hat makes the Paint Horse—along with the Quarter Horse—the best
sprinter in the equine world? As it turns out, it’s all in the genes. In a recent study, researchers discovered a genetic mutation responsible for producing a higher proportion of fast-twitch muscle fibers, making horses better at quicker, explosive movements. Published in PLOS Genetics in
January 2013, this was the first large-scale study to investigate how selective breeding has shaped the equine genome. Randomly selected horses were genotyped and analyzed, comparing differences and similarities between breeds and within a breed. By identifying areas of the genome shared by horses of the same breed— but that were different in horses of other breeds—researchers were able to pinpoint areas of the genome respon- sible for alternative gaits, like the pace and amble, and several regions that might play a role in determining size. But perhaps the most significant finding was the identification of genetic variants in the Paint and Quarter Horse closely associated with altered muscle-type proportions favorable for increased sprinting ability.
Compare and Contrast Jessica L. Petersen, Ph.D., a post-doctoral research fellow in the University of Minnesota’s Veterinary Population Medicine department, was one of the lead researchers in the study. “While horses were domesticated 5,000 or 6,000 years ago, the breeds
were developed recently,” Jessica said. “Breeds, when you compare them, are pretty different from one another. But when you compare individuals within a breed, they are very similar for sev- eral reasons. One is a small number of founding individuals. But it’s also largely due to selection. Breeders select for things that they can see or for the horse’s ability to do a specific task. What traits are selected depend upon the breed. We tried to find, genetically, what was driving those breed-specific traits.”
Collaborating with scientists from
more than 20 institutions around the world, she and her colleagues spent two years collecting and analyzing data from 744 horses representing 33 breeds, collected through the Equine Genetic Diversity Consortium. The study included 25 Paints and
40 Quarter Horses. Researchers did not select horses specifically bred for racing because so many of them share the same bloodlines. “This is a random sample of each of
the breeds, and that’s important,” she said. “If we have related horses, they are going to share portions of their genome just because they are related and not because this was a trait breed- ers selected for.”
After analyzing the data, they came
across an exciting discovery. “What we found in the Paint Horse was a signature of selection—a region in the genome—that was identical across all the Paint Horses in the study,” Jessica explained. “The Quar- ter Horses also shared this signature of selection. The question then was, ‘What trait is this telling us about?’
All of the Paints in the study—and most of the Quarter Horses—carried the C/C gene and an extra piece of DNA nearby that seems to indicate enhanced sprinting ability. This mutation confirms a previous discovery that indicates sprint- ing suitability in Thoroughbreds.
Sampling nearly 750 horses across 33 breeds, the study should help identify additional genetic “signatures” that could predict racing ability.
The gene that happened to be in the center of this region was myostatin, the gene involved in the double mus- cling in cows. And it has variants that can help you predict if your Thor- oughbred is a better distance racer or sprinter.” In an earlier study of the myostatin
gene in Thoroughbreds, researchers had discovered that C/C horses were best suited for fast, short-distance races. Horses that were C/T were best at middle-distance races. And T/T horses had the most stamina in long- distance races. Jessica says the mutations found in high frequency in the Paint and Quarter Horse—C/C—as well as a nearby region of extra DNA, were the same that suggest a Thoroughbred would be a better sprinter. “So the question then is, ‘How are these mutations working to make the horse a better sprinter?’ ” she said. To answer that question, they looked at muscle biopsies of 79 Quar- ter Horses. Some carried the “sprint- ing” mutation while others did not. They discovered that those Quarter Horses who carried the same muta- tion as the sprinter Thoroughbreds had a higher proportion of Type 2B muscle fibers. “Those are your fast-twitch muscle fibers,” Jessica said. “Type 2B muscle fibers contract more powerfully for