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The three-spot wrasse, Halichoeres trimaculatus, can change sex from female to male (i.e. protogyny) due to sharp decrease in endogenous estrogen. During the sex change, ovarian tissue degenerates and testicular tissue arises newly. Finally, ovarian tissue disappears completely and replaces into mature testis. In order to predict the molecular mechanisms controlling the processes of sex change, we investigated the expression patterns of four genes (rspo1, figla, sox9b and amh), which have been thought to be associated with ovarian/testicular differentiation in vertebrates. Expression levels of rspo1 and figla, which play important roles for ovarian differentiation in vertebrates, were stable until the middle stage of the sex change, and subsequently down-regulated. Therefore, it was indicated that decrease in rspo1 and figla could result from ovarian degeneration. On the other hand, basis on the expression pattern …