Sex determination in animals generally occurs at an early developmental stage, and the sex of most animals remains fixed throughout its life. However, some teleosts can change sex as adults in response to changes in social status (Devlin and Nagahama,2002). Numerous endocrine studies of sex change in fish indicate that estrogens (e.g., estradiol-17β; E2) play crucial roles in bringing about gonadal sex change (Frisch,2004). Estrogens are mainly synthesized in the gonad by enzymes using testosterone as the substrate. The terminal enzyme in the steroidogenic pathway is cytochrome P450 aromatase (Simpson et al.,1994).
The three-spot wrasse, Halichoeres trimaculatus, which is common to coral reefs in Okinawa, Japan, is a protogynous hermaphrodite. Individuals mature initially as either males or females. Under appropriate social conditions, initial phase (IP) males and females can become terminal phase males (TP) (Hourigan et al.,1991; Kuwamura et al.,2007). Ovaries of the female, which initially have no detectable testicular tissue, are restructured into fully functional testes during sex change (Hourigan et al.,1991). Previously, the suppression of aromatase activity via the administration of Fadrozol, a nonsteroidal aromatase inhibitor, in females induced a complete female-to-male sex change, and the gonads of AI-induced females were similar to those of normal males (Nozu et al.,2009). In addition, we induced a reversed sex change (male-to-female) by treatment of adult IP males with a high dose of estrogen (Kojima et al.,2008). These results indicate that estrogen is a key factor in sex change in the three-spot wrasse. However, recent studies have revealed that estrogen is important not only as a female hormone but also as a hormone in the male reproductive tract in all vertebrate classes (Carreau et al.,2008; Schulz et al.,2010). Indeed, no significant sexually dimorphic gonadal expression of estrogen receptors was observed in the three-spot wrasse (Kim et al.,2002). Similarly, high levels of aromatase and of forkhead transcriptional factor 2 (Foxl2), a transcriptional regulator of aromatase, were observed in the testis of IP and TP males under natural conditions (Kobayashi et al., 2010). Furthermore, a constant production of estrogen, albeit at a lower level than in the spawning female, was observed during the sex change process in the saddleback wrasse, including TP males (Nakamura et al.,1988). These observations indicate that estrogens may have a role in spermatogenesis in the three-spot wrasse. To explore this possibility and investigate the role of estrogen in spermatogenesis, we examined in this study the effect of exemestane, an AI, or E2 on IP males of the three-spot wrasse.
To evaluate the effect of AI or E2 on spermatogenesis, we analyzed changes in plasma sex steroid hormone levels and cell proliferation rate in the testis of IP males. We conclude that estrogens play an important role during the meiotic phase of spermatogenesis in IP males of the three-spot wrasse. The recognition that estrogen is involved in males will undoubtedly help advance our understanding of the mechanisms driving sex change in protogynous wrasses.