gray ones, but the accounts I read said that no offspring resulted from the mating.
I assumed that the two species could not hybridize, and I still have not seen any literature that even suggests hybridization could occur until I read this paper.
More work is going to be needed to see exactly how this mutation originated and if there are other traits that originated in one species that now are found in the other.
And yes, there is that old wives’ tail that says that gray squirrels castrate fox squirrels to reduce the competition. What actually happens is that when squirrels are hunted in the early part of the season, the testicles shrink in size, so that they appear to have been castrated.
But I have never heard of these two species hybridizing. Indeed, it may be that the hybridization that transferred that particular mutation onto Eastern gray squirrels happened far back in the evolutionary history of both species, when they were still chemically interfertile.
However, they might still be able to hybridize. It is just that no one has ever documented a true hybrid between the two species.
But I am certainly open to the possibility.
So it is likely that black gray squirrels resulted from introgression, just as black wolves and coyotes do.