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The average height and overall body structures are broadly consistent between human races. However, that is not the case for our canine companions. As a matter of fact, body size difference in dogs transcends all the land mammals. While petite breeds like Chihuahua and Pekingese weigh only a few kilograms, mid-sized dogs such as Golden Retrievers and German Shepherds typically weigh between 27 to 45 Kg, and giants like Grate Danes, St. Bernards, and Alaskan Malamutes can weigh over 80 Kg.
Despite having wild wolves as common ancestors, the extraordinary size difference between domesticated dog breeds have intrigued dog-lovers and scientists for decades. A recent study on the DNA of approximately 200 modern dog breeds and nearly 1,400 canids, including ancient dogs, wolves, jackals, and coyotes, have thrown light on a major genetic candidate that impacts size variation between dog breeds.
Compared to approximately two hundred genes that control height variation in humans, our canine companions have only about two dozen of genes regulating body size. The research study highlighted above has pinpointed that of these 25 genes, a growth hormone-regulating gene controls about 15% of body size variations in domesticated dogs and canids.
Distinct alterations in the DNA sequence of this growth hormone-regulating gene gave rise to different versions or variants of the gene, and each variant was linked to a particular body size in dogs. Most notably, only two varieties of this gene were found to be the major driving force behind body size variation. For the sake of simplicity, let us call them “small’ and “large” variants. Researchers found that any dog with two copies of the “large” gene weighed more than 25 Kg, dogs possessing a pair of the “small” gene weighed less than 15 Kg, whereas intermediate sized dogs tend to harbor one copy of each variant.
Findings of the current study also challenges the prevailing view that small body size arose during domestication and genetic changes that favors small body size are fairly new. Scientists discovered that 53,000-year-old Siberian wolf DNA harbors the “small” genetic variant of the growth hormone-regulating gene. Since domestication of dogs began only about 20,000 years ago, this new finding suggests that the small dog gene variant did not arise due to selective breeding and potentially stayed dormant in wolves.
Additionally, the “small” version was not just found in dogs and wolves, but also in most other canids explored in the present study. In fact, DNA of many coyotes, foxes, and jackals possessed two copies of the “small” variant, indicating that this variant potentially existed in a mutual ancestor of these canids.
Scientists have only started to piece together the puzzle of body size variation in dogs. It is critical to note here that the growth hormone regulating gene is not the singular determinant factor for dog body size and accounts for only 15% of size variation across breeds. Future work on other canine genes known to participate in body size regulation would help us uncover the extraordinary continuum of dog body sizes.
These pieces are being published as they have been received – they have not been edited/fact-checked by ThePrint.
Source: The Print
