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Dog Genome Mapped, Shows Similarities to Humans

Stefan Lovgren
for National Geographic News
December 7, 2005
 
Researchers have finished mapping the genome of the domestic dog.

The results show among other things that dogs, mice, and humans share a core set of DNA.

A rough sequencing of a poodle's genome was reported in 2003, but it covered only about 75 percent of the genes.

This time, "we have sequenced 99 percent of the genome of a female boxer," said Kerstin Lindblad-Toh, a geneticist at the Broad Institute of Harvard University and MIT in Cambridge, Massachusetts. She is the lead author of the dog-gene study, which appears tomorrow in the journal Nature.

The researchers also examined DNA from ten different dog breeds to spot genetic differences between them.

The comparison could help scientists find the genetic roots of dog behavior and physiology and—perhaps most importantly—help them identify genes that cause diseases in both dogs and humans.

Dogs, Mice, and Humans

The researchers obtained the gene data from Tasha, a female boxer picked from a selection of dogs made available by breeding clubs and veterinary schools.

The geneticists sequenced the 2.4 billion "letters" of the dog's DNA code, representing 39 chromosome pairs. Humans by comparison have 23 chromosome pairs.

"The boxer genome sequence is big step towards the goal of having a complete reference sequence for a dog genome," said Ewen Kirkness, a molecular biologist at the Institute for Genomic Research in Rockville, Maryland, who led the 2003 poodle study and co-authored the new study.

Scientists had previously found that about 5 percent of the human genome sequence appears in the mouse genome. The new study shows that 5 percent of the human genome is also shared with dogs.

Significantly, the sequences that are conserved in all three species are virtually the same.

"For genes this is not unexpected, since most mammals share a rather similar set of genes," said Hans Ellegren, an evolutionary biologist at Uppsala University in Sweden.

"However … this indicates that there is a core set of noncoding sequences needed to make a mammal."

The researchers have also found that many of the conserved sequences are clustered around developmental genes.

"You could imagine these could be genes [that are] really important for body planning and development," Lindblad-Toh said.

Genetic Disorders

Dogs were the first animals to be domesticated some 15,000 years ago. They all originate from a single species, the wolf, but modern breeds display a wide diversity of traits.

To understand the genetic basis for this diversity, the researchers compared DNA samples from another ten dog breeds.

They also compiled a catalogue of 2.5 million genetic units that vary between different dogs. The units will help researchers identify nearby genes that are responsible for traits such as disease susceptibility.

"The fact that only a few breeds typically show a disease means that by searching for genetic dissimilarities between breeds, there is a decent chance of finding genes that cause the [disease] between breeds," said Ellegren, the Uppsala University biologist.

Dogs suffer from more than 350 genetic disorders, many of which resemble human conditions. The most common diseases among purebred dogs include cancer, epilepsy, heart disease, allergies, retinal disease, and cataracts.

"[This research] increases the possibility of using dogs as a model for human disease," Ellegren said.

The study could also help researchers identify genes that govern behavioral traits, such as aggressiveness or kindness.

In addition, the data suggests how dogs were domesticated. The amount of genetic diversity across breeds is consistent with a "bottleneck"—a strong reduction in genetic variety—that occurred between 10,000 and 15,000 years ago.

"This bottleneck is likely to have been [due to] domestication," Ellegren said.

Within breeds there are also signs of strong bottlenecks just 50 to 200 years ago, suggesting most dog breeds have a very recent origin.

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