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Changes in androgen‐producing cell size and circulating 11‐ketotestosterone level during female–male sex change of honeycomb grouper Epinephelus merra

11‐ketotestosterone (11‐KT), a potent male‐specific androgen in fish, has important roles on spermatogenesis, male behavior, and nuptial coloration. The site of 11‐KT synthesis and its role on male germ cell development during protogynous sex change is not clearly understood. We examined the dynamics of steroidogenic enzymes immunolocalization, viz cholesterol side‐chain cleavage (P450scc), biomarker of steroids and cytochrome P45011β‐hydroxylase (P45011β), downstream to 11‐KT production, throughout the process of sex change in honeycomb grouper (Epinephelus merra). In female, P450scc immunoreactivity (‐ir) was observed in the theca layer and tunica near blood vessels (BV). During the onset of sex change, P450scc reactive cells were observed in the remaining follicle layer of degenerated oocyte of the ovo‐testis in early transitional (ET) and late transitional (LT). In male, P450scc‐ir was localized in the interstitial Legdig cells of testis. P45011β reactive cells were observed in the tunica near BV in female but not in theca layer. In ET and LT phases gonads, P45011β localized in remaining follicle layer of degenerated oocyte and tunica near BV. On the other hand, in male, both interstices and tunica near BV showed strong signals against P45011β. Moreover, in vivo and in vitro levels of 11‐KT related with the changes in the nuclei diameter of P45011β‐positive cells in both tunica near BV and remaining follicle layer of degenerated oocyte to interstices during the progress of sex change. The present results suggest that 11‐KT produced in the tunica near BV may provide the stimulus for female to degenerate oocytes and initiate sex change. However, 11‐KT produced both in tunica near BV and remaining follicle layer of degenerated oocyte possibly plays critical role during testicular differentiation as well as gonadal restructuring at mid to late phases (ET to LT) of sex change in honeycomb grouper. Mol. Reprod. Dev. © 2005 Wiley‐Liss, Inc.