Chihuahuas, Pomeranians or Yorkshires do not exist by mere whim of man . The genetic mutation that makes these dogs so small first originated in wolves around 53,000 years ago, according to a study conducted by the US National Institutes of Health and published in the journal Current Biology . .
An unusual mutation
The researchers identified a mutation in a canine growth hormone regulatory gene that is associated with small body sizes. Specifically, in the gene for insulin-like growth factor 1 (IGF1), as they extracted from data from a citizen science project in which owners collect DNA samples from their companion dogs. The catalog comprised 1,431 genomes , including ancient canines, dogs of modern breeds, and wild canids.
The mutation in question is found in the DNA that regulates the expression of this IGF1 gene, which had eluded discovery for more than a decade.
The experts achieved this milestone when they searched for genetic sequences that were placed backwards and that were also present in other canids such as wolves and also in ancient DNA . This approach produced a reverse form of the insulin-like growth factor 1 (IGF1) gene, which was found to have variants that correlated with overall body size.
“Domestic dogs ( Canis lupus familiaris ) are the most variable-sized mammalian species on Earth, showing a 40-fold difference in size between breeds,” said Jocelyn Plassais, a postdoctoral researcher at the National Human Genome Research Institute at the National Institutes of Health and study leader. “Although dogs of variable size are found in the archaeological record, the most dramatic changes in body size are the result of selection over the last two centuries, when dog breeders selected for and propagated extreme phenotypes within closed breeding populations. ”.
A new evolutionary narrative
This mutation was already present in the genetic code of a steppe wolf that lived in Siberia about 53,000 years ago. In this mutation, they found two versions, or alleles, of this DNA fragment: one allele has an extra cytosine (C) base that causes smaller body size, and the other allele has an extra thymine (T) base that causes smaller body size. a larger body size.
This discovery contradicts the idea that small dogs stem from domestication 20,000 years ago, as one of the main genetic mutations responsible for size in certain breeds evolved long before humans began to breed these household companions. .
“It is as if nature had kept it in its back pocket for tens of thousands of years until it needed it,” say the authors.
This twist to the evolutionary narrative holds true not only for dogs and wolves, but also for coyotes, jackals, African hunting dogs, and other members of the family of animals known as canids.
The researchers plan to continue investigating the genes that regulate body size in dogs; Only 25 genes, compared to several hundred that we humans possess.
Note: 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 particular protein.
Referencia: Jocelyn Plassais et al. 2022. Natural and human-driven selection of a single non-coding body size variant in ancient and modern canids. Current Biology 32: 1-9; doi: 10.1016/j.cub.2021.12.036
