In Dog DNA, Small Size Has an Ancient Pedigree

Big dogs get all the glory. I don’t have a peer-reviewed paper to prove it, but I can quote the greatest country singer of all time, Hank Williams, who is certainly some kind of authority.

To wit: “Move over little dog cause the big dog’s movin’ in.”

Not so fast, big dogs. A group of researchers who may not sing as well as Hank, but who make some impressive DNA charts, reported Thursday that part of the dog DNA responsible for small size is likely older than a variation that contributes to large size. The small-size version seems to be ancestral.

This is good news for the maltipoos and their owners. Even if Fluffy himself doesn’t care about his DNA, Fluffy’s designated human may appreciate a little ammunition the next time someone starts huffing and puffing about “real” dogs.

To be clear, the research report in Current Biology did not make any mention of country music or Fluffy. Jocelyn Plassais and Elaine Ostrander, two members of a team of canine science specialists from the United States and Europe, were far more interested in the details of canine genomics when I spoke to them in a joint Zoom call.

There’s no single gene that determines dog size, according to Dr. Ostrander, a dog genomics expert at the National Institutes of Health. There are about 25. She and other scientists found the first known dog size gene, and one of the most significant, in 2007. It’s called IGF1. The importance of the gene has been confirmed in many studies. But how it affected size wasn’t clear.

“The thorn in my side,” Dr. Ostrander said, was that no one could find a mutation in the gene or the DNA stretches that controlled it to explain what the actual changes in DNA were that affected size. “It’s not an exon, it’s not a promoter. It’s not an enhancer. It’s not a splice site,” she said, referring to sections of DNA that control genes. “We couldn’t find it. And Joc (pronounced Joss) came on the scene.”

“And I found it,” he said, with a grin.

Dr. Plassais, who was working at the N.I.H. during the research and is now at the University of Rennes 1, in France, said he had the great good luck to be working at a time when so many genomes of dogs and other canids, modern and ancient, are available. So by comparing more than a thousand genomes from more than 200 different breeds, he found a DNA stretch that came in two versions, or variants, that were tied to size.

This bit of DNA is not a gene, because by definition a gene has to contain the instructions to make a protein. But many other stretches of DNA have the instructions for bits of RNA that help to control genes. He found a DNA stretch that has instructions for what is called anti-sense RNA, which plays a significant role in controlling the production of proteins specified by genes.

His find is called IGF1-AS and it comes in two variants. Each dog has two copies of this DNA. Two large variants make a big dog, like a German shepherd. Two small variants make a small dog, like a miniature schnauzer, and one of each make a medium-sized dog, like one of the ubiquitous dusty-colored village dogs seen around the world.

But Dr. Plassais wanted to go beyond dogs, and he collaborated with other researchers to determine that the variants had the same effect in wolves, coyotes and foxes, both within a given species and between species. In dogs the effect is profound, perhaps accounting for 15 percent of dog’s size.

The variant is not the result of a mutation that emerged during domestication, as might have happened. Ancient DNA, and the evidence that the small size variant is present in small modern day wolves, in coyotes and in foxes, suggest that it is the ancestral version. The large variant is a relatively recent mutation, perhaps 50,000 or so years old, that the authors think contributed to large size in wolves during periods of glaciation.

Of course, all dogs have an equally ancient heritage, as do all people. No breed of dog is better than any other breed of dog in any fundamental way, unless you happen to breed or own that kind of dog. And science should never be used to gain a leg up socially in a vicious (I’m thinking of the people) environment like a dog park.

Nonetheless, at least in urban areas, the use of dogs in stag hunting and sheep defense is in decline. Little dogs are quite good at fulfilling the needs of humans for companionship. And they have an unarguable advantage when it comes to poop scooping. You with the plastic bag over your hand — which would you rather: Boston terrier or Mastiff? Your choice.

Not that science should be used as a basis for morality, or melody, but maybe large dog chauvinists should just sit and stay for once?

“Move over big dog, cause the little dogs are movin’ in.”

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