The role of Myostatin polymorphisms in the Finnhorse and Shetland pony breeds
Abstract: Myostatin, encoded by the MSTN gene, is a member of the transforming growth factor β that normally acts to limit skeletal muscle mass by regulating both the number and growth of muscle fibers. Natural mutations that decrease the amounts of myostatin and/or inhibit its function have been identified in several cattle, sheep and dog breeds, where loss-of-function mutations cause increased skeletal muscle mass and produce a phenotype known as “double-muscling”. This gene has also been associated with racing performance in Thoroughbreds, where studies have found two Myostatin polymorphisms (PR3737 and PR8604) to be strongly associated with genetic potential and athletic phenotype, affecting both speed and muscularity, although no such associations have been found in harness-racing breeds such as the Swedish-Norwegian Coldblooded trotter. Single nucleotide polymorphisms have also been shown to have an effect on conformation, where these SNPs have been found to have different genotype distributions between horse breeds with different origin, morphology and uses. Such is the case with Icelandic horses, where significantly different genotype distributions for another SNP, PR5826, were observed between horses used for different purposes. This study investigated the association of these MSTN polymorphisms with harness racing performance in the Finnhorse and body conformation in the Shetland pony. Finnhorses used for different disciplines were genotyped for three SNPs (PR5826, PR3737 and PR8604) and were divided into three categories based on their use: harness racing horses (n=223), representing those horses with at least one start in harness racing; riding horses (n=79) used for recreational riding of which owner questionnaire information was available and horses with no performance data (n=112). An association analysis was performed on the raced group, where the genotypes were evaluated for association with life-time racing performance results for: number of starts, victories, placings (1-3) and unplaced, along with proportions for each of these traits, as well as earnings, earnings per start and race times for volt- and autostart. Additionally, the genotypes were evaluated for association with these performance results obtained between 3 and 6 years of age (n=207) and 7 to 10 years of age (n=183). Significant associations were found in the 3 to 6 years of age group as well as the 7 to 10 group. In both cases, TT horses for PR3737 earned more money, were faster for both racing methods and presented a lower number of disqualifications than the CT horses. Concerning PR8604, a similar effect was observed, where the TT horses also earned more money, were faster in auto start racing method and presented a lower number of disqualifications. This study concluded that MSTN sequence polymorphisms seem to have an effect on harness racing performance in the Finnhorse. Concerning body conformation on the Shetland pony, a breed which presents a “Heavy” body type and a “Light” body type, no significant associations were found between conformation and MSTN genotype. However, further investigation is needed before drawing the conclusion that MSTN has no effect on Shetland pony conformation. This is due to the small sample size and classification method, which could be expanded to include morphological measurements.
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