In North America and Europe, both of which have a long racing history, the Thoroughbred population is wide and varied with a strong genetic pool. In Japan and some other countries, however, the sport is relatively new and founded on a limited number of imported horses. While that limited gene pool hasn’t put Asian racehorses at a risk of losing genetic diversity, selective breeding and low foal numbers have, researchers in Japan recently learned.
“As long as the current trend of reproduction continues, the genetic diversity of the Japanese Thoroughbred population will tend to gradually decrease, as we’ve shown it to be doing already over the past decade,” said Hironaga Kakoi, PhD, of the Laboratory of Racing Chemistry, in Utsunomiya, Tochigi.
He and colleagues recently investigated the genotypes of more than 100,000 Thoroughbreds registered in Japan from 2002 to 2016.
“The number of offspring has decreased to less than 80% over the last 15 years, and popular stallions tend to be used frequently for breeding, which possibly induces a decrease in genetic diversity,” Kakoi said. “Preferring particular horses or pedigrees is ineluctable in order to focus on good performance as racehorses—that is, selective breeding through the horse racing system. So I think that the current change of genetic diversity is a process of adaptation for racing performance.”
This doesn’t necessarily indicate a risk of inbreeding, however, Kakoi added. His team’s recent research showed that there’s still good variety in the overall gene pool, and that more than 200 Thoroughbreds, including stallions and broodmares, continue to be imported into the country annually. Breeders should take advantage of these new genetic pools, he said: “There is the potential for an increase in genetic diversity as a result of these importations.”
“Furthermore, by continuing the international exchange of horses, the robustness of genetic resources could be developed,” he added. “Through such genetic resources, the transition of the stallions and broodmares along the adaptation of racing performance can be expected, and, consequently, we can maintain genetic diversity.”
Kakoi said he doesn’t believe the worldwide ban on artificial insemination (AI) in Thoroughbred breeding is a contributing factor to the dropping genetic diversity rates in Japanese Thoroughbreds, however.
“This ban is based upon the definition and the integrity of Thoroughbred,” he said. “The current rule against AI has a role in avoiding the risk of inbreeding in terms of genetic diversity. AI might promote the development of breeding programs and rapid adaptations for racing performance; however, it could possibly actually increase the bias of reproductive use of particular horses or families over the long term.”
Going forward, the research team hopes to continue to oversee the rates of genetic diversity in Japanese Thoroughbreds in order to maintain the breed’s “integrity,” Kakoi explained.
“We intend to continuously monitor genetic diversity to maintain the effectiveness of parentage testing,” he said. “This contributes to exact pedigree registration—in other words, the definition and the integrity of the Thoroughbred.”
The study, “Evaluation of recent changes in genetic variability in Japanese thoroughbred population based on a short tandem repeat parentage panel,” was published in the Animal Science Journal.