| Summary: | Our current understanding of crustacean immunity lacks sufficient scope because it mainly comes from research on insects and decapods. Little is known specifically about the immunity of amphipods, which constitutes a large proportion of crustacean species. The goal of my thesis is to develop a model system for studying crustacean immunity using the amphipod Parhyale hawaiensis, which is an emerging animal model in many research areas. P. hawaiensis was challenged with Vibrio bacteria to elicit an immune response. P. hawaiensis was resistant to Vibrio bacteria under normal conditions but injury significantly increased mortality rate due to increased bacteria entry. Whole-body transcriptomic response after 24h bacterial challenge yielded hundreds of differentially expressed genes. Additional in vivo bacteria, qPCR, and RNA-seq measurements across the infection time course provided a comprehensive overview the immune system after bacterial challenge. The expression pattern of Relish closely matched that of in vivo bacteria. Key effector genes (i.e., TEP, ALFs, and crustin) upregulation was critical in determining survival. At 4h, upregulation involved pathogen recognition receptors and Toll/IMD pathway components. At 8h, immune regulation and effector activities peaked with the aid of JAK-STAT pathway components. At 24h, peak bacterial load pushed the immune system to the limit. Immune system returned to near normal at 168h. Based on in vivo bacterial measurements and stronger transcriptomic response, females seemed to exhibit higher immunocompetence. Only some immune related genes (e.g., IMD and JAK-STAT pathway components) were differentially expressed in both sexes. Toll pathway components were only differentially expressed in females. Hemocytes imaged using imaging flow cytometry were divided into five subpopulations based on size and granularity. The size of larger hemocytes increased by 9.3% after bacterial challenge and these cells may be involved in antioxidant defense and phagocytosis bacterial clearance based on orthologs. The hemocytic transcriptome was very distinct from whole animal and hemocytic response to bacteria was much stronger with 2077 upregulated and 1599 downregulated genes. Key immune genes (e.g., scavenger B, GNBPs, and Tollip) with hemocytic pathogen recognition functions in other taxa were identified. Findings from my thesis better our understanding of the understudied amphipod immunity and provide additional insight into the immune system of the phylogenetically close decapods.
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