Le déterminisme du sexe de l’huître creuse Crassostrea gigas au cours de son développement et du cycle gamétogénétique adulte : recherche de déterminants moléculaires

The pacific oyster Crassostrea gigas is a successive protandric irregular hermaphrodite. The factors involved in the first gonadic differentiation in juveniles and those involved in sex reversal in adults are still unknown. This thesis consisted in the identification of molecular factors of the oyster’s gonadic differentiation during its adult cycle and its development. Thus, the respective orthologs to a DM-family factor and to Foxl2, Cg-DMl and Cg-Foxl2, have been characterized in the oyster by RT-PCR. Their study by real-time PCR, in situ hybridization and immunohistochemistry, has highlighted a ubiquitous expression of Cg-DMl in potentially proliferating / differentiating cells in the gonads of both sexes. The Cg-Foxl2 expression is specific to the gonads (excepted in the labial palps) in both sexes. The gonadic expression of these factors is in agreement with the seasonal development of the adult gonad. The significant increase of Cg-DMl in stage III male and of Cg-Foxl2 in stage II female may suggest a temporal window of sex reversal variable according to sexes, at the end of the cycle preceding the sexual rest, more precocious in female than in male. During the development, a peak of expression of Oyvlg and Cg-DMl was observed after metamorphosis in 1-1.5 old month spats whereas Cg-Foxl2 expression decreased significantly. This key temporal window of the development may correspond to the differentiation of primordial germ cells in germinal stem cells during the first gonadic establishment.