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Sex change in fishes provides a good experimental model for understanding the mechanisms and plasticity of sex determination and differentiation. The three‐spot wrasse, Halichoeres trimaculatus is a protogynous hermaphrodite. During sex change from female to male, the ovary is replaced by the testis through the degeneration of oocytes and subsequent spermatogenesis. In the present study, we cloned a cDNA‐encoding gonadal soma‐derived factor (GSDF) from protogynous wrasse and examined its expression pattern in the sexually mature gonads and the sex‐changing gonad induced experimentally by aromatase inhibition. Expression of gsdf was predominantly observed in the testis, and it was mainly localized to the supporting cells surrounding the spermatogonia. In the ovary, only slight expression of gsdf was observed in morphologically undifferentiated supporting cells in contact …