Morphological and Physiological Studies on Gonadal Sex Differentiation in Teleost Fish

Over the past 40 years, gonadal sex differentiation in teleost fish has been studied from histological and physiological viewpoints, for its application in fisheries. The morphological characteristics of gonadal sex differentiation in teleost fish were discussed in Section 2, and a summary of sexual processes, such as gonadal sex differentiation and sexual maturation in triploid salmonid fishes, was given. Subsequently, the effects of sex hormones/steroidal analogues on sex differentiation in several fish were examined. It was concluded that there is a critical period during physiological sex differentiation— before morphological sex differentiation—in which the induction of artificial sex reversal by exogenous sex hormones can occur. It was also demonstrated that endocrine environmental disruptors have the potential to induce sex reversal in genetic males. In Section 3, it was clarified that ultrastructural steroid-producing cells appeared in the gonads of tilapia around the time of sex differentiation, indicating that endogenous steroid hormones play an important role during sex differentiation in fish. In Section 4, the expression of various steroidogenic enzymes including aromatase was immunohistochemically proven to occur in the gonads during the period of morphological ovarian differentiation in genetically female tilapia. In contrast, there was no expression in the gonads during differentiation, or in early testicular differentiation in genetically male tilapia. Aromatase inhibitor induced sex reversal from females to phenotypic males in tilapia and golden rabbitfish. It was also demonstrated that androgen treatment suppressed the expression of steroidogenic enzymes in the gonads of genetic females, and induced sex reversal. It was concluded that endogenous estrogen functions as an ovarian inducer, whereas lack of estrogen induces testicular differentiation.