Scientists have long understood that dogs with flat faces like pugs and bulldogs are the result of out-of-control selective breeding. But they have yet to discover the exact genetic mutation that's responsible for the physical traits of these dogs. A new study has gone a long way towards finding the answer and could have implications for the health of these beleaguered canines.
Skull shape tends to be fairly consistent in the various species of mammals. Natural selection has done its work to make sure that a species adapts to its environmental, dietary and respiratory needs, and the shape of the skull is a byproduct of those requirements. But selective dog breeding has thrown natural selection some curve balls when it comes to canines. Whereas the shape of a bear skull is relatively the same, there's a huge variety of dog skulls. Smooshed-faced dogs, aka brachycephalic dogs, may be cute, but their wide face and bug eyes aren't doing them any favours. Respiratory distress is common, and they tend to get more ocular injuries than other dogs.
Previous studies of doggos' cranial shape haven't really dived into the genetic causes of why some breeds are different than others. A new study published in Current Biology aims to fix that. In order to determine what causes canine brachycephaly, researchers used an X-ray process called computed tomography to analyse 374 dogs that were brought into a veterinary service. Among the subjects were 84 Kennel Club recognised breeds and 83 mixed-breed dogs. Then, the researchers reconstructed detailed 3D models of each skull that had been scanned. Analysis of the data showed that skull shape (obviously) correlated with the individual breeds and the breed with the most smooshed-up face was the pug. Smooth collies were found to have the most elongated snouts.
Smooth collie. Photo: Wikimedia
Looking at genetic variations between the breeds, the researchers were able to zero in on a section of DNA (QTL) that appears to be associated with brachycephaly. They then re-sequenced 28 brachycephalic dogs and compared the results with 319 other canid genomes. Several variants in smooshed-face dogs were discovered, the most significant of which was a long interspersed nuclear element (LINE-1) inside what's known as the SMOC2 gene. This mutation suppresses SMOC2 which has been found to play a role in the cranial development of fish and mice.
Other genetic factors are surely at play. The researchers have concluded that suppression of SMOC2 explained up to 38% of the variation in test subjects and more resources are need to continue their studies.
Not only would this research add to our understanding of evolutionary biology but it could also help in the development of human and veterinary medicine. While I'm sure people might miss the distinct way these dogs look, different breeds tend to have different temperaments. If we could isolate what's causing the health problems of these little guys, maybe we could fix it at a genetic level and still preserve what gives them their specific personalities.