Dog breeds

A genetic mutation that helps make ‘toy’ dog breeds so small existed in wolves 54,000 years ago

A genetic mutation partly responsible for the small stature of ‘toy’ dog breeds like Chihuahuas has been discovered in 54,000-year-old wolf DNA, highlighting that the potential for small dogs existed long before humans began to raise them. (Image credit: Shutterstock)

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One of the main genetic mutations responsible for the small size in some dog breeds, such as Pomeranians and Chihuahuas, evolved into dogs long before humans began breeding these miniature companions. The researchers found that the mutation could even be traced back to wolves that lived over 50,000 years ago.

The researchers discovered the mutation, which is in the gene for insulin-like growth factor 1 (IGF1), by studying data collected as part of the National Institutes of Health (NIH) Dog Genome Project, a project of citizen science in which owners collect DNA samples of pet dogs. This “unusual” mutation, found not in the IGF1 gene itself, but rather in the DNA that regulates the expression of this gene, had previously eluded researchers for more than a decade.

After consulting with scientists in England and Germany, the researchers found that the mutation was present in 54,000-year-old DNA from Siberian wolf fossils (Canis lupus campestris), as well as in the DNA of all canid species living today, including jackals, coyotes, and African hunting dogs.

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“It’s as if nature kept it hidden away in its back pocket for tens of thousands of years until it was needed,” said lead author Elaine Ostrander, an NIH geneticist specializing in in dogs. said in a press release. The discovery helps connect what we know about the domestication of dogs and their size, she added.

Unusual mutation

Genes are sections of DNA that serve as a blueprint for building specific proteins. Each gene is made up of a unique combination of four bases – adenine (A), guanine (G), cytosine (C) and thymine (T) – which code for a certain protein. To make a specific protein, cells must unpack double-stranded DNA to read the bases of the strand that contains the corresponding gene. Special machinery inside the cell then copies the DNA and creates RNA – a single-stranded molecule similar to DNA with a different sugar (ribose instead of deoxyribose) and the base uracil (U) instead of thymine (T) – which is then used to make proteins. This process is known as transcription.

The new mutation is located in a section of DNA near the IGF1 gene and regulates its expression, which in turn influences the dog’s body size. There are two versions, or alleles, of this DNA extract: one allele has an extra cytosine base (C) which causes a smaller body size, and the other allele has an extra thymine base (T) which causes a larger body size, Ostrander said. Live Science. Each dog inherits two alleles of the gene (one from each parent), meaning it can have two versions of the small allele (CC), one from each (CT) or two of the large allele (TT), a-t -she adds.

The researchers looked at the DNA of different dog breeds and found a major correlation between alleles and height: small dogs were CC, medium dogs were CT, and large dogs were TT.

A figure from the new study showing how canines got smaller thanks in part to the IGF1 mutation. (Image credit: Plassais et al./Current Biology)

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Become smaller

After finding the mutation, NIH researchers wanted to know how far the alleles could be tracked in the canine. evolutionwhich led them to search for the mutation in the DNA of ancient wolves from genomes published in previous studies.

“We were surprised to find the mutation and delighted to find that both variants [C and T] were present over 54,000 years ago,” Ostrander told Live Science. The researchers had predicted that the short allele was much newer than the tall one, but that was not the case, she added.

The IGF1 mutation appears to have played a key role in the evolution of small canids such as jackals, coyotes and African hunting dogs, which all have two copies of the small allele (CC). However, it is extremely unlikely that small dogs naturally evolved to be as small as they are without the intervention of human domestication and breeding, she added.

“The small allele was kept low [in dogs] for tens of thousands of years until it was selected during or around the time of domestication,” Ostrander said. This breeding was done to create smaller dogs that could better hunt small prey, such as rabbitsshe added.

The first slightly smaller dog breeds, which were eventually bred into the extremely miniature versions we see today, appeared between 7,000 and 9,500 years ago, researchers say.

Understanding Body Size

The IGF1 gene is not the only gene that affects a dog’s body size. At least 20 known genes code for body size, but this particular gene has an outsized influence: it is responsible for around 15% of the variance in body size between dog breeds, a large amount for a single gene, a Ostrander said.

In comparison, hundreds of genes affect body size in humans, Ostrander said. But it’s no surprise that dogs have fewer genes linked to body size given that most dog breeds have only been around for a few hundred years, she added.

Researchers will continue to study more body size genes in dogs to better understand how the genes work together to determine the exact size of each breed, from Chihuahuas to Great Danes. “The next step is to understand how all the proteins produced by these genes work together to make big dogs, little dogs and everything else,” Ostrander said.

The study was published online January 27 in the journal Current biology.

Originally posted on Live Science.