Remarquable heterogeneity in the genetic architecture of resistance to a key bacterial pathogen in commercial populations of rainbow trout

Background Bacterial cold-water disease (BCWD), caused by Flavobacterium psychrophilum , remains a major challenge for rainbow trout ( Oncorhynchus mykiss ) aquaculture, due to the absence of effective vaccines and increasing concerns over antibiotic use. Genetic selection for disease resistance offers a sustainable alternative. In this study, we investigated the genetic architecture of BCWD resistance in two French commercial rainbow trout populations using a standardized waterborne infection model and high-density SNP genotyping. Results Survival following experimental infection varied significantly between populations, with population B showing higher resistance (71.3% vs 50.7% of survival at 29 days post-infection). Genome-wide association studies (GWAS) were performed using a Bayesian sparse linear mixed model (BSLMM), separately in each population and in a combined dataset. Eleven quantitative trait loci (QTLs) were identified across the analyses, with limited overlap between populations, highlighting the complexity and partial divergence of resistance architectures. Several candidate genes located within QTL regions were involved in immune signalling, inflammation, macrophages/neutrophils biology, and soluble factors important for antibacterial defences. Notably, two QTLs contained genes from the complement system (e.g., C3, Cfb), highlighting their central role in resistance to F. psychrophilum . Conclusions Our findings underscore the polygenic nature of BCWD resistance, the influence of host genetic background, and provide valuable targets for selection for BCWD resistance in rainbow trout breeding programs.