Prevalence of the Fragile Foal Syndrome (FFS) type I mutation in Italian equine breeds and genealogical study on the introgression of the allele in the Maremmano population

Fragile Foal Syndrome (FFS) is a lethal autosomal recessive disorder of the connective tissue caused by the PLOD1:c.2032G > A mutation. Although widely investigated in warmblood populations, information on its prevalence in Italian breeds is limited. This study assessed the prevalence of the FFS allele in the Maremmano horse and other Italian breeds and reconstructed the genealogical introgression dynamics within the Maremmano population. A total of 332 Maremmano horses, along with samples from seven Italian breeds were genotyped using a PCR-RFLP approach. A pedigree analysis was performed to successfully reconstruct the variant's genealogical transmission, estimate inbreeding coefficients and calculate Thoroughbred blood percentage over time. In the Maremmano, the carrier frequency was 10.24% (allele frequency 5.12%). The allele was also detected in Tolfetano (6.25%) and Anglo-Arab (1.69%). The genealogical study in the Maremmano horse revealed that all 34 carriers trace back to a common ancestor: a key mare born post-WWII with documented Thoroughbred ancestry. Pedigree reconstruction identified a parsimonious ancestral transmission pathway compatible with historical Thoroughbred introgression facilitated by historical breeding practices. The relatively high carrier frequency in the Maremmano population reflects the persistence of a historical introgression event within a structured breeding system. Our results emphasise the importance of integrating genetic testing with pedigree-informed breeding strategies to reduce the risk of FFS in foals and improve the health of horse populations.