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.