A morphological and experimental study of gonadal sex differentiation in the rainbow trout, Salmo gairdneri.

Gonadal sex differentiation in rainbow trout takes place between day (D) 45 and D 55 after fertilization. Until D 400 the male GSI is maintained at about 0.4%, and that of the female at about 1%. Treatment with N,N-dimethylformamide (DMF) influences the sex ratio and GSI, and is therefore unsuitable as a solvent for steroids to be added to the aquarium water for examination of their effects on gonadal sex differentiation. Triton X-100 has no such effect, and is used as the steroid solvent in the present experiments. Progesterone (300 microgram/l) administered for 4 weeks from hatching, or from D 43, significantly affects the sex ratio in favour of females. Methyltestosterone given from hatching results in sterilization of the gonads, especially when the treatment is carried out for 8 weeks, or when a relatively high dose (300 microgram/l) is used for 4 weeks. When given from D 43 methyltestosterone has a masculinizing effect. It is concluded that progesterone and androgen(s) are important in inducing the onset of ovarian and testis development, respectively.