Evolutionary Map of Dog Breeds Yields Insights into Breed Origins

Investigators from the National Institutes of Health have created a genetic map of dog breeds, classifying modern breeds into clades (groups sharing common ancestors). The work clarifies the effects of human migration on dog breed development and will allow researchers to track disease-causing gene mutations among breeds, say the authors. The study was published in Cell Reports.

The researchers sequenced DNA samples from 1346 dogs representing 161 breeds. To examine the effects of geography and migration on breed development, they sampled dogs from 4 continents: North America, Europe, Africa, and Asia.

Nearly all of the breeds could be sorted into one of 23 clades. Most breeds are closely related only to others within their own clade, say the authors. A few, however, share large amounts of genetic material with dogs in multiple clades.

• The pug is close kin to the Brussels griffon, a member of its own clade, and is also strongly related to small breeds in many other clades. Pugs were exported from Asia and used to develop other small breeds, write the authors.
• The chinook was created in the early 1900s from multiple breeds. The genomic map underscores this history, showing that the chinook shares genetic material with breeds in several clades.

Some clade groupings illustrate the effects of human migration and of breeding for specific purposes. Over thousands of years, common traits appeared in dogs around the globe as they were selected to fill particular functions, such as herding or flock guarding. In the past 200 years, the number of breeds surged as breeding was fine-tuned to select for certain physical traits or behaviors.

• All the breeds created by crossing terrier and bully breeds are genetically related to terriers of Ireland dating to the 1860s, say the authors. They attribute this finding to crossbreeding to produce fighting dogs.
• The xoloitzcuintli (Mexican hairless) is related to the German shepherd. The authors write that this unexpected pairing probably reflects colonization of the Americas by Europeans, who brought along both livestock and dogs to manage the livestock. Other hairless breeds native to South and Central America are also genetically related to herding dogs.
• Dogs were originally brought to the Americas by the earliest humans migrating from east Asia. The original New World dogs were superseded by dogs of European and other Asian ancestry. However, the researchers found evidence that genetic material from New World dogs is still present in some modern breeds from the Americas.
• The researchers sampled Tibetan mastiffs, salukis, and cane corsos from the United States and from each breed’s country of origin. For all three breeds, the US gene pool was less diverse (more inbred) and was also more admixed with other breeds.

The investigators also found evidence of genetic mutations shared across clades.

• Collie eye anomaly affects herding breeds such as the collie, Border collie, and Australian shepherd. The disease has also been found in Nova Scotia duck tolling retrievers, a breed not obviously related to collies. The analysis revealed that the collie did in fact contribute genetic material to the Nova Scotia duck tolling retriever, explaining the presence of the disease in this breed.
• MRD1 gene mutations cause severe drug reactions in affected German shepherds and chinooks, breeds found in the analysis to share genetic material. The authors suggest that the xoloitzcuintli may also carry the mutation via its relationship with the German shepherd.

Adding more breeds to the dataset will produce a denser evolutionary map, say the authors. They conclude that the genomic information gathered to date can help researchers identify gene variants responsible for breed development, behavior, and disease.

Dr. Laurie Anne Walden received her doctorate in veterinary medicine from North Carolina State University. After an internship in small animal medicine and surgery at Auburn University, she returned to North Carolina, where she has been in small animal primary care practice for over 20 years. Dr. Walden is also a board-certified editor in the life sciences and owner of Walden Medical Writing, LLC. She works as a full-time freelance medical writer and editor and continues to see patients a few days each month.