From Greater Denmark to Little Chihuahua, a leap that has made a huge difference in the size of dogs

From Chihuahua to Great Dean, dogs are different in size than other mammals on Earth. The mutation responsible for this difference has been identified somewhere unexpected: the ancient wolves.

The mutation is found near a gene called IGF1, which researchers identified 15 years ago as an important factor in the difference in the size of domestic dogs. It was the first of about 24 known genes related to dog body size; Attempts to accurately identify the gene variant responsible for it were unsuccessful.

Elnie Ostrander, a geneticist at the US National Institute of Human Genome Research in Maryland, led the 2007 IGF1 determinant of the size of dogs, as well as a new study published Jan. 27 that finally answered that question. had.

“IGF1 has been a thorn in our side,” says Ostrander. Ancient dogs, domesticated from wolves 30,000 years ago, were somewhat different in size. But the current dramatic differences (the largest dog breeds are up to 40 times larger than the smallest breeds) in the last 200 years have been the result of human efforts to introduce new breeds.

Ostrander and colleagues, including Joslan Plasi of INSERM University in Rennes, France, studied the genomes of more than 1,400 dogs in the canine family, including ancient dogs, wolves, coyotes, and 230 modern dog breeds.

Growth control

After comparing the differences in the area around the IGF1 gene and body size in dogs and wild dogs, a variant attracted attention. This variant was found in a piece of DNA that translates non-coding RNA that is responsible for controlling the level of the protein IGF-1 (a strong growth hormone).

Researchers have identified two versions or alleles of this variant. According to Ostrander, among all breeds, dogs with two copies of one of the gods weighed less than 15 kg, while two copies of the other allele were common in dogs weighing more than 25 kg. Dogs that had a copy of each allele were mediocre in size. Dogs with two copies of the large allele also had higher levels of the IGF-1 protein in their blood than species with two copies of the small allele.

Researchers have observed this relationship in other species of the canine family. “It was not just about dogs,” says Ostrander. This was the story of wolves, foxes, coyotes and others. “This is a big issue for the whole family of dogs.”

Tiny ancestors

Researchers think that the allele associated with the microorganism is evolutionarily much older than the large allele. The coyotes, jackals, foxes, and most of the dogs examined bore two copies of the microscopic version, which means that this copy is present in the common ancestor of these animals.

The head of an ancient wolf that lived 30,000 years ago, in the last ice age. This head was discovered in 2018 under the currently melting layers of Siberian icy soil.

It is not clear when the large allele formed. Researchers have found that an ancient wolf that lived in Siberia 53,000 years ago had a copy of this version. Other ancient wolves and modern gray wolves carry two copies of this allele, indicating that large limbs are beneficial to wolves.

According to Robert Wayne, an evolutionary biologist at the University of California, Berkeley, the prevailing view among scientists was that the small limb may be linked to a newer genetic mutation that is unique to domestic dogs. He says:”[یافته‌های جدید] It completely changed the story. “It’s amazing.”

According to Elnor Carlson, a geneticist at Chan University School of Medicine in Massachusetts, the study could be a sign that dogs have been domesticated by smaller wolves than modern gray wolves. “We don’t even know what the wolves that have become dogs today look like,” he says.

Researchers point out that the story of the size of the dogs is not yet complete. Plasi wants to know how variants affect IGF-1 protein levels. Of course, this variant is not the only factor that determines the size of dogs. The IGF1 gene is only responsible for 15% of the differences between the different races.

“We’re not talking about a mutation that made a wolf the size of a Chihuahua,” Carlson said. “We are talking about one of the many mutations that make you a little smaller.”

The study was published in the January 27, 2022 issue of the journal Current Biology.