Ecological factors and potential endocrine mechanisms regulating sex change in Thalassoma duperry

In teleosts, the proximal mechanisms by which external cues are transduced into internal signals that alter reproductive biology throughout sex change are largely unknown. To address this issue, this work sought to identify potential mechanisms by investigating ecological factors and their effects on ovarian physiology in the protogynous wrasse Thalassoma duperrey. Inhibition of sex change in T. duperrey appears to result from the stimulation of ovarian function by the presence of terminal phase males. When compared with isolated females and females housed with initial phase males, females housed with small terminal phase males showed no histological evidence of sex change. The large gonadosomatic indices of these females suggest a stimulation of ovarian function. Estrogens are major mediators of ovarian function in teleosts. Assessment of estrogen production throughout sex change, therefore, represents a means to investigate the relationship between external cues and changes in ovarian physiology. Induction of sex change does not affect the gonad's ability to produce steroid hormones. Cytochrome P450 cholesterol-side-chain-cleavage, the enzyme responsible for production of steroidal precursors, immunolocalized in gonadal tissues throughout sex change. The gonad's ability to produce estrogens, however, is permanently lost in the earliest stages of sex change. Aromatase mRNA, the enzyme responsible for converting androgens to estrogens, was undetectable by Northern and in situ hybridization analysis in testes and transitional gonads. Without a source of gonadal estrogens, ovarian function is lost. The down-regulation of aromatase mRNA may be affected by alteration of gonadotropin release profiles. Tonic, in vitro exposure to gonadotropin analogs appears to shift steroid hormone production from estrogens to androgens in ovarian fragments. To summarize, terminal phase males may maintain female-specific gonadotropin profiles. Absence of terminal phase males may alter this pattern. The resulting release profile may down-regulate gonadal aromatase and estradiol, thus causing the loss of ovarian function. Once ovarian function is lost, sex change occurs. In conclusion, these studies identified gonadotropin release profile as a potential mechanism which tranduces an environmental cue (absence of terminal phase males) in a critical physiological step (loss of gonadal estrogens) in the process of sex change.