Review and perspectives of physiological mechanisms underlying genetically-based resistance of the Pacific oyster Crassostrea gigas to summer mortality

Abstract

French oyster farming has been subject to severe mortalities during the summer months. Results from the research program "Morest", which ran from 2000 to 2006 and examined the possible causes of these mortalities, led to the construction of a model to explain the interaction between environmental factors, oyster physiology and different opportunistic pathogens underlying oyster summer mortality. Temperature, food, reproduction and stress were the main factors required for oyster mortality. Genetically-based resistance ("R" oysters) or susceptibility ("S" oysters) to summer mortality was revealed by divergent selection. Building on these results, a literature search was made in 2007 on the molecular origin of genetic resistance to such a complex mortality risk. The objectives were to lay a foundation for the preparation and orientation of future research directions and to improve understanding of the underlying physiological mechanism leading to summer mortality. Three years later, the resulting conceptual analysis reported here was presented as an introductory lecture to Physiomar 2010, a conference where many new results contributing to this research field were also reported. The literature review highlighted two major review articles: the first dealing with nutrition and reproduction (Schneider 2004), the second with reproduction, temperature, oxidative stress and mortality (Heineinger 2002). The effect of nutrition level on energy orientation to growth or reproduction is controlled by endocrine factors. Among these, neuropeptide Y (NPY), ghrelin and leptin neuropeptides appeared to be potential candidates involved in germ-soma orientation in relation to trophic conditions. Depending on reproductive effort and temperature, a metabolic stress resulting from the germ-soma conflict can appear, characterized by mitochondrial reactive oxygen species (ROS) production. Such an excess of ROS induces perturbations in mitochondrial activity leading to cell death. Many organisms, such as annual plants or the Pacific salmon, do not survive their first reproduction. In contrast, others increase stress resistance by selection of antioxidant processes (superoxide dismutase SOD, catalase, etc.) through evolution, and survive first reproduction. A similar difference was observed in the comparison made between R and S oysters, which differed in ROS production, SOD and catalase levels. Such factors controlling reproduction and ROS detoxification processes could therefore provide new markers for selection of oysters with better resistance to non-specific pathogens, complementing other classic selection approaches against specific pathogens or for improved immunity. This antioxidant defence mechanism is found in many organisms including vertebrates and in some invertebrates, including oysters. Its role needs to be considered in pathology events involving other aquaculture species and it may also contribute to explaining the increase in marine pathologies under anthropogenic environmental changes. © EDP Sciences, IFREMER, IRD 2011.