It's all in the gene: myostatin and racehorses

"Horses ain't like people, man, they can't make themselves better than they're born. See, with a horse, it's all in the gene. It's the f#cking gene that does the running. The horse has got absolutely nothing to do with it." --- Paulie (Eric Roberts) in The Pope of Greenwich Village.

For more on myostatin mutations in people and dogs, see here.

The genetic origin and history of speed in the Thoroughbred racehorse

Nature Communications 2012 Jan 24;3:643. doi: 10.1038/ncomms1644

Abstract
Selective breeding for speed in the racehorse has resulted in an unusually high frequency of the C-variant (g.66493737C/T) at the myostatin gene (MSTN) in cohorts of the Thoroughbred horse population that are best suited to sprint racing. Here we show using a combination of molecular- and pedigree-based approaches in 593 horses from 22 Eurasian and North-American horse populations, museum specimens from 12 historically important Thoroughbred stallions (b.1764-1930), 330 elite-performing modern Thoroughbreds and 42 samples from three other equid species that the T-allele was ancestral and there was a single introduction of the C-allele at the foundation stages of the Thoroughbred from a British-native mare. Furthermore, we show that although the C-allele was rare among the celebrated racehorses of the 18th and 19th centuries, it has proliferated recently in the population via the stallion Nearctic (b.1954), the sire of the most influential stallion of modern time, Northern Dancer (b.1961).

From the paper:

Athletic phenotypes are influenced markedly by environment, management and training; however, it has long been accepted that there are underlying genetic factors that influence a horse’s athletic performance capabilities. Indeed, selection and breeding of racehorses is predicated on the belief that racing performance is inherited. Although the physiological adaptations to elite athleticism and exercise are well described for the Thoroughbred, few genes have been identified to explain these traits. In humans more than 200 genes have been reported to be associated with fitness-related health and exercise traits17, and it is likely that racing performance in the Thoroughbred is also polygenic and is influenced by genes that contribute to the wide range of anatomical, metabolic and physiological adaptations that enable elite-racing performance. The athletic potential of a racehorse will therefore depend on a favourable environment as well as inheriting the optimal combination of DNA variants at loci that significantly affect exercise.

Recently, variation at the MSTN locus has been found to be highly predictive of genetic potential for race distance aptitude in Thoroughbred racehorses18–21 and contributes to morphological type in other horse breeds22. The MSTN locus is associated with muscle hypertrophy phenotypes in a range of mammalian species23–27 and a single nucleotide polymorphism (SNP, g.66493737C/T) located in the first intron of the MSTN gene influences speed in the Thoroughbred19. Thoroughbred homozygous C/C horses are best suited to fast, shortdistance, sprint races (1,000–1,600m); heterozygous C/T horses compete favourably in middle-distance races (1,400–2,400m); and homozygous T/T horses have greater stamina (>2,000m). Evaluation of retrospective racecourse performance, physical growth and stallion progeny performance has demonstrated that C/C and C/T horses are more likely to be physically precocious and enjoy greater racecourse success as 2-year-old racehorses than T/T horses19,28.

These findings have been subsequently validated in three independent genome-wide association (GWA) studies18,20,21 in populations of Thoroughbreds originating in Ireland, Great Britain, New Zealand20, USA18 and Japan21. The singular, genomic influence on optimum race distance at the MSTN locus in the Thoroughbred is supported by a high heritability for race distance (h2=0.94)29. Further evidence for the role of chromosomes containing the C-allele in influencing speed comes from association tests with field-measured speed indices30 and from previous analysis of g.66493737C/T genotypes in the Quarter Horse, for which a high frequency of C/C homozygotes (0.83) has been reported19. The Quarter Horse is a North-American breed that excels at sprinting over distances of a quarter of a mile (400m) or less. Because of the specificity of its role in short-distance racing, this breed has undergone intense selection for speed since its foundation in the mid-1800s.