Self/nonself recognition in bivalves

Allorecognition is the ability to distinguish between self and nonself in conspecifics. Allorecognition is described in vertebrates and several species of colonial invertebrates, including Chordata, Cnidaria, and Porifera. Responses involve highly polymorphic histocompatibility loci that encode allodeterminants governing fusion or rejection, depending on the degree of shared alleles, and the presence of this system in bivalves is unknown. This thesis provided evidence of allogeneic response in bivalves. It observed xenorecognition across classes (bivalve vs. gastropod) and found similarities between this response and pathogen recognition. By combining the outcomes of explant cultures, molecular responses in mantle and haemocytes, and caspase-mediated apoptosis, responses were explored. Specifically, chapter two developed novel methods using explant cultures to explore allorecognition and observe fusion in auto-, allo-, and xenogeneic treatments, and found lower fusion levels in allogeneic treatment than in the other two treatments. Based on this finding, chapter three investigated molecular responses in these three treatments using transcriptomics and the identification of DEGs. Although low levels of DEGs were observed, two paralogous genes were downregulated in allogeneic and xenogeneic treatments, thought to be related to biomineralization and are possible evidence of the return to normal processes in autogeneic treatment and limitations of that in allogeneic and xenogeneic treatment. Due to the low levels of DEGs identified, injections of allogeneic and xenogeneic haemocytes were analysed in chapter four, comparing results from a viral mimic (for pathogen recognition) and an assumed apoptosis inducer. Findings of downregulated enriched processes in allogeneic treatment indicated an allogeneic response. Chapter five found no evidence of caspase-mediated apoptosis in allogeneic challenge, however caspase-mediated apoptosis is found in xenogeneic treatment. This foundational knowledge of allogeneic and xenogeneic response has advanced our understanding of self/nonself recognition in bivalves and provides a clear direction for future studies in this area of bivalve innate immunity.