The earliest record of color variation in dogs is from 10,000 year old archeological evidence. Now, finally, the intensity of this variation has been explained genetically due to the work of researchers at the University of California-Davis. In this research, the use of genome-wide association was used for specific dogs to determine the number of copies of a copy number variant (CNV), which is a repeating section of the genome, present in their DNA. A genome-wide association study involves rapidly scanning a genome to find areas where there is genetic variation. By using this method, the researchers discovered a genetic region that controls color intensity in a number of dog breeds.

They found that a CNV in the KITLG gene on chromosome 15 controls color intensity in dogs. The higher the number of copies of this CNV, the more intense the color of the dog’s coat. 

Pigmentation in dogs can come from the chemicals pheomelanin (yellow) and eumelanin (black). One gene that has been found to control the variation of these pigments in dogs is the KITLG gene. To evaluate the genetic basis for light and dark red phenomelanin, the UC-Davis researchers conducted a genome-wide association study in 35 Nova Scotia Duck Tolling Retrievers. These dogs were classified as light (n=23) or dark (n=13). 

Blood and saliva were collected from these dogs through the UC-Davis Veterinary Medical Teaching Hospital. To compare the CNVs in the genetic data from different Nova Scotia Retrievers, the researchers used the PCR method, where DNA is broken down into pieces and and then multiplied to attain a number of copies of a specified portion of the DNA. Next, they performed the genome-wide association study across the KITLG gene and then compared the number of CNVs. The results showed that dark red hair Nova Scotia Retrievers had significantly more CNV copies compared to light red hair Retrievers. 

Image from malp007 via Flickr. A Nova Scotia Retriever training with a wooden duck.
These dogs were originally known as the little river duck dog. They have webbed feet and a double coat so they are well adapted to water and so they have been used for waterfowl hunting for two centuries.

These comparisons were repeated in multiple different breeds such as light grey breeds like the Bearded Collies and Old English Shepherds, and also in black breeds like Rottweilers and Black Coated Retrievers. The results showed that there was a correlation between high CNV copy numbers and dog hair color intensity.  

This correlation was not found in Poodles, a breed with longer hair. Analysis along the hair shaft showed that Poodles with higher CNV copy numbers had more uniform color intensity and a higher average hair color intensity along the length of their hairs . Poodles with lower CNV copy numbers had less color intensity in the roots of their hairs, more color intensity at the tips,  and less overall color intensity. This led to the hypothesis that higher CNV copy numbers lead to a more uniform color intensity and a higher average color intensity. 

Prior to these discoveries, the understanding of pigmentation in dogs was that it was controlled through the pigment switching genes MC1R and ASIP. The alteration of MC1R and ASIP leads to the different variations in color. For example, the appearance of solid red/ yellow means that there is a loss of function of MC1R which results in the sole production of pheomelanin.  

In this new study, by comparing genetic data, the researchers have demonstrated that the CNV in the KITLG gene is a newly discovered portion of DNA that controls the intensity of eumelanin and pheomelanin intensity in a number of dog breeds with a spectrum of coat colors. However, there are some breeds where this correlation has been found to be inconclusive; namely in Golden Retrievers and Labrador Retrievers, where there is no significance in CNV copy numbers. Due to large scale artificial breeding practices focused on varying coat colors in dogs, it is not a surprise that there are many different regions of DNA that affect pigmentation in dogs. This would explain why the results seen in these two Retriever breeds are inconclusive as there might be a different DNA region that controls their color. 

 

Weich, K., Affolter, V., York, D., Rebhun, R., Grahn, R., Kallenberg, A., and Bannasch, D. 2020. Pigment Intensity in Dogs is Associated with a Copu Number Variant Upstream of KITLG. Genes 11(1): 75.  

For more information, go to:  https://www.mdpi.com/2073-4425/11/1/75.