Heat shock treatment for improving productivity and disease resistance in aquaculture

The dataset associated with this thesis is available in ORE at: https://doi.org/10.24378/exe.4824Aquaculture plays an important role in meeting current food demands and will experience growth as a result of increasing human population and per capita fish consumption in conjunction with plateaued capture fishery yields. However, factors including disease outbreaks and ubiquitous stress significantly limit production and welfare such that aquaculture’s contribution to current and future food security is compromised. This thesis explores the hypothesis that embryonic heat shock exposure can influence epigenetic regulation of transcription, and impact the phenotype of an aquaculture model species, the rainbow trout (Oncorhynchus mykiss). The epigenome and its regulation of gene expression is susceptible to manipulation by environmental conditions, particularly those experienced during early embryo development. If this process could be harnessed successfully, modification of the epigenome through controlled environmental manipulation during early embryo development could provide a simple method of phenotypic alteration to increase fitness and robustness in aquaculture species. Embryonic heat shock exposure was correlated with greater induction of heat shock protein 70a (hsp70a) transcription in response to thermal stress exposure in larval rainbow trout, indicating a greater response ability as a result of the embryonic pre-exposure. In addition, embryonic heat shock exposure resulted in alteration of the rainbow trout methylome and may prime the methylome for subsequent thermal stress response at later life stages. Differentially methylated regions resulting (DMRs) resulting from embryonic heat shock were related to genes involved in signalling pathways regulating metabolic, neuronal, developmental and immune response processes. Embryonic heat shock exposure did not alter susceptibility or responsiveness to disease when juvenile rainbow trout were infected with bacterial pathogen Yersinia ruckeri, the causative agent of enteric redmouth disease. However, it impacted the response to aquaculture related handling stressors, indicated by transcriptomic trends suggesting increased unfolded protein response (UPR) activity, including induction of hsps, as well as greater transcription of genes involved in immune response and oxidative stress. Building on these findings, further exploration of impact of embryonic heat shock and handling stress response in larval rainbow trout revealed suppressive impacts on transcription of non-essential genes and pathways, particularly immune response processes, which were more pronounced in heat shocked compared to naïve larvae. Heat shocked fish also displayed faster recovery from handling stress, indicating greater resilience in comparison to naïve larvae. Together, this thesis demonstrates that embryonic heat shock exposure results in persistent alteration of the rainbow trout epigenome that is capable of altering regulation of the transcriptomic response to aquaculture stressors, verifying the main hypothesis this thesis aimed to test. Despite this, the exact embryonic heat shock conditions required to generate positive change, particularly with regards to producing positive alterations in phenotype, remain unconfirmed. As such further research is required to establish whether this approach is suitable for promoting robustness in farmed fish species. Overall, alteration of the epigenome and its regulation of transcriptomic response through embryonic heat shock exposure holds potential to alter overall fitness and robustness of aquaculture species. When combined with other methods such as genetic improvement to promote greater domestication, disease prevention, and enhancement of husbandry practices, could be successful in promoting greater productivity and improved welfare of aquaculture finfish production.Centre for Environment, Fisheries and Aquaculture Scienc