What makes a dachshund different from a Doberman? A new study sheds light on the genetics underlying the low-slung dachshund, the powerful Doberman and other breeds through history.
By analyzing genetic samples from dogs of many shapes and sizes, a Seattle-based research team has shown how a dog’s genes can reveal its breed. A genetics-based classification system for breeds will allow researchers to study dog genes for diseases that have counterparts in humans and to piece together the evolutionary history of our closest companions.
Such a system may also mean that owners of purebred dogs and mutts alike may soon be able to document which breeds their dogs come from by simply sending a cheek swab or blood sample to a genetics lab.
The researchers found that classifying breeds according to their genetic similarities produced many results similar to traditional groupings but also revealed some unexpected connections. The findings appear in the journal Science, published by AAAS, the nonprofit science society.
Breed barriers
The rarefied world of dog competitions involves exacting requirements for a purebred’s physical appearance, but the pedigree is the bottom line. A dog can be registered with a certain breed only if both its parents were registered.
This meticulous attention to ancestry has erected “breed barriers,” ensuring that each breed is a relatively closed genetic pool. The genetic similarities within a breed should be a boon for efforts to find genes associated with disease, behaviors or other traits.
“Each breed is like the human population of a Finland or an Iceland, meaning there have been limitations on what’s gone into gene pool. So there may be 100 heart disease genes in dogs, but just one or two for a German shepherd or a border collie,” said study author Elaine Ostrander of the Fred Hutchinson Cancer Research Center and the University of Washington.
Understanding human diseases
Ostrander and her colleague Leonid Kruglyak, also of the Fred Hutchinson Cancer Research Center and the University of Washington, have proposed that dogs can offer unique opportunities for understanding human diseases, including cancer, heart disease and epilepsy, and conditions such as blindness and deafness.
Some of these diseases, such as certain cancers, occur naturally in dogs, whereas they must be induced in mice and rats.
These research efforts will also benefit from the sequencing of the dog genome, which is currently under way. In the meantime, Ostrander, Kruglyak and their colleagues can still study the genetic basis for diseases and other traits in dogs through comparing different breeds.
The Science authors frequented dog shows and other venues where dog owners allowed the researchers to collect cheek swabs and blood samples from their pets.
The researchers then analyzed various “marker” regions throughout the dogs’ genomes. In most every case, they could identify the dog’s breed solely from the variations in its genetic sequences. Identifying a mutt’s multiple breeds is a more complex problem, but Ostrander thinks this will be possible some day.
All in the family
Ostrander’s group used their genetic data to construct an evolutionary tree showing which breeds were most closely related. The tree contained three relatively recent groups and one ancient one.
The oldest group includes dogs whose origins date back to antiquity — the very oldest ones being from Asia, such as the shar-pei and the chow, and from Africa, such as the basenji. Others, like the Afghan, come from the Middle East, while the Siberian husky and others come from the Arctic. These dogs are also the most genetically similar to wolves.
It may seem surprising that dogs with such different appearances and geographical origins are so closely related, but it’s consistent with one hypothesis that dogs were first domesticated from wolves in Asia. Some researchers have proposed that the early dogs then migrated with nomadic human groups to Africa and the Arctic and around Asia.
Looks can be deceiving
The other breeds outside this group didn’t emerge until the around the 1800s, underscoring the fact that appearances can be deceiving. For example, a small fluffy Shih Tzu, which the authors assigned to the ancient group of Asian dogs, is more closely related to the wolf than a German shepherd is.
The authors’ results also turned up a couple of breeds that are in fact much younger than previously thought. The pharaoh hound and the Ibizan hound, for example, are commonly believed to have ancient origins. Not so, according to their genetics. These dogs appear to be “re-creations” bred more recently from combinations of other breeds.
“There are stories and histories for every breed. It’s interesting to figure out how much of that lore is accurate and what new relationships you can pull out that aren’t necessarily known from the history books,” said Kruglyak.
Genes and behavior
The three more recent breed groups the authors identified from their genetic data generally correspond to the type of work the dogs were bred for. The mastiffs, bulldogs and other bulky dogs in one group typically make good guard dogs. The herding group includes agile dogs like the border collie and the Shetland sheepdog. The third group of dogs includes terriers and a variety of other dogs bred for hunting.
Many of the dogs in each group share certain behavioral traits as well as physical characteristics.
The tenacious herding dogs, for example, have a variety of tricks for controlling a flock, including a steely stare known as “eye,” nipping at the heels and barking.
“The herding dogs use different behavioral mechanisms to herd, but they all have ingrained the desire to collect and contain and to move herds long distances. They have body strength and stamina, plus the intelligence to be trained quite easily,” said Ostrander.
Ostrander and Kruglyak think that eventually the scientists will be able to identify the genes underlying certain canine behaviors.
“Whatever we say about disease can extend to interesting features of morphology and behavior. Now we have a real formula for grouping breeds together and statistical power for tracking down the genetic basis for these characteristics,” Ostrander said.