Sex reversal in the genus Oreochromis: 1. Immersion of eggs and embryos in oestrogen solutions is ineffective

Failure to achieve feminization of tilapia under conditions where masculinization is successful was attributed to earlier determination of testis development than of ovary development. Success in obtaining sex inversion by immersion of eggs and fry of salmonids in hormone solutions suggested the possibility of obtaining sex inversion by treatments applied at early life stages. Thence, tilapia embryos were exposed to oestrogens as early as possible after fertilization, by incubating the eggs in oestrogen solutions. Further treatments consisted of rearing the fry in oestrogen solutions for various durations. Eggs of Oreochromis mossambicus (L.) and of O. mossambicus×O. urolepis hornorum Trewavas were collected from the mouths of females immediately after spawning and incubated in Erlenmeyer flasks set in a water bath shaker, at 27°C. The following oestrogenic hormones (dissolved in ethanol) were added to the flasks: 17β-oestradiol, progesterone, flutamide, and flutamide + progesterone. Initial treatments were given at concentrations of 1-50αg/l, for different durations ranging from 7 to 30 days after spawning. Due to high mortalities, higher doses (i.e. 100-5000αg/1) were tested for shorter periods (from 2 hours to 12 days). No sex inversion to female or to intersex gonads was achieved in any of the immersion treatments.