Saturday, February 6, 2010

Genetic Structure of Contemporary Wolves and Dog Breeds

The genetic structure of wolves and dogs was finally mapped from 2000 to 2004.  The origin of domestic dogs from wolves is firmly established.  Dogs are related more closely to East Asian wolves than to European or North American wolves (1).  All wolves and dogs belong to the same species.  Other canids, such as coyotes, jackals, and foxes, are genetically more distant and distinct from wolves and dogs.

By tracing mitochondrial DNA, inherited only from mothers, researchers found several wolf origins for contemporary dogs.  The study looked at the DNA of 654 dogs from 83 dog breeds and 38 Eurasian wolves. Three maternal DNA patterns accounted for more than 95% of dog genotypes, and these three sources came from East Asian wolf populations.  Based on number of mutations found in the DNA sequences, researchers estimate that dogs became domesticated in several events (at least 5 unique mothers), beginning  about 15,000 years ago.

Another group of researchers (2) studied 96 gene loci in 414 purebred dogs representing 85 breeds.  They plotted the genetic relatedness of contemporary dogs and wolves.  They found that genotypes of ancient Asian and Arctic breeds more closely resemble contemporary Eurasian wolves than they resemble other dog breeds.  Specifically, Shiba Inu, Chow Chow, Akita, Alaskan Malamute, Basenji, Shar-pei, and Siberian Husky breeds cluster with contemporary wolf genotypes more closely than with other dogs breeds.  Afgan Hound, Saluki, Tibetan Terrier, Llasa Apso, Samoyed, Pekingese, and Shih Tsu breeds are intermediate, sharing genotypes with both contemporary wolves and with other dog breeds.

Most dog breeds were created by human selective breeding for specific tasks or appearances. Breed isolation, however, is a relatively new phenomenon.  Most dog breeds have existed as isolated breeding populations for less than 200 years, many for less than 100 years. Following wars, famines, and natural disasters, some dog breeds that became nearly extinct were re-established by interbreeding several related breeds.  So, what are the genetic resemblances and differences among dog breeds today?

To refine further the relatedness of dog breeds today, researchers looked for distinctive genetic patterns among breeds.  Genetic differences among breeds account for about 30 % of all genetic differences among the dogs, which shows a high degree of genetic isolation among breeds (as breeders intend).  The degree of genetic difference among dog breeds is far greater than differences among human populations or breeds of other domestic animals, such as cattle and sheep. By looking only at genes, researchers could classify 99% of individual dogs into their correct breed.

Do dog breeds cluster?  To some extent. Beginning with samples of wolves' DNA, the Asian Spitz-type dogs form a distinct cluster: Shar-pei, Shiba Inu, Akita, and Chow Chow.  A second cluster includes only Basenjis, an ancient African breed.  A third cluster contains two Artic breeds, Alaskan Malamute and Siberian Husky, and a fourth cluster includes two Middle Eastern sight hounds, the Afgan and Saluki.

Structure analysis shows additional breed groups not readily apparent from other analyses.  If a three-group structure is used, a cluster of Mastiff-type dogs appears -- Mastiff, Bulldog, Boxer, Rottweiler, and their close relatives.  A four-group structure produced a cluster of herding dogs.  No additional groups could be reliably found in the data.

Remaining breeds showed few consistent relationships, except for breeds that are not (yet) genetically distinct (e,g., Belgian Sheepdog and Belgian Turvuren) or have a recent history of interbreeding,  In other words, more than 70 breeds of modern European and North American invention are now genetically distinct populations, but the breeds show no pattern of genetic relatedness.  They are equally related to each other and to modern wolves.

I would guess that few pet owners look at their Poodles, toy terriers, and Chihuahuas and think "wolf".  Yet, these small dogs are just as much wolves as dogs that bear more obvious resemblances to species-brothers.  Small size in dogs is caused by a single gene (3) that has been imported into many breeds to downsize them.  The rest of the genotype is still wolf.

Dogs would thank their owners for thinking "wolf" when they consider how to feed them.  Perhaps, it is obvious that an Alaskan Malamute or a German Shepherd would appreciate the whole-prey diet their wolf-brothers thrive upon.  It is not as obvious that toy dogs need the same diet, scaled down to their size.

Don't let "experts" in veterinarian clothing, or minced-veggie purveyors online, tell you dogs are omnivores, whose diet has been shaped by human leftovers. Wolves and dogs are "opportunistic carnivores", who kill and eat whole prey and scavenge off other predators' kills.  They will eat human garbage, but a healthy diet for wolves and dogs is principally meats and meaty bones.  Pet owners can easily provide a wolf-diet in appropriate amounts and sizes for their friendly domestic wolves.

(1) Savolainen, P., Zhang, Y, Luo, J, Lundeberg, J., & Leitner, T. (2002).  Genetic evidence for an East Asian Origin of Domestic Dogs, Science, 298, 1610-1613.

(2) Parker, H., Kim, L.V., Sutter, N.B., Carlson, S., Lorentzen, T.D., Malek, T., Johnson, G.S., DeFRance, H.B., Ostrander, E.A., & Kruglyak, L. (2004). Genetic structure of the purebred domestic dog. Science, 304, 1160-1164.

(3) Sutter, N.B.and 20 coauthors, (2007). A single IGF1 allele is a major determinant of small size in dogs. Science, 316, 112-115.

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