Two faces of lethal gene found in study of Swedish Warmblood horses

Researchers have investigated the carrier frequency of the gene responsible for Warmblood fragile foal syndrome in Swedish Warmbloods. Image: General file photo
Researchers have investigated the carrier frequency of the gene responsible for Warmblood fragile foal syndrome in Swedish Warmbloods. File image

Swedish Warmblood horses who carry the gene responsible for fragile foal syndrome tend to have better movement and dressage traits than non-carriers, researchers report.

However, if the recessive gene is inherited from both parents, foals will generally be lost to abortion. Those who make it to full term will be affected by thin and fragile skin, lacerations, bruising and hyperextensible joints. Affected foals are invariably euthanized soon after birth.

Despite its harmful potential, a relatively high frequency of carriers has been found in Warmblood breeds around the world.

Recent studies have reported carrier frequencies ranging from 11 to 30% among several Warmblood breeds. In contrast, a much lower frequency of 2.4% was observed among 716 tested Thoroughbreds.

Researchers with the University of Parma in Italy and the Swedish University of Agricultural Sciences noted that, in a 2020 study, the fragile foal syndrome allele was reported to be associated with estimated breeding values (the genetic merit for each trait) for conformation, gaits, and dressage ability in Warmblood horses.

The study team set out to examine the performance of Swedish Warmblood horses that carry the gene, and breeding prospects.

In total, 2288 Swedish sport horses born between 1971 and 2020 were tested for the Warmblood fragile foal syndrome mutation and had estimated breeding values for 10 traditional evaluating and 50 linear descriptive traits.

Michela Ablondi and her fellow researchers, writing in the journal Genetics Selection Evolution, found that the frequency of carriers calculated from a pool of 511 randomly selected Swedish Warmbloods born in 2017 was equal to 7.4%.

It is clear, they said, that the fragile foal syndrome allele was present in the Swedish Warmblood population in the mid-1980s.

The effect of the carrier genotype was significant for several estimated breeding values, mainly related to improved movement and dressage traits, especially for horses not bred for showjumping.

They said the frequency of the harmful gene in Warmbloods appears to have been driven by what is known as balancing selection, with selection of horses for favourable traits helping to maintain the gene in the population.

“While it is possible for any allele to increase in frequency due to genetic drift, a recessive lethal allele is unlikely to become common by genetic drift alone, even in a small population,” they said.

They noted that a genetic test for the allele is currently used in Swedish Warmbloods to avoid mating between carriers. This policy, they said, improves animal welfare by eliminating the risk of affected foals.

The authors ran simulations to investigate the effect of balancing selection and selection against carriers of a recessive lethal allele, using a population modelled on Swedish Warmbloods.

The work showed that balancing selection can maintain a recessive lethal allele in populations such as the Swedish Warmblood breed over generations, whereas the frequency is expected to slowly decrease in the absence of balancing selection.

The results showed that selection against carrier sires can substantially reduce the frequency of the lethal allele, even when not all carrier sires are excluded.

Discussing their findings, the researchers said that while the policy of using genetic testing for the gene to avoid mating between carriers to eliminate the risk of affected foals being born is beneficial for animal welfare, the simulations showed that such testing and mating strategies are not expected to decrease the frequency of carriers over time.

“If, instead, the aim is to decrease or eliminate the Warmblood fragile foal syndrome allele in the population, selection against carriers would be needed because the decline of such a recessive lethal allele is slow, even in the absence of balancing selection, with the allele still persisting in the simulated populations at a frequency of about 5% after 20 generations.”

The simulations showed that selection against carrier sires led to a much more rapid decline of the lethal allele, decreasing the frequency of carriers to less than 1% within four generations.

“Selection against carrier sires does not need to be complete, since allowing a few carrier sires to be included in the breeding program did not make much difference in the rate of decline of the frequency of carriers.

“However, in practice, the effect of allowing carrier sires will also depend on to what extent they are used in breeding.

“Simulations have shown that the use of popular sires increases the risk of lethal alleles becoming frequent in a population. Thus, an alternative suggestion could be to limit the number of mares that are mated to … carrier sires.

By selecting only carrier stallions if they are in the top 10% in terms of breeding goals, the frequency of the harmful allele would decrease substantially in a few generations, while maintaining genetic gain.

The authors said they also detected an association between the lethal gene and conformation traits, with carriers having longer body conformation.

“In the future,” they said, “we will aim at gaining more knowledge about performance traits under selection in sport horses by identifying associated genetic markers or novel causative mutations which might serve as new tools in horse breeding to select for healthy, sustainable, and better-performing horses.”

The study team comprised Ablondi and Alberto Sabbioni, with the University of Palma; and Martin Johnsson, Susanne Eriksson, Åsa Gelinder Viklund and Sofia Mikko, all with the Swedish University of Agricultural Sciences.

Ablondi, M., Johnsson, M., Eriksson, S. et al. Performance of Swedish Warmblood fragile foal syndrome carriers and breeding prospects. Genet Sel Evol 54, 4 (2022).

The study, published under a Creative Commons License, can be read here

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