Effect of Sex Steroids on Gonadal Differentiation and Sex Reversal in the Frog, Rana curtipes

The bicolored frog, Rana curtipes, is endemic to Western Ghats of Southern India, having a prolonged larval life. In this species, gonadal differentiation is of the semidifferentiated type. The gonads initially differentiate into ovaries in all the individuals at Gosner stage 25. Later, in genetic males, the oocytes degenerate and testicular differentiation occurs at stages 30–31. Exposure of R. curtipes tadpoles to 50 μg/L of testosterone (T) or estradiol-17β (E2) during stages 24–26 or 29–32 did not affect gonadal sex differentiation and proportion of males and females at metamorphosis. In all the groups, the sex ratio was almost 1:1 as in the controls. Likewise, exposure of tadpoles to low concentration of steroids (12.5 μg/L T or E2) throughout larval development was ineffective in altering the sex ratio or the gonadal sex differentiation. On the other hand, exposure to higher concentrations of steroids (25 and 50 μg/L) throughout the larval development (stages 25–45) significantly skewed the sex ratio toward the male or female direction, depending upon the sex steroid used. Thus, exposure to T or E2 throughout the larval period could produce 93% males or 79% females, respectively, indicating the ability of these steroids to cause sex reversal. This study shows a possible absence of a “critical stage” that is sensitive to sex steroids for gonadal sex reversal in this frog.