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Sex determinism in the pacific oyster Crassostrea gigas during its development and the adult gametogenetic cycle : search of molecular markers

Team 2: Reproduction and development of aquatic organisms: evolution, adaptation, regulations


Year of defence: 

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.

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