September 14, 2009

Three Gene Variants Account for Most Dog Coat Differences

Photos of long-haired and  short-haired Chihuahuas The Chihuahua on the left is an example of a dog with a long coat. The Chihuahua on the right is the short-haired variety. Tyrone Spady, NHGRI.

The wide range of coat textures seen in dogs—from the poodle's tight curls to the beagle's straight fur—stems from variants in just 3 genes acting in different combinations, according to a new study. The finding shows how an array of traits can be reduced to the effects of just a few genes.

Modern dog breeds diverged from wolves some 15,000 years ago. But in just the past 200 years, selective breeding by humans has created dogs with a broad range of qualities. This rapid genetic change makes dogs ideal for studies of how genes can affect physical characteristics.

Researchers have long understood the genetic basis of dogs' coat colors. However, relatively little was known about the genes influencing coat length, growth pattern and texture. To investigate, a team led by researchers from NIH's National Human Genome Research Institute (NHGRI) performed a genome-wide scan of tiny variations in DNA, called single-nucleotide polymorphisms (SNPs), in 1,000 dogs representing 80 breeds.

The scientists compared the SNPs with 3 distinct coat characteristics. The first was wiry hair in a growth pattern that makes the dogs look like they have a mustache and eyebrows. Examples include Scottish terriers, Irish terriers and schnauzers. The second characteristic was long, silky or fluffy hair like that found in Cocker spaniels, Pomeranians and long-haired Chihuahuas. The third was curly hair, such as that of the Irish water spaniel. The scientists reported their findings on August 27, 2009, in the advance online edition of the journal Science.

Remarkably, they identified a single genetic change accounting for each coat characteristic. An alteration in the RSPO2 gene (encoding the R-spondin-2 protein) results in wiry hair. Long hair was linked to a variant in the FGF5 gene (which encodes the fibroblast growth factor-5 protein). A variant in the KRT71 gene (encoding the keratin-71 protein) produces curly coated dogs. Combinations of the 3 variants can produce dogs, such as poodles and Portuguese water dogs, with all 3 characteristics. The researchers calculated that these 3 mutations in different combinations can explain the coat characteristics of 95% of dogs.

None of these 3 mutations are found in gray wolves or short-haired dogs. That strongly suggests that a single mutation occurred for each trait and was transferred through selective breeding, creating a remarkable diversity from simple genetic underpinnings.

"This study is an elegant example of using genomic techniques to unravel the genetic basis of biological diversity," says NHGRI Scientific Director Dr. Eric Green.

In addition, canine studies may help uncover biological mechanisms that are relevant to human disease."The carefully controlled breeding of dogs offers advantages in pinpointing the genes that determine particular traits, which may have immediate application to the study of diseases, like cancer, that are common to both dogs and humans," says lead author Edouard Cadieu, a graduate student in NHGRI's Cancer Genetics Branch.

—by Harrison Wein, Ph.D.

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