Diversité moléculaire des effecteurs antimicrobiens chez l'huître creuse Crassostrea gigas : mise en évidence et rôle dans la réponse antimicrobienne

This work contributed to the knowledge of the molecular bases of oyster immunity by the characterization of the diversity of three antimicrobials of C. gigas and the understanding of the role played by their diversity in the oyster antimicrobial response. Phylogenetic analyses of two antimicrobial peptides (AMPs), Cg-Defensins (Cg-Defs) and Cg-Proline rich peptide (Cg-Prp), and one Bactericidal Permeability Increasing protein, Cg-BPI, led us to the identification of a high diversity for both AMPs. Further analyses showed that this diversity is generated by gene duplication, allelic recombination and directional selection pressures, suggesting their functional diversification. The biological meaning of AMP diversity was investigated for the three major variants of Cg-Defs, which revealed a strong but variable potency against Gram-positive bacteria. We evidenced that oyster defensins kill S. aureus through binding to the cell wall precursor lipid II, resulting in the inhibition of peptidoglycan biosynthesis. Finally, transcript expression and localization of oyster antimicrobials after a pathogenic infection evidenced a complex network in their expression profiles in hemocyte populations and oyster tissues, suggesting a potential interplay between antimicrobials as a result of their colocalization. Indeed, the combination of oyster antimicrobials produced strong synergistic activities that enlarged their antimicrobial spectra. Thus, the diversity of oyster antimicrobials may provide significant means in acquiring functional divergence, probably concerned in the evolutionary arms race between hosts and their pathogens.From our data, it would provide oysters with a higher protection against the potential pathogens from their environment.