Unraveling the genomic architecture of hock conformation defects in Pura Raza Española horses

Conformational traits define a horse's shape and morphology, heavily influencing its functionality, sport performance, health, and appearance, making conformation a critical selection criterion in horse breeding. However, the genomic basis of conformational defects, specifically those affecting the hock, remains largely unexplored. This research aimed to identify the genomic regions associated with hock defects using a large cohort of Pura Raza Española (PRE) horses. A total of 58,930 horses were evaluated for hock angle, with 4,122 genotyped on high- or medium-density arrays. For this purpose, weighed single-step genomic REML analysis methodology was used with the hock evaluation assessed as a continuous variable (lateral view hock defect, LVHD, and rear-view hock defect, RVHD) and divided into two different opposite defects: closed and open and convergent and divergent, respectively (multinomial approach). Some deviation from the optimum hock angle was observed in 49,386 horses, of which 10.1% were evaluated as having a severe or very severe defect. The most prevalent hock defect in the PRE population evaluated was open hock (6.67% considered severe or very severe), while the least prevalent was closed hock (0.2%). The estimated heritability for the continuously evaluated traits was 0.18 for LVHD and 0.25 for RVHD, while the estimated heritability of the traits evaluated with a multinomial approach ranged from 0.25 to 0.42 for open and divergent defects, respectively. A genome-wide association study revealed eight genomic regions associated with hock defects. In these, 17 genes related to musculoskeletal diseases, bone malformations, joint disorders, cartilage defects, or bone fragility were identified. These findings could support the development of selective breeding strategies aimed at reducing conformational defects in the hock of the PRE and related breeds. Furthermore, they provide insight into the genetic and molecular mechanisms involved in the occurrence of conformation defects. However, further studies are needed to shed further light on these conformational defects in the equine species.