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Gene changeover that helps make 'toy' dog breeds so small existed in wolves 54,000 years ago

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One of the key genetic mutations responsible for the small size of certain dog breeds, such as Pomeranians and Chihuahuas, emerged in dog relatives long before humans started breeding these diminutive companions. The mutation can even be traced back to wolves that existed more than 50,000 years ago, according to researchers.

The mutation was detected in the insulin-like growth factor 1 (IGF1) gene by reviewing data collected as part of the National Institutes of Health (NIH) Dog Genome Project, a citizen science project in which owners provide DNA samples from their pet dogs. This "unique" mutation, discovered not in the IGF1 gene but in the DNA that governs its expression, had previously eluded researchers for over a decade.

The researchers discovered the mutation in 54,000-year-old DNA from fossils of Siberian wolves (Canis lupus campestris), as well as in the DNA of every canid species alive today, including jackals, coyotes, and African hunting dogs, after conferring with scientists in England and Germany.

"It's as if nature had kept it tucked away in her back pocket for tens of thousands of years until it was needed," said senior author Elaine Ostrander, an NIH geneticist who specialises in dogs. She went on to say that the discovery helps to tie together what we know about canine domestication and body size.

Exceptional mutation
Genes are DNA segments that serve as the blueprint for the creation of certain proteins. Each gene is made up of a unique combination of four nucleotides that code for a specific protein: adenine (A), guanine (G), cytosine (C), and thymine (T). Cells must unzip the double-stranded DNA to read the nucleotides of the strand containing the corresponding gene to produce a specific protein. 

The DNA is then copied by special machinery within the cell, which produces RNA — a single-stranded molecule similar to DNA but with one different sugar (ribose instead of deoxyribose) and the nucleotide uracil (U) instead of thymine (T) — which is then used to construct proteins. This is referred to as transcribing.

The novel mutation is found in a stretch of DNA near the IGF1 gene and governs its expression, which influences the dog's body size. This piece of DNA has two variations or alleles: one allele has an extra cytosine base (C), which produces reduced body size, and the other allele has an extra thymine base (T), which promotes bigger body size, according to Ostrander. According to her, each dog receives two alleles of the gene (one from each parent), which means it can have two versions of the small allele (CC), one of each (CT), or two of the large allele (TT).

The researchers examined the DNA of various dog breeds and discovered a significant association between alleles and size: little dogs were CC, medium-sized dogs were CT, and large dogs were TT.
Reducing in size

After discovering the mutation, the NIH researchers wanted to know how far back the alleles might be traced in canid evolution, so they looked for it in the DNA of ancient wolves from genomes released in earlier studies.

"We were astonished to detect the mutation and happy to discover that both variations [C and T] existed over 54,000 years ago," Ostrander told Live Science. The researchers assumed that the allele for shorter stature would be considerably younger than the one for higher stature, but this was not the case, she continued.

The IGF1 mutation appears to have had an important role in the evolution of smaller canids like jackals, coyotes, and African hunting dogs, which all have two copies of the tiny gene (CC). However, she adds, it is exceedingly unlikely that little dogs would have evolved to be as small as they are without the intervention of human domestication and breeding.

"The tiny allele [in dogs] was kept at a low level for tens of thousands of years until it was selected on during or around the time of domestication," Ostrander added. She went on to say that the breeding was done to generate smaller canines that could better hunt small game like rabbits.

According to the experts, the first somewhat smaller dog breeds, which were subsequently bred into the exceedingly diminutive ones we see today, appeared between 7,000 and 9,500 years ago.

Understanding Body Dimensions
The IGF1 gene is not the only one that influences the growth of a dog's body. There are at least 20 known genes that code for body size, however, this one has a disproportionate impact: According to Ostrander, it is responsible for around 15% of body size variation across dog breeds, which is a significant amount for only one gene.

In humans, hundreds of genes influence body size, according to Ostrander. However, given that most dog breeds have barely been established for a few hundred years, it is expected that dogs contain fewer body-size-related genes, she adds.

More body-size genes in dogs will be studied in the future to better understand how the genes work together to determine the exact size of every breed, from Chihuahuas to Great Danes. "The next step is to figure out how all of the proteins created by these genes interact to create huge dogs, little dogs, and everything in between," Ostrander said.

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