Gene expression profiles of white bass (Morone chrysops) and hybrid striped bass (M. chrysops x M. saxatilis) gill tissue following Flavobacterium covae infection

Columnaris disease is a prevalent disease in freshwater environments caused by the ubiquitous aquatic pathogen Flavobacterium species. Adhesion to the external mucosal surfaces of fishes is the initial stage of infection, and the gills specifically have been identified as both a primary target and release site for this pathogen. Demonstrated here and in previous research, the hybrid striped bass (Morone chrysops x M. saxatilis), a prominent United States aquaculture product, is more susceptible to infection with F. covae than the maternal white bass (M. chrysops) parental species. To further elucidate the mechanisms underlying differences in resistance between these fish we examined gill gene expression profiles using RNA sequencing at different timepoints after F. covae infection. Patterns of differential gene expression and association with key enrichment terms indicate the effective immune response observed in white bass includes the up-regulation of multiple cytokines (IL-1β, IL-17C, TNF-α, G-CSF, IL-8, CCL16, CXCL9), hepcidins (HAMP and HAMP2), ribosomal subunit components (e.g., RPL13), and, interestingly, the down-regulation of leptin (LEPA) during initial (1 – 4 h) stages of infection. Conversely, up-regulation of the same genes was not detected in hybrid striped bass until 24 h after infection, indicating a delay in immune response mechanisms that is ultimately ineffective in protecting the host, as this was concurrent with the onset of mortality in these fish. Collectively, the presented results include several putative pathways and candidate genes for further investigation toward characterizing immune defense mechanisms underlying the resistance (white bass) and susceptibility (hybrid striped bass) for selective breeding efforts and/or biotechnological intervention.