Single-Step Genomic Reaction Norm and ssGWAS Reveal Heat-Load-Dependent Genetic Architecture of Age at Puberty in Yorkshire Gilts

Heat stress adversely affects porcine reproduction, but genetic variation in the response of age at puberty (AP) to heat exposure remains poorly characterized in gilts. We analyzed 4438 AP records from Yorkshire gilts using adjusted meteorological data and compared three heat-exposure gradients across five prepubertal windows. Pedigree- and single-step genomic reaction-norm models were fitted, followed by single-step genome-wide association analyses of the intercept and slope. Among the 15 exposure scenarios, the best-fitting model used the proportion of sustained heat-stress days in the 14-day window before first observed estrus; a one-standard-deviation increase in this measure was associated with a 7.64-day increase in AP. Both reaction-norm models detected significant genetic variance for environmental sensitivity (p < 0.001). The genomic intercept–slope genetic correlation was −0.524, and genetic variances and correlations varied across the exposure gradient. GEBVs rankings at the 10th and 90th percentiles of the exposure gradient were negatively correlated (Spearman’s ρ = −0.742), and the most favorable 10% of gilts did not overlap between these two exposure levels. Intercept and slope window contributions were strongly correlated (Spearman’s ρ = 0.822), although component-specific regions were also detected. Functional annotation highlighted candidate genes and biological processes related to neuroendocrine regulation, ovarian development, mitochondrial and redox homeostasis, and cellular stress responses. These findings reveal genetically heterogeneous heat sensitivity in AP, supporting environment-specific genomic evaluation for reproductive robustness in gilts.