The short curved legs, characteristic of today's dachshunds, corgis, basset hounds and at least 16 other breeds of dogs, is due to a single evolutionary event in the past, according to researchers at the National Human Genome Research Institute (NHGRI)
The discovery could offer new clues about how physical differences may arise within species and suggests new approaches to understanding a form of human dwarfism.
In the study, the researchers led by NHGRI's Dr. Elaine Ostrander, examined DNA samples from 835 dogs, including 95 with short legs.
They conducted a survey of more than 40,000 markers of DNA variation and uncovered a genetic signature exclusive to short-legged breeds.
After follow-up DNA sequencing and computational analyses, the researchers determined that the dogs' disproportionately short limbs could be traced to one mutational event in the canine genome - a DNA insertion - that occurred early in the evolution of domestic dogs.
"Every species, including canine and human, carries an amazing record of evolution scripted in its genome that can teach us about the mechanisms at work in biology, as well as about human health and disease. This work provides surprising evidence of a new way in which genome evolution may serve to generate diversity within a species," said NHGRI Scientific Director Dr. Eric Green.
The researchers specifically found that in contrast to other dog breeds, all short-legged dog breeds have an extra copy of the gene that codes for a growth-promoting protein called fibroblast growth factor 4 (FGF4).
Although functional, the extra gene lacks certain parts of the DNA code, called introns, found in normal genes.
Owing to these characteristics, the researchers concluded that the extra gene is a so-called retrogene that was inserted into the dog genome some time after the ancestor of modern dog breeds diverged from wolves.
In case of short-legged dogs, the inserted retrogene results in the overproduction of the FGF4 protein, which according to researchers could turn on key growth receptors at the wrong times during foetal development.
"Our findings suggest that retrogenes may play a larger role in evolution than has been previously thought, especially as a source of diversity within species. We were surprised to find that just one retrogene inserted at one point during the evolution of a species could yield such a dramatic physical trait that has been conserved over time," said the study's first author, DR. Heidi G. Parker.
The dog findings are the first example of a retrogene that has spurred significant and long-lasting variation within a single species.
The findings may also have implications for understanding human biology and disease as well as help in the uderstanding of a similar appearing growth disorder, called hypochondroplasia, or dwarfism.
The study has been published in the advance online edition of the journal Science.