Abstract: Polydactyly is a common hereditary limb malformations in vertebrates.Equine polydactyly presents additional digits on limbs, which can be classified into three categories:Unilateral polydactyly, bilateral polydactyly, and four limb polydactyly according to the number and positions of toes.The type of unilateral polydactyly is the most popular, accounting for 58% in all.Hereditary variations are the main factors attributed to polydactyly, and the inheritance pattern of polydactyly are relatively complex.Evidence had shown that polydactyly is associated with chromosomal variation in horses.However, due to the scarcity coupled with singleton and long generational intervals, it is difficult to construct large-scale lineages to identify mutations using genome-wide association study (GWAS) and forward-genetic methods.Consequently, the molecular mechanisms and genes of horses polydactyly are still not clear till now.The growing experimental evidence in human and other species illustrated that the signaling pathways controlling limb development are conserved and gene expression is temporal.A total of 199 polydactyly-associated genes are known in different species, and some of them have pleiotropy.Gene similarity analysis revealed 178 homologous genes in horses, of which 56% had >90% similarity and 96% had >80% similarity.Therefore, the polydactyly genes found in other species could be the candidate genes for equine polydactyly, offering valuable insights into the molecular mechanisms of this condition.

Key words: horse; polydactyly; morphological analysis; signal pathway; candidate gene