Abstract
Improving swine climatic resilience through genomic selection has the potential to minimize welfare issues and increase the industry profitability. The main objective of this study was to investigate the genetic and genomic determinism of tolerance to heat stress in four independent purebred populations of swine. Three female reproductive traits were investigated: total number of piglets born (TNB), number of piglets born alive (NBA) and average birth weight (ABW). More than 80,000 phenotypic and 12,000 genotyped individuals were included in this study. Genomic random-regression models were fitted regressing the phenotypes of interest on a set of 95 environmental covariates extracted from public weather station records. The models yielded estimates of (genomic) reactions norms for individual pigs, as indicator of heat tolerance. Heat tolerance is a heritable trait, although the heritabilities are larger under comfortable than heat-stress conditions (larger than 0.05 vs. 0.02 for TNB; 0.10 vs. 0.05 for NBA; larger than 0.20 vs. 0.10 for ABW). TNB showed the lowest genetic correlation (-38%) between divergent climatic conditions, being the trait with the strongest impact of genotype by environment interaction, while NBA and ABW showed values slightly negative or equal to zero reporting a milder impact of the genotype by environment interaction. After estimating genetic parameters, a genome-wide association study was performed based on the single-step GBLUP method. Heat tolerance was observed to be a highly polygenic trait. Multiple and non-overlapping genomic regions were identified for each trait based on the genomic breeding values for reproductive performance under comfortable or heat stress conditions. Relevant regions were found on chromosomes (SSC) 1, 3, 5, 6, 9, 11, and 12, although there were important regions across all autosomal chromosomes. The genomic region located on SSC9 appears to be of particular interest since it was identified for two traits (TNB and NBA) and in two independent populations. Heat tolerance based on reproductive performance indicators is a heritable trait and genetic progress for heat tolerance can be achieved through genetic or genomic selection. Various genomic regions and candidate genes with important biological functions were identified, which will be of great value for future functional genomic studies.
Highlights
Heat stress (HS) is a major welfare issue in the swine industry, especially as global temperatures trend upwards
Our results indicate that heat tolerance based on reproductive traits is heritable and genetic progress for HT can be achieve through genetic or genomic selection
Higher heritability estimates were observed in comfortable environments, indicating that selection for improved heat tolerance based on reproductive traits is feasible, genetic progress will be slow
Summary
Heat stress (HS) is a major welfare issue in the swine industry, especially as global temperatures trend upwards. Genetic selection has played a major role in improving productive and reproductive performance in pigs (Merks, 2000; Hill, 2016; Zak et al, 2017). In addition to creating welfare concerns, HS is responsible for significant economic losses to the swine industry (St-Pierre et al, 2003; Johnson and Baumgard, 2019). HS leads to a reduction in the number of piglets born alive and weaned as well as litters produced per sow per year (Bertoldo et al, 2012). HS has been linked to reduced embryo establishment, resulting in a smaller number of piglets born per litter, in gilts (Tummaruk et al, 2010)
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