The honeycomb grouper shows protogynous hermaphroditism. The endocrine mechanisms involved in gonadal restructuring throughout protogynous sex change are largely unknown. In the present study, we investigated changes in the gonadal structures and levels of serum sex steroid hormones during female to male sex change in the honeycomb grouper. On the basis of histological changes, entire process of sex change was assigned into four developmental phases: female, early transition (ET), late transition (LT), and male phase. At the female phase, the oocytes of several developmental stages were observed including gonial germ cells in the periphery of ovigerous lamellae. At the beginning of ET phase, perinucleolar and previtellogenic oocytes began degenerating, followed by proliferation of spermatogonia toward the center of lamella. The LT phase was characterized by further degeneration of oocytes and rapid proliferation of spermatogenic germ cells throughout the gonad. At the male phase, no ovarian cells were observed and testis had germ cells undergoing active spermatogenesis. Serum levels of estradiol-17beta (E2) were high in females in the breeding season, but low in the non-breeding female, transitional and male phase, and those of 11-ketotestosterone (11-KT) and testosterone (T) were low in females and gradually increased in the transitional and male phase. The present results suggest that low serum E2 levels and degeneration of oocytes accompanied by concomitant increase in the 11-KT levels and proliferation of spermatogenic germ cells are probably the events mediating protogynous sex change in the honeycomb grouper.

The honeycomb grouper, Epinephelus merra , is a protogynous hermaphrodite fish. Sex steroid hormones play key roles in sex change of this species. A significant drop in endogenous estradiol-17β β β β (E2) levels alone triggers female-to-male sex change, and the subsequent elevation of 11-ketotestosterone (11KT) levels correlates with the progression of spermatogenesis. To elucidate the role of an androgen in sex change, we attempted to induce female-to-male sex change by exogenous 11KT treatments. The 75-day 11KT treatment caused 100% masculinization of pre-spawning females. Ovaries of the control (vehicle-treated) fish had oocytes at various stages of oogenesis, while the gonads of the 11KT-treated fish had transformed into testes; these contained spermatogenic germ cells at various stages, including an accumulation of spermatozoa in the sperm duct. In the sex-changed fish, plasma levels of E2 were significantly low, while both testosterone (T) and 11KT were significantly increased. Our results suggest that 11KT plays an important role in sex change in the honeycomb grouper. Whether the mechanism of 11KT-induced female-to-male sex change acts through direct stimulation of spermatogenesis in the ovary or via the inhibition of estrogen synthesis remains to be clarified.

Androgen plays an important role in the developing ovaries of female fish. However, little is known regarding either the sites of production of androgen or its functional roles. In the present study, we investigated immunohistochemically the localization of cholesterol-side-chain-cleavage (P450scc) and cytochrome P45011beta-hydroxylase (P45011beta) with antibodies P450scc and P45011beta in the ovary of the female honeycomb grouper Epinephelus merra during its reproductive cycle. Clusters of strongly immunopositive cells, with 100-1000 cells in each cluster, against both P450scc and P45011beta, were observed throughout the annual reproductive cycle in tissue near blood vessels in the tunica ovary surrounding the outer periphery of the ovary. The ultrastructural characteristics of these cells showed that they were steroid-producing cells. In contrast, immunopositive cells against P450scc but not against P45011beta were localized in the theca layer surrounding the outer periphery of oocytes. These results suggest that two distinct steroid biosynthesis sites exist in the ovary and that cells at the two sites differ functionally. The only cells that biosynthesize 11-ketotestosterone are found in clusters in the vicinity of blood vessels; they possibly play a physiological role in oocyte growth and gonadal restructuring during the sex change of individuals of this species.

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