The impact of outbreeding on the immune function and disease status of eight hybrid Chinook salmon crosses after Vibrio anguillarum challenge

Abstract

Aquaculture in North America is currently dominated by Atlantic salmon, but there has been an increasing interest in the production of species native to the Pacific coast. Chinook salmon is relatively new to production; therefore, the selection of appropriate stocks is critical. Often genes from wild populations are incorporated into farmed stocks to avoid performance decreases associated with inbreeding. The present study focuses on assessing the immunological performance of one inbred stock and seven outbred/hybrid crosses after challenge with the marine pathogen, Vibrio anguillarum. Throughout exposure challenge to both pathogen and sterile PBS, significant differences in mortality were observed between crosses. Fish were also assessed for major histocompatibility complex (MHC) class II β1 genotype, and although particular alleles did not confer resistance, crosses with better survival had more individuals presenting a heterozygous genotype. The stress induced during infection resulted in several individuals presenting signs of Bacterial Kidney Disease (Renibacterium salmoninarum) indicating that chronic co-infection may have contributed to susceptibility. When spleen samples from the highest and lowest performing hybrid crosses were assessed for cytokine and respiratory burst gene expression throughout bacterial challenge, high surviving crosses presented lower expression of inflammatory cytokine transcripts (IL-1β, IL-6 and TNFα) when compared to low performing crosses. Interestingly, microsatellite analysis revealed no significant differences in inbreeding coefficient or allelic richness between crosses despite observed variation in immune performance. Understanding the impact of outbreeding on the immune function of farmed, and often inbred, Chinook salmon could aid in future development of high-quality aquaculture stocks for this species.