Sexual plasticity in fish: a possible target of endocrine disruptor action.

Various genetic and molecular approaches have been used to investigate the mechanisms of sex determination, gonadal sex differentiation, and sex change in fish. We identified, for the first time in nonmammalian vertebrates, DMY, as the sex-determining gene of medaka. In tilapia, endogenous estrogens act as the natural inducers of ovarian differentiation, while DMRT1 may be important for testicular differentiation. In the protogynous wrasse, a rapid decline in serum estradiol-17beta levels may be an initial trigger of the female-to-male sex change. Both sex steroids and endocrine disrupters do not seem to act at the level of the sex-determining gene, but during gonadal sex differentiation. The Japanese flounder exhibits temperature-dependent sex determination. Some of the estrogenic endocrine disrupters induce feminization of the flounder larvae reared at the masculinizing temperature. The actions of these sex steroids and endocrine disrupters may be mediated by the actions of somatic cells within gonads. Thus, sexual plasticity of gonads during sex differentiation may be implicated through the somatic cells within gonads. Cloning and sequencing of a number of genes that are considered to be associated with gonadal sex differentiation have been performed and some are still in progress. These molecular probes provide useful tools for understanding not only the molecular mechanisms of sex determination and gonadal sex differentiation but also provide important basic information for studying the effects of endocrine-disrupting chemicals during these periods.