The role of innate immunity in cnidarian-dinoflagellate symbiosis Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/sn00b237w

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  • Cnidarians, such as corals and sea anemones serve as hosts to a variety of organisms including symbiotic dinoflagellates, bacteria, virus, and apicomplexans. As corals are vital to the health and productivity of the reef ecosystem it is important to understand how these organisms interact with each component of the holobiont. Cnidarians possess members of several innate immunity pathways, but there is little is known about how the role these molecules play in balancing mutualistic and pathogenic associations. The complement system represents one innate immune pathway that has been characterized in cnidarians and there is preliminary evidence to suggest that C3, the central molecule in the pathway, plays a role in both symbiosis and immunity. However, the role of other complement proteins, such as Factor B and MASP is unknown. Therefore, the purpose of the research presented in this dissertation was to (1) determine the role of Factor B and MASP in cnidarian-dinoflagellate symbiosis and ancestral immunity using the model anemone Aiptasia and (2) investigate the evolution of innate immune proteins in cnidarians and invertebrates as a whole. In Chapter 2, the TLR/Interleukin-1 Receptor (TIR)-domain-containing repertoire of nine anthozoan species was characterized, which revealed the presence of a diversity of sequences including Toll-like receptor (TLR)-like, MyD88, IL-1Rlike, and TIR-only proteins. Corals have an expanded TIR-only protein repertoire compared to anemones, and the complexity is greatest in the acroporids and pocilloporids. This work also revealed the existence of TIR_2-domain-containing proteins in anthozoans, which at this time have an unknown function. In Chapter 3, the role of the complement system in the onset and maintenance of cnidarian-dinoflagellate symbiosis was explored using the anemone Aiptasia. Three Factor B and one MASP transcripts were characterized in Aiptasia and functional work was performed on Ap_Bf-1, Ap_Bf-2b, and MASP. Gene expression studies revealed that Ap_Bf-2b is upregulated at the onset of symbiosis and is more highly expressed in the gastrodermis than the epidermis, suggesting that it may interact with symbionts. However, it was also found to be suppressed in the symbiotic state suggesting that presence of symbionts alters the host immune response. The results for Ap_Bf-1 and Ap_MASP, were inconsistent and therefore the role of these molecules in symbiosis is not clear. Phylogenetic analysis of invertebrate complement sequences provided evidence for lineage-specific expansions, and potentially differences between corals and anemones that require further investigation. In Chapter 4, complement gene expression in response to immune challenge was investigated. Challenge with the individual immune stimulants lipopolysaccharide or peptidoglycan induced very few changes in expression, but dramatic changes were observed in response to the coral pathogen S. marcescens. In general Ap_Bf-1 and Ap_MASP were upregulated in response to S. marcescens, while Ap_Bf-2b showed little change or was downregulated, suggesting functional divergence between Aiptasia complement molecules. Overall, the work presented here indicates that cnidarian complement is involved in both symbiosis and immune challenge. The results indicate that Ap_Bf2b is more involved in symbiosis, and in contrast Ap_Bf-1 and Ap_MASP are more responsive to challenge with the coral pathogen S. marcescens. This suggest functional divergence in the Aiptasia complement system and provides information on how cnidarians may mediate interactions with the diverse microbial community in their environment.
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