Dr. Luigi Vezzulli, Università di Genoa, rapporteur
Dr. Carlos Rodriguèz Osorio, Universidade de Santiago de Compostela, rapporteur
Dr. Delphine Destoumieux-Garzon, Université de Montpellier/CNRS/Ifremer, examinatrice
Pr. Marcelino Suzuki, Sorbonne Université - Observatoire océanologique de Banyuls/mer, examinateur
Dr. Frédérique Le Roux, Sorbonne Université/Ifremer - SBR, directrice de thèse
Global change, including anthropogenic activities such as aquaculture, have been associated with an increase in the incidence of Vibrio-associated illnesses and basic knowledge about vibrios in the environment is urgently needed. Investigating pathogen eco-evolutionary history and exploring their virulence mechanisms is an essential step to understand and further manage environmental vibrio-associated diseases. In this PhD project, we addressed two main questions: i) How widespread is pathogenic potential of vibrios in the wild? ii) Does oyster farming select for emergent pathogen(s)? To address these questions, the Pacific oyster Crassostrea gigas was used as a model of infection and we combined experimental ecology, a high throughput infection assay, genome sequencing and inverse genetics. We found that a number of environmental populations are virulent toward oysters thanks to virulence mechanisms likely acquired during ecological differentiation. We observed that diseased juvenile oysters are always colonized by diverse vibrios and proposed a scenario on the role of vibrios in mortality events. V. crassostreae was found to be abundant in diseased oyster. This plasmid is only detected in high-density oyster farming areas, suggesting that intensive aquaculture settings have selected higher virulent genotypes. We further showed that plasmid selection is enhanced in oyster compared to in vitro conditions. This result suggests that oysters represent a niche for HGT and emergence of pathogens.