Effects of Styrene Monomer and Trimer on Gonadal Sex Differentiation of Genetic Males of the Frog Rana rugosa

To ascertain whether styrene monomer and trimer induce an estrogen-like effect, all-male tadpoles of Rana rugosa were exposed to dilute solutions of styrene monomer and trimer (2,4,6-tripfenyl-1-hexene) at concentrations of 0.1, 1, or 10 μM during days 19–23 after fertilization, which is the critical period of gonadal sex differentiation. As positive and vehicle controls, tadpoles were exposed to dilute solutions of estradiol-17β at concentrations of 0.01, 0.1, or 1 μM and 0.01% ethanol, respectively, during the same period. The influence was estimated by examining the inner structure of the gonads of 40-day-old tadpoles. All the gonads of tadpoles in the vehicle and untreated controls showed histological characteristics of testes, whereas all those of tadpoles treated with 1 μM estradiol-17β showed a structure typical for ovaries, having an ovarian sac and many meiotic germ cells. Of the genetically male tadpoles treated with styrene monomer and trimer, 97 and 96%, respectively, showed normal testicular structure of the gonads. In contrast, the remaining tadpoles had gonads showing coexistence of testicular and ovarian structure, irrespective of the strength of styrene concentration. These findings suggest that the styrene monomer and trimer induced a weak estrogen-like effect on the pathways of testicular differentiation in genetically male tadpoles.