Canine morphology: hunting for genes and tracking mutations

Abstract

In this essay, Abigail Shearin and Elaine Ostrander discuss the proposed genomic mechanisms for the extraordinary level of phenotypic variation observed in the domestic dog and the evidence detailing the variants responsible for the many shapes, sizes, textures, and colors of man's best friend.

Figure 1. Morphological variation in the dog.

Dog breeds display extremes of morphological variation including body size and proportion, head size and shape, coat texture, color, and patterning. Clockwise from the left: the Bloodhound, the Chinese-crested, the Dandie Dinmont terrier, the Scottish deerhound, the long-haired Chihuahua, and the French bulldog. (Image: Mary Bloom, American Kennel Club).

Figure 2. Linkage disequilibrium in the dog.

The average LD distances were established by Gray et al. for several breeds based on the distance, where r² decays to two . LD distances for breeds denoted with an (*) were established previously by Sutter et al. and are based on the distance where D′ falls to half its maximum value . The degree to which LD varies between breeds is remarkable and the fine mapping of traits is greatly facilitated when data from multiple breeds can be combined. The level of LD within a breed can be attributed to a number of factors: the historical use and popularity of the breed; the effective population size; bottlenecks due to size of the starting population; popular sire effects; and breeding practices which allow matings between closely related individuals.

Figure 3. Size variation in the dog.

Variation in skeletal morphology in the dog is a complex phenotype, with IGF-1 as a major determinant of small size . The difference in overall body size between a Cane Corso and a Yorkshire terrier is over 30-fold, yet both are members of the same species, Canis familiaris.

Figure 4. Coat variation in the dog.

Coat texture and length are features that distinguish between breeds and between varieties of the same breed . Clockwise from the left are shown the Vizsla with a short, straight coat. These dogs and others like them have wild-type alleles for the three critical genes controlling coat texture, length, and curl, which are RSPO-2, FGF5, and KRT71, respectively. The giant Schnauzer displays the eyebrows and moustache characteristic of the trait called “furnishings” and carries the variant form of RSPO-2. Dogs with furnishings usually exhibit wiry coats as well. The Cocker spaniel has long straight hair, demonstrating the variant form of FGF5, but wild-type alleles at other loci. The Bichon frise has variant alleles at all three critical loci, RSPO-2, FGF5, and KRT71, and displays a coat that is long, curly, and with furnishings. (Image: Giant schnauzer and Bichon frise pictures provided by Mary Bloom, American Kennel Club.)