Déterminismes génétiques de l'allocation à la reproduction chez les huîtres creuses (Crassostrea gigas) triploïdes

In diploid Pacific oyster Crassostrea gigas, gonadic development can be important in terms of energetic investment. Phenotypic and genetic negative correlations have also been observed between gonadic occupation and traits related to survival and growth, and this fact suggests that this trait could be deleterious for its aquaculture production. Triploidy can be been artificially induced to improve the production of Pacific oysters as it results in a general diminution of mean gonadic development. This trait is however highly variable. In this context, this study aimed to quantify and qualify the allocation to reproduction in triploid oysters and to study its genetic bases. Variability for allocation to reproduction and growth was first studied in 3 sets of 96 families produced following a factorial crossing design and reared together under common condition, including diploids, triploids obtained by 4n x 2n crosses or chemical induction. Gonadic development in triploid oysters was divided by two compared with diploids. The maturation of gonadic tissues showed perturbations characterized with high occurrence of hermaphroditism, and high inter-individual variation in maturation stage. A moderate genetic determinism has been found for the allocation to reproduction in the three groups, probably influenced by environmental heterogeneity due to competition between individuals. Significant interactions have been detected between triploidy and genetic familial value for soft-tissue weight and allocation to reproduction. The genetic basis for the variation of the allocation to reproduction in diploid and triploid oysters was then studied with the analysis of the response to a divergent selection experiment. Diploid breeders showing contrasted gonadic development were chosen to induce a directional selective pressure. Allocation to reproduction measured in diploid and triploid offspring showed a clear response to selection in these two groups. The possible causes of the observed variation of allocation to reproduction in diploid and triploid oysters, and its consequences on correlated phenotypic traits, as well as the sensibility of phenotypic traits and related genetic parameters estimates to environmental heterogeneity are discussed in the light of our results.