TORONTO — Dogs come in all shapes, sizes and colours. A new study has shed some light on certain canine colour patterns, including the discovery that one of these patterns originated in a now-extinct dog-like species millions of years ago.
The study, published this week by the journal Nature Ecology & Evolution, revealed structural variants that control expression of the agouti signalling protein (ASIP) gene at two separate locations to produce five distinctive dog colour patterns. These patterns were labelled dominant yellow, shaded yellow, agouti (think greyish; salt and pepper), black saddle and black back, and they occur in hundreds of dog breeds.
What surprised researchers, however, was the origin of one of these patterns. They discovered that the genetic combination for dominant yellow is shared with arctic white wolves, and based on phylogenetic analysis, originated from an extinct canid that existed more than two million years ago. Meanwhile, dogs are thought to have been domesticated only 30,000 years ago.
“While we think about all this variation in coat colour among dogs, some of it happened long before ‘dogs’ were dogs,” Danika Bannasch, the Maxine Adler endowed chair in genetics at the University of California, Davis, School of Veterinary Medicine, said in a news release. “The genetics turn out to be a lot more interesting because they tell us something about canid evolution.”
Lighter coat colours might have given an edge to the extinct canid ancestor in an arctic environment during glaciation periods 1.5 to two million years ago, according to the researchers. Natural selection would have caused that coat pattern to persist as these animals would eventually give rise to dogs and wolves.
“We were initially surprised to discover that white wolves and yellow dogs have an almost identical ASIP DNA configuration,” Chris Kaelin of the HudsonAlpha Institute for Biotechnology in Alabama said. “But we were even more surprised when it turned out that a specific DNA configuration is more than two million years old, prior to the emergence of modern wolves as a species.”
Wolves and dogs can make two different types of pigments, a black one called eumelanin and a yellow one called pheomelanin. The production of these two pigments at different times and places on the body can result in very different coat colour patterns. The production of yellow pigment is controlled by the agouti signalling protein, which is produced by the ASIP gene.
No single genetic mutation could account for the five colour patterns, according to the study. Dogs need mutations in two areas of the ASIP gene to get different coat patterns.
The black back pattern, for example, was identified in a dog sample that was 9,500 years old, showing that the rich variation in dog coat colours was present in the earliest canine companions.