Genetic and genomic analyses of resistance to yersiniosis in Atlantic salmon (Salmo salar) assessed by tank challenge

Abstract

The bacterial disease yersiniosis caused by the bacterium Yersinia ruckeri (non-Hagerman strain) has previously caused significant loss to the Atlantic salmon aquaculture industry in Tasmania (Australia). Despite an ongoing vaccination strategy, the disease can impact commercial freshwater hatcheries and following introduction of smolt to seawater. Genetic selection for disease resistance is a long-term strategy that can be considered to manage yersiniosis, requiring a consistent challenge model. Utilising fish of known pedigree in a biosecure tank challenge facility we exposed two consecutive year classes of juvenile Atlantic salmon to Yersinia ruckeri serotype O1b biotype 1 (TCFB 2282) by intraperitoneal injection and recorded challenge test survival. Fish showed clinical signs of yersiniosis and cumulative mortality reached 72.7–72.9%. Pedigree analysis of binary survival (TS) and continuous days to death (DD) confirmed that mortality to yersiniosis is a heritable trait (h2 = 0.24 and 0.29, respectively) and that consecutive year classes were highly correlated (rg = 0.99), evidence of the development of a robust challenge model. The accuracy of genomic prediction was assessed under two different prediction scenarios, estimating a 9–14% improvement in accuracy over classic pedigree-based selective breeding. GWAS confirmed that resistance to yersiniosis is a polygenic trait with no major QTL. Candidate genes identified within 100 Kb of SNPs having the largest effects (> 0.5% genetic variation) suggest altered host interactions to break down pathogen virulence, altered inflammatory response by key innate immune cells such as neutrophils and macrophages and modified clearance of Y. ruckeri infection by macrophage phagocytosis and adaptive immunity.