FUNCTIONAL CONSEQUENCES OF APOPTOSIS GENE DIVERSIFICATION IN EASTERN OYSTER IMMUNITY

Abstract

The eastern oyster is an ecologically and economically important filter feeding bivalve mollusc endemic to the east coast of the United States. Outbreaks of disease, such as Dermo disease caused by the parasite Perkinsus marinus, threaten natural oyster populations, and oysters have evolved a complex innate immune system containing large, expanded gene families to combat the diverse array of pathogens in their environment. Apoptosis, or programmed cell death, is a critical immune response to Dermo disease involving a series of expanded gene families, including the Inhibitor of Apoptosis (IAP) family of apoptosis regulatory molecules. The goal of this research is to investigate the role of apoptosis in immunity and disease resistance in oysters and characterize the role of IAP diversity in disease response. To address these gaps in knowledge, this dissertation first annotated the full repertoire of apoptosis genes in the eastern oyster, revealing the major apoptosis and regulated cell death pathway proteins in the eastern oyster and providing a detailed resource for future researchers. Second, this dissertation characterized oyster IAP gene family diversification and potential evolutionary mechanisms of expansion, and the role of this diversification in oyster disease response. Research on oyster IAP characterization showed that: 1) oyster IAPs have highly diverse Baculoviral IAP Repeat (BIR) domains and domain architecture; 2) IAPs likely expanded through tandem duplication and retroposition; 3) oysters express unique assemblages of IAPs to diverse disease challenges; and 4) IAP expression is directly correlated with apoptosis pathway response. Third, this dissertation investigated eastern oyster apoptosis mechanisms in response to P. marinus challenge. In vitro P. marinus challenge and novel use of IAP and caspase inhibitors indicated basal hemocyte apoptosis may be IAP-dependent, apoptosis suppressed by P. marinus may involve caspase-independent pathways, and hemocyte apoptosis suppression following P. marinus challenge involves oxidation-reduction, TNFR, and NF-kB pathways. Finally, this dissertation investigated the connection between apoptosis phenotype and resistance to Dermo disease. In vivo P. marinus challenge across six selectively bred families revealed families differed in their P. marinus resistance in terms of parasite load change over time, apoptotic responses to P. marinus significantly differed across families, and apoptotic response at 7 days after challenge was correlated with resistance in the most resistant family. Future research is needed to confirm the role of apoptosis on resistance and the utility of apoptosis as measure of Dermo resistance remains unclear. The relationship between apoptosis gene expression and Dermo resistance should be