Caractérisation et diversité des mécanismes du syndrome de mortalité affectant les juvéniles de Crassostrea gigas

nfectious diseases are very often explored using reductionist approaches, despite repeated evidence showing them to be strongly influenced by numerous interacting host and environmental factors. Many diseases with complex etiology therefore remain misunderstood. In this thesis, by developing a holistic approach to tackle the complexity of the interaction, (i) we deciphered the complex intra-host interactions underlying the Pacific oyster mortality syndrome affecting juveniles of Crassostrea gigas, the main oyster species exploited worldwide and (ii) we validated this mechanism in different infectious environments and oyster genotypes. Using ecologically realistic experimental infections combined with thorough molecular (metabarcoding, transcriptomics, pathogen monitoring) and histological analyses on oyster families with contrasting susceptibilities, we demonstrated that the disease is caused by a multiple infection whose initial and necessary step is the infection of oyster haemocytes by a herpesvirus. Viral replication leads to an immune-compromised state of the host, evolving toward subsequent bacteremia by opportunistic bacteria. By identifying critical intra-host interactions between microorganisms and host immunity, this study cracks the code of the Pacific oyster mortality syndrome and provides important molecular data for the design of prophylactic measures and breeding programs dedicated to the production of oysters resistant to the mortality syndrome. We believe that such a systems biology approach could be applied to decipher other multi-factorial diseases that affect non-model invertebrate species worldwide.