Are sex steroids essential for gonadal differentiation of the ornate frog, Microhyla ornata?

- Sex steroids are not essential for gonadal differentiation in M. ornata.
- Exposure to E2, TP, CA, TAM and FOR was ineffective in inducing sex reversal.
- Sex ratios were comparable among control and treated groups.
- A sensitive window for gonadal sex reversal is absent in M. ornata.
- E2 and TP are necessary for ovary and testis development in M. ornata.

Knowledge about sensitivities and responses of amphibian larvae to sex steroids and the chemicals alike is the first step towards understanding and assessing the effect of diverse chemicals that interfere with gonadal development and other endocrine functions. Herein, we used Microhyla ornata to determine the role of sex steroids on its gonad differentiation and sex ratio. Our results show that the exposure to increasing concentrations of estradiol-17β throughout larval development did not affect gonad differentiation resulting in 1:1 sex ratio at metamorphosis. But, females emerging from estradiol-17β treatment had larger ovaries with larger sized follicles. Further, testes of some males contained lumens, the number of which was dose dependent. Similarly, exposure to testosterone propionate had negligible effects on gonad differentiation. However, the mean diameter of the largest follicles was smaller in treated ovaries. Treatment of tadpoles with tamoxifen had no effect on gonad differentiation and ovary development while testicular development was accelerated at the highest concentration. Similarly, treatment of tadpoles with cyproterone acetate had little effect on gonad differentiation as well as development, hence the sex ratios at the end of metamorphosis. Further, in tadpoles exposed to increasing concentrations of formestane, gonad differentiation was normal resulting in 1:1 sex ratio. Thus, in M. ornata, both estradiol and testosterone are essential for the development of ovaries and testes respectively but, they are not critical to gonadal differentiation. Hence, the effects of sex steroids and other endocrine disrupting chemicals could be species-specific; different species may have differential sensitivities to such chemicals.