Critical period of androgen-inducible sex differentiation in a teleost fish, the European sea bass

Twenty-day exposures to 17 alpha -methyltestosterone (MT)(10 mg kg(-1) food ) starting at 86 or 106 days post fertilization (DPF) resulted in a complet e masculinization of sea bass Dicentrarchus lubrax. as opposed to 46% femal es present in the controls. Earlier exposures failed to suppress ovarian de velopment, resulting in a variable number of females (range 10.5-49.5%). Al l treatments assayed between 110 and 210 DPF induced a complete masculiniza tion. regardless of the androgen or the dose rested. However. a dose depend ent increase in the number of fish with intratesticular oocytes was observe d after MT but not after 17 alpha -methyldihydrotestosterone (MDHT) adminis tration. This might be a reflection of the capability of MT and not of MDHT to be converted into oestradiol by the action of aromatase. One hundred-da y exposures to either of the androgens (10 mg kg(-1) food) starting at 60 o r 160 DPF resulted in a total suppression of ovarian development and a part ial induction of sterility. Complete sterility was accomplished after 200 d ays of exposure starting at early ontogenesis (60-260 DPF). These deleterio us effects on gonadal development were later confirmed by a dramatic reduct ion of the gonadosomatic index. In addition, around the first year of age, growth was significantly depressed in all groups except those for which tre atments started at the latest (160-260 DPF: experiment 1) or the earliest a nd shortest (46-66 DPF; experiment 2). Hence, the critical period of androg en-inducible masculinization is located between 96 and 126 DPF and coincide s with a rapid proliferation of primordial germ cells in the sexually undif ferentiated gonad. (C) 2001 The Fisheries Society of the British Isles.