An international research team on Wednesday announced the completion of sequencing the dog genome, which sheds light on both the genetic similarities between dogs and humans, and the genetic differences between dog breeds.

Releasing their findings in the Dec. 8 issue of the journal Nature, scientists said that comparison of the dog and human DNA reveals key secrets about development, disease and evolution.

"Of the more than 5,500 mammals living today, dogs are arguably the most remarkable," said Eric Lander, director of the Broad Institute and senior author of the study.

"The incredible physical and behavioral diversity of dogs - from Chihuahuas to Great Danes - is encoded in their genomes. It can uniquely help us understand embryonic development, neurobiology, human disease and the basis of evolution."

Dogs, sitting at a key branch point in the evolutionary tree, have 19,300 genes, the researchers found. Humans have 3,000 more, but almost all of the dog genes can be found in people in a different version.

By tracking evolution's genetic footprints through the dog, human and mouse genomes, the researchers found that humans share more of their ancestral DNA with dogs than with mice, confirming the utility of dog genetics for understanding human disease.

Most importantly, the comparison revealed the regions of the human genome that are most highly preserved across mammals.

Roughly 5 percent of the human genome has been well preserved by evolution over the past 100 million years and must encode important biological functions.

The researchers discovered that the most highly conserved of these sequences are not randomly distributed throughout the genome. Instead, they are crowded around just about 1 percent of the genes that encode crucial regulatory proteins involved in development.

Comparison among dogs also reveals the structure of genetic variation among breeds, which can now be used to unlock the basis of physical and behavioral differences of dog breeds, as well the genetic sources of common diseases of dogs and humans, scientists said.

Dogs were domesticated from gray wolves as long as 100,000 years ago, but selective breeding over centuries has made modern dog breeds a testament to biological diversity. Different dog breeds may have contrasting sizes and habits.

The researchers embarked on a two-part project to assemble a complete map of the dog genome. First, they acquired high-quality DNA sequence from a female boxer named "Tasha," covering nearly 99% of the dog's genome.

Using this information as a genetic 'compass,' they then sampled the genomes of 10 different dog breeds and other related canine species, including the gray wolf and coyote.

By comparing these dogs, they pinpointed about 2.5 million individual genetic differences among breeds, called single nucleotide polymorphisms (SNPs), which serve as recognizable signposts that can be used to locate the genetic contributions to physical and behavioral traits, as well as disease.

Finally, the researchers used the SNP map to reconstruct how intense dog breeding has shaped the genome.

They discovered that selective breeding carried large genomic regions of several million bases of DNA into breeds, creating 'haplotype blocks' that are 100 times larger than seen in the human population.

Breeding programs had unintendedly led many dog breeds to genetic diseases, including heart disease, cancer, blindness, cataracts, epilepsy, hip dysplasia and deafness.

With the dog genome sequence and the SNP map, scientists now have the tools to identify these disease genes. This effort may help trace the genetic sources of many similar human diseases, the researchers said.

"The genetic contributions to many common diseases appear to be easier to uncover in dogs," said Kerstin Lindblad-Toh, lead author of the paper.

"If so, it is a significant step forward in understanding the roots of genetic disease in both dogs and humans."

Source: Xinhua