In vitro steroid secretion during early development of mono-sex rainbow trout: sex differences, onset of pituitary control, and effects of dietary steroid treatment.

Sex differentiation in many teleost species can be controlled by treatment with steroids. To investigate the development of steroidogenesis during both natural and controlled sex differentiation, the production of androstenedione, testosterone, and estradiol were determined in tissues from populations of all-female and all-male rainbow trout (Oncorhynchus mykiss). At various times from hatching through gonadal sex differentiation, explants of steroidogenic tissues were incubated in vitro alone or in the presence of partly purified salmon gonadotropin and the resulting media were assayed for steroids. Androstenedione and testosterone were produced at higher levels in media from testes than from ovaries within 2 weeks of the onset of feeding (before any dramatic gonadal differentiation). Gonadal estradiol secretion was nondetectable until about 1 month after the onset of feeding when females produced more than males. Gonadotropin stimulated gonadal steroid production only after differentiation, but stimulated anterior kidney (interrenal) production of androstenedione much earlier in development. Dietary treatment of rainbow trout with either estradiol or 17 alpha-methyltestosterone (MT) inhibited in vitro gonadal steroid production and this effect persisted in MT-fed fish even after withdrawal of dietary steroids.