A significant rise in inbreeding levels has occurred in the global Thoroughbred population over the last 45 years, according to researchers.
The research, published in the peer-reviewed journal Scientific Reports, analysed the genomes of more than 10,000 thoroughbreds – the largest set of horses examined to date.
The research linked the increase in inbreeding to selection for favourable racing traits and the influence of popular sire lines – 97% of horses in the study traced to Northern Dancer, and 35% and 55% of European and Australasian horses had Sadler’s Wells and Danehill, respectively, in their pedigrees.
Selection for popular sire lines reduces genetic diversity which can lead to inbreeding depression.
The research team was led by University College Dublin’s Professor Emmeline Hill, who specialises in equine genomics. Hill is also chief science officer with equine DNA testing company Plusvital, which was involved in the study.
“Inbreeding has always been high in Thoroughbreds,” Hill says, “but it is getting higher.
“It is likely that, unchecked, inbreeding in the Thoroughbred will continue to increase in a market where there is high demand for particular sire lines.
“The problem with inbreeding is that it can compromise overall population fertility and health. This is a highly significant issue akin to global warming, where inbreeding is accumulating in the population, that must be addressed at an industry-wide level.”
Unlike most managed animal production systems, there is no systematic, industry-mediated genomic selection or population management for the Thoroughbred.
However, Hill stressed that solutions to this issue now exist.
“Our new data can assist the industry to monitor inbreeding and DNA-based tools have been developed that individual breeders can use to reduce the problem by choosing genetically diverse stallions for their mares.
“Pedigree is not powerful enough to help anymore. Pedigree can be useful in highlighting broad trends in breeding practices, such as the predominance of certain sire lines leading to a high degree of relatedness.
“However, since Thoroughbreds are now so closely related, there is no longer the resolution in a pedigree to accurately infer relatedness between individuals,” she says.
“Multiple studies show that pedigree-based estimates of relatedness are less accurate than DNA-based methods which measure the true genetic relationship between individuals. Suitable outcrosses will be best identified using genetic data.”
Higher inbreeding is not associated with superior racing performance, and Hill reiterated that the research should be seen as a positive for the industry.
“The purpose of this research is not to upset or disrupt an industry that works every day in the best interest of the horse, but rather to provide information that, if harnessed in the appropriate way, could be beneficial to ensure the future sustainability of the breed.”
In their paper, Hill and her colleagues say their findings show that centuries of selection for favourable athletic traits among Thoroughbreds had acted on genes with functions in behaviour, musculoskeletal conformation and metabolism.
A genomics-based approach to identify genetic outcrosses will add valuable objectivity to augment traditional methods of stallion selection, they say.
“Genomics-based methods will be beneficial to actively monitor the population to address the marked inbreeding trend.”
They characterised the increase in inbreeding in the global Thoroughbred population during the last five decades as highly significant.
They say it is unlikely to be halted with current breeding practices.
Such inbreeding results in a mutational load in populations that may negatively impact on population viability.
“‘Genetic rescue’ of highly inbred populations may be possible by the introduction of genetically diverse individuals,” they wrote.
However, rescuing genetic diversity in the Thoroughbred will be challenging due to the limitations of a closed studbook and the disproportionate influence on the genetic composition of the breed by a limited number of stallions.
McGivney, B.A., Han, H., Corduff, L.R. et al. Genomic inbreeding trends, influential sire lines and selection in the global Thoroughbred horse population. Sci Rep 10, 466 (2020) doi:10.1038/s41598-019-57389-5