Molecular characterization of clam (Mercenaria mercenaria ) immune responses against Quahog Parasite Unknown (QPX): Effect of host and environmental factors

Quahog Parasite Unknown (QPX) is a fatal protistan parasite that infects wild and cultured hard clams (Mercenaria mercenaria) along the northeastern coast of the US. The severity of clam mortality events and resulting economical losses caused by QPX emphasize the urgent need for the development of effective disease control measures, which require a good understanding of disease pathobiology and host resistance mechanisms. This dissertation explored clam immunity and host and environmental factors that affect M. mercenaria resistance against QPX infection. The investigation of molecular immune mechanisms using multiple high-throughput transcriptomic methods revealed an extraordinary complexity of M. mercenaria's defense system, featured by highly diversified immune recognition receptors and pathways involved in immune cell activation, signal transduction, wound repair and apoptosis. Results also showed that QPX infection induces a tailored immune response in clams with focal up regulation of variable pattern recognition receptors (e.g. lectin, fibrinogen-related proteins, complement-1q-domain containing proteins) and signaling networks involved in the activation of focal adhesion, inflammation, apoptosis and extracellular killing, in contrast to a systemic response characterized by an overall immune suppression and up regulation of stress proteins. Comparative analysis between resistant and susceptible clam strains highlighted a transcriptome-wide deficiency in QPX-responsive immune factors in susceptible clams, suggesting a primary role of host genetic make-up in QPX resistance. Furthermore, results showed that temperature influences disease development mainly through the alteration of the expression of immune genes. Low temperatures resulted in the suppression of the clam immunome thus facilitating the establishment of QPX infection, while higher temperatures supported much higher levels of immune gene expression, which were correlated with host resistance. Finally, given the importance of temperature in regulating disease development, the study evaluated the role of heat shock treatments on the progress of pre-established infections. Results showed a reduction of QPX disease in clams submitted to temperature shocks. Best mitigation results were found in conditions with short exposures to moderately elevated (27°C for 2 hours) temperatures. The heat shock treatment has great potential for field-application in hard clam fisheries. Overall, the findings of this dissertation represent a valuable addition to the currently limited molecular data on M. mercenaria, and contribute to a better understanding of the mechanisms of clam resistance to QPX. The results provide solid guidelines and preliminary data that facilitate future research targeting clam selective breeding and the development of disease mitigation strategies.