From the 80-kilogram Great Dane to the 1-kilogram tiny teacup poodle, there seems to be a dog for everyone. Now, the largest genetic analysis to date has figured out how those breeds came to be, which ones are really closely related, and what makes some dogs more susceptible to certain diseases.
"They show that by using genetics, you can really show what was going on as [breeders] were making these breeds," says Elinor Karlsson, a computational biologist at the University of Massachusetts Medical Center in Worcester who was not involved with the work.
After dogs were initially domesticated—likely between 15,000 and 30,000 years ago—people picked the best hunters, house guards, and herding animals to be their best friends, depending on their needs. There were dogs for war and for cuddling, for fur and meat, and for being good companions. Today dogs come in 350 or so breeds, each with specific traits and behaviors. Many arose in the past 200 years. Some studies have defined the genetics of a relatively small number of breeds, but none has been comprehensive enough to show how and when most came into existence. "The whole period in between [domestication and today] has been a black box," Karlsson says.
Elaine Ostrander and Heidi Parker, geneticists at the National Human Genome Research Institute in Bethesda, Maryland, and their colleagues spent 20 years going to dog shows, writing dog fanciers, and getting help from all corners of the world to collect DNA samples; in some cases they used already collected data. They weren’t interested in determining how and when dogs were domesticated, but how all the breeds developed. Their sample now includes 1346 dogs representing 161 breeds, or not quite half of all kinds of dogs. By comparing the differences at 150,000 spots on each dog's genome, they built a family tree. "The scope of the analysis is very impressive, [a] tour-de-force on breed evolution," says evolutionary biologist Robert Wayne of the University of California, Los Angeles, who was not involved with the work.
Almost all the breeds fell into 23 larger groupings called clades, the team details today in Cell Reports. Although genetically defined, the clades also tended to bring together dogs with similar traits: Thus boxers, bulldogs, and Boston terriers—all bred for strength—fall into one clade; whereas herders like sheepdogs, corgis, and collies fall into another; and hunters like retrievers, spaniels, and setters fall into a third. The grouping of different breeds that share particular jobs suggests that ancient breeders likely bred dogs for specific purposes, choosing to care for those that were best at guarding or herding. Then, in the past 200 years, people subdivided those larger groups into breeds.
But the data also show how some breeds helped create others, as they share DNA with multiple clades. As one of the earliest small dogs, the pug, which hailed from China, was used in Europe from the 1500s onward to shrink other breeds. Thus, pug DNA is part of many other toy and small dog genomes, Parker explains.
"This is very exciting!” says Peter Savolainen, an evolutionary geneticist at the Royal Institute of Technology in Solna, Sweden, who was not involved with the work. “It shows how attractive traits from one breed [have] been bred into new breeds."
Having these clades will help veterinarians spot potential genetic problems, Parker says. For example, before vets couldn't really understand why a genetic disease called collie eye anomaly, which can distort different parts of the eye, and shows up in collies, border collies, and Australian shepherds, also occurs in Nova Scotia duck tolling retrievers. But the genetic analysis shows that this retriever has either collie or Australian shepherd ancestors that may have passed on the defective gene. "Mixing has resulted in the sharing of specific genomic regions harboring mutations which cause disease in very different breeds," Wayne says.
Wayne and Karlsson both stress that to provide more details, the researchers should work to compare whole genomes—the entire 2.5 billion bases. And as Savolainen points out, the work "is a very good first step into the origins of all dog breeds, but half of all breeds are still missing." Ostrander and Parker say they see this publication as a midpoint, not an endpoint. "We had reached a point where we could begin to do some of the things we wanted to do," Ostrander explains. "By no means are we done."