Dog Breeds and Skull Shape
At the Westminster dog show, trainers walk animals that uphold a breed’s standards for size and shape. A perfect pug has a squished-in face, a collie an elongated snout. The dog’s skull is a useful signifier for breed, because of the sheer diversity of shapes. What is the genetic mechanism behind this variance?
The domestication of the dog from the wild grey wolf produced staggering morphological diversity. Researchers Drake et al found that the diversity of domestic dog skull shapes exceeds the range of not only wolves, but the entire Carnivora group containing foxes, big cats, weasels, and pandas. They used a technique called principal component analysis, which overlays skull shape data and determines which parameters contribute most to the diversity in the group. The most impactful spatial constraint, PC1, represents the range between Brachycephaly (a squished face), and Dolichocephaly (an elongated face). PC2 charts the skull’s squish and stretch on the vertical axis. PC3 is the difference between a tapered muzzle and a cylindrical muzzle. Extracting these constraints tells us what traits breeders were looking for in a dog’s skull when they created new offspring.
The team used a dataset called CanMap, with DNA information from 915 dogs in 80 breeds. This information includes mutation data called Single Nucleotide Polymorphisms (SNPs). The mutation only switches out one nucleotide, but this change can produce noticeable physical differences between individuals. In dogs, the SNPs that have the most power over the body are responsible for changes in body composition. In humans, the strongest SNPs are responsible for hair color and metabolic adaptation. The process of breeding accounts for this unique arrangement in dogs.
The rapid, human-driven process of dog domestication simplified the genetic architecture to foster a distinctly morphological control system. A small number of genes have disproportionate control over variability in skull shape. This may have come about because Victorian breeders made decisions based on aesthetic rather than behavior or metabolism. When they spotted a noticeable shape change in an offspring’s skull, they promptly bred this individual with other breeds to stir the genetic pot. Breeders also diminished the pressure on survival constraints, because their dogs don’t live in the wild. It’s unlikely that a pug arose out of natural selection, with its ungainly trot and hindered breathing.
When scientists have a sense for how genes act in one species, they are able to extrapolate conclusions to a wider group of animals. The domestic dog is becoming a model organism, because of the abundance of data available to evolutionary biologists. Understanding which genes contribute most to skull shape in dogs can help us pinpoint the mechanisms of birth defects in humans, because the mechanisms are shared among mammals.