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Untangling Canine Coiffures
27 August 2009 (All day)
It doesn't take much, genetically, to turn the buzzcut of a bloodhound into the silken mane of a bearded collie. Although dog hair seems to come in an infinite variety of lengths, thicknesses, and styles, most breeds' looks are dictated by the particular versions of just three genes in that breed's genetic makeup, according to a new study. That so few genes are involved speaks to the power of artificial selection among dogs and to the ability of just a few genetic changes to make a big difference in appearance.
Ever since researchers pinpointed a gene for short stature in purebred dogs, they have been mining canine genomic data for genes underlying other traits. In the new study, Elaine Ostrander of the National Human Genome Research Institute in Bethesda, Maryland, and her colleagues sought to understand such canine curiosities as why schnauzers have bushy eyebrows and mustaches, poodles have the Shirley Temple look, and basset hounds are clean-cut types.
The study, reported online today in Science, had three main components. First, to begin to pin down some of the genes involved, the researchers assessed genetic variation at about 50,000 spots in the genome of more than 100 dachshunds--which come in short-hair, long-hair, and wiry-hair varieties--and correlated the sequence differences at those spots with the type of hair. Variations at one spot on chromosome 13 were associated with wiry hair, mustaches, and eyebrows--known as furnishings among dog fanciers. With additional study, the researchers pinpointed the gene responsible. Called R-spondin-2, it encodes a protein that interacts with other proteins that are important for the development of hair follicles and of hair-follicle tumors that are often seen in dogs with mustaches.
With respect to hair length, the researchers confirmed that a mutation in a gene called FGF5 coincides with long hair. They determined that a base change that swaps an amino acid in the FGF5 protein seems to be responsible. Looking at a larger set of dog breeds, the researchers saw that change in 91% of the long-haired dogs they evaluated, 4% of short-haired dogs, and 30% of dogs with medium length hair.
In the second part of the study, the researchers did a similar genetic analysis with 76 Portuguese water dogs, about half with curly hair and half with wavy hair. A mutation in a gene for keratin, a protein found in hair, was associated with curliness.
The team had studied 903 dogs from 80 breeds to pin down these mutations, and in the final part of the study, they examined these and additional dogs to see how mutations at these three genes worked together to produce various coat types. Combinations of variations in R-spondin-2, FGF5, and keratin genes explain the seven coat types that characterize most of the dogs sampled, which represented 108 breeds. Short-hair breeds such as basset hounds, for example, have none of the three mutations; dogs with just the R-spondin-2 change had furnishings--picture an Australian terrier--and those with that change and the change in FGF5 have furnishings and long, soft coats, like bearded collies. All three mutations are found in dogs with long, curly hair and mustaches--such as the bichon Frisé.
That the same genes are at work in many breeds indicates that these mutations are very old. "All the unique characteristics are something that occurred once a long time ago and have been preserved ever since," notes Gregory Acland, a geneticist at Cornell College of Veterinary Medicine. When breeders shuffled these genes, they created dogs with new types of coats.
Åke Hedhammar of the Swedish University of Agricultural Sciences in Uppsala would now like to know how these genes exert these effects and whether they are at work in other species. That's a logical next step, says Acland: "Once you have a candidate gene in one species, it's easier to ask if it's involved in [a different] species."