The many breeds of domestic dogs vary enormously in body size, even though all domestic dogs belong to one species. An international team of scientists has carried out a wide-ranging study of genes that influence body size in domestic and wild dogs. A gene named IGF1 is already known to have a strong influence on body size. The research team studied the genomes of 230 modern breeds, wild dogs and village dogs and found two gene variations, named C and T that made a difference in body size. These were not in the IGF1 gene itself, but in a region of DNA next to the gene which regulates its activity. The T variant promoted larger size, while the C variant resulted in smaller size.
As each dog carries two copies of the IGF1 gene and its regulators, they can also have a combination of both variants. Researchers found most large size breeds, such as Irish wolfhounds and great Danes, had two copies of T, and small breeds had two copies of C. Medium size breeds usually had one of each variant.
The research team also collected DNA from dog and wolf bones buried in archaeological sites. To their surprise they found ancient dogs, including the Pleistocene Siberian wolf had both the C and T variants. They were also intrigued to find that coyotes, jackals, and foxes (all classified as canids) only had the C variant.
The standard story of dog evolution and domestication is that domesticated dogs were derived from large wild wolves, and as domestic dogs were bred intensively for desirable features, mutations occurred that have resulted in small breeds that are now called “toy” size, e.g. toy poodle. Adam Boyko, an expert on canine genetics at Cornell University, commenting on the origin of the C variant said: “This research suggests that we didn’t create it—it was already there.”
Editorial Comment: The diseases and problems seen in small domestic dogs can be related to growth gene mutations, but these two variants don’t seem to cause any disease – just the variation in size. Therefore, they are probably built-in variations that were always there. This would fit with them being found in both domestic and wild dogs, especially as disease causing mutations are usually rapidly weeded out by natural selection in wild animals.
The fact that the C variant was found in jackals and dingos, which are not particularly small, is also a reminder that genes do not work alone. It takes more than the C variant to make “toy” size in some domestic dogs, and more than the T variant to make extra-large dogs, which are also the result of domestication and breeding.
We may not have created the C and T variants, but the intense inbreeding carried out by dog breeders to please frivolous people did set up conditions where built-in variants have interacted with mutations and led to some of the problems seen in some extremely small and large breeds. So beware – breeding for smaller or bigger dogs can be cruel to the dog if you don’t know what side effects it will cause.
Creation Research News 23 February 2022
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