Performance of Swedish Warmblood fragile foal syndrome carriers and breeding prospects

Abstract

Background Warmblood fragile foal syndrome (WFFS) is a monogenetic defect caused by a recessive lethal missense point mutation in the procollagen‐lysine, 2‐oxoglutarate 5‐dioxygenase 1 gene (PLOD1, c.2032G>A). The majority of homozygous WFFS horses are aborted during gestation. Clinical signs of affected horses include fragile skin, skin and mucosa lacerations, hyperextension of the articulations, and hematomas. In spite of its harmful effect, a relatively high frequency of WFFS carriers has been found in Warmblood horses, suggesting a heterozygote advantage. Thus, in this study our aims were to: (1) estimate the frequency of WFFS carriers in the Swedish Warmblood breed (SWB), (2) estimate the effect of WFFS carrier genotype on performance traits in two SWB subpopulations bred for different disciplines, and (3) simulate the potential effects of balancing selection and different selection strategies on the frequency of carriers. Methods In total, 2288 SWB sport horses born between 1971 and 2020 were tested for the WFFS mutation and had estimated breeding values (EBV) for ten traditional evaluating and 50 linear descriptive traits. Results The frequency of WFFS carriers calculated from a pool of 511 randomly selected SWB horses born in 2017 was equal to 7.4% and ranged from 0.0 to 12.0% among the whole set of tested SWB horses, starting from 1971 till 2020. The effect of the WFFS carrier genotype was significant for several EBV mainly related to movements and dressage traits and especially for horses not bred for the show jumping discipline. Using simulation, we showed that balancing selection can maintain a recessive lethal allele in populations such as the SWB breed over generations and that the frequency is expected to slowly decrease in absence of balancing selection. Finally, we showed that selection against carrier sires can result in a more rapid decrease of the frequency of the mutant allele over time. Conclusion Further research is needed to confirm the apparent association between equine performance and the WFFS carrier genotype. Identification of such associations or new causative mutations for horse performance traits can serve as new tools in horse breeding to select for healthy, sustainable, and better performing horses.

767. Haplotype blocks and heterozygosity rich regions on ECA2 in Swedish Warmblood horses

“…Despite its harmful effect, a relatively high frequency of FFS carriers (7.4%) was found in the Swedish Warmblood breed (SWB), suggesting the presence of a heterozygous advantage. The presence of balancing selection in SWB was corroborated in a recent study where significant positive effects of the FFS allele were found mainly on movement related traits (Ablondi et al, 2022). Balancing selection causes various types of signatures in the genome such as: increased diversity around the target of selection, differentiation between populations departing from the same genome wide background, and increased linkage disequilibrium around the target of selection (Fijarczyk and Babik, 2015).…”

“…In NS horses, we found a haplotype block containing the FFS variant which was not detected in the SJ horses. A potential reason behind the failure to detect the haplotype in the SJ may be the lower frequency (5.8% in SJ vs 14.6% in NS) of the FFS variant in SJ horses as previously found by Ablondi et al (2022) or that the selection pressure for heterozygosity in PLOD1 is lower in SJ horses. In NS horses, the FFS variant was in strong LD with a SNP located within the Mitofusin 2 (MFN2) coding gene.…”

Breeding of Swedish Warmblood horses towards specialization in show jumping and dressage

“…It is also important to take notice to traits that have a linear relationship with performance, because selection towards more extreme expression of certain traits may have negative consequences on the welfare of the horse. For example, it has been shown that horses that are carriers of the Fragile Foal Syndrome mutation have more elastic trot than non-carriers, indicating that breeding for that trait could increase the prevalence of the deleterious mutation in the population (Ablondi et al, 2022). Thus, monitoring of health and durability in the population is highly recommended, which would require cooperation between the breeding organization, veterinarians, insurance companies, and horse owners.…”

764. Recent advances in genomics of equine health, welfare and performance

“…A likely causal variant is known in 99 of these traits or disorders, where 54% of them represent coat colours, 41% are disease-causing variants, and 4% of the variants are associated with performance traits. Among the most recent studies of important monogenic diseases are Fragile Foal Syndrome (Ablondi et al 2022), Dwarfism and joint laxity in Frieasian horses (Leegwater et al 2016), and Ocular Squamous Cell Carcinoma (Chen et al 2021).…”