Involvement of Gonadal Steroids During Natural and TemperatureInduced Sex Differentiation of the Tilapia Oreochromis niloticus
J.F Baroiller1,2, H. D´Cotta1,2, A. Toguyeni3, F. Clota1 and A. Fostier2
1: CIRAD-EMVT / Unité de Recherche Aquaculture, c/o CEMAGREF, 361 rue J.F. Breton, BP
5095, 34033 Montpellier-Cedex 1 (FRANCE)
jfb@beaulieu.rennes.inra.fr
2: INRA / Station Commune de Recherches en Ichtyophysiologie, Biodiversité et
Environnement (SCRIBE), Campus de Beaulieu, 35042 Rennes-Cedex (FRANCE)
3: Institut du Développement Rural / Université Polytechnique de Bobo-Dioulasso, 01 BP
1091 Bobo-Dioulasso 01 (BURKINA FASO)
ABSTRACT
Populations composed exclusively of males are extremely useful in the commercial
production of tilapias. They take into advantage the high growth-rate of males and avoid the
rapid over-population of rearing facilities brought about by the efficient reproduction of
females. This aquaculture requirement has encouraged research on sex determinism and
differentiation of the tilapia group. In the case of Oreochromis niloticus reared at 27ºC,
differentiation of the female gonad can be detected histologically at 28 days post-fertilisation
(DPF) by the appearance of the first meiosis features. The analysis of the gonadal
steroidogenesis has put into evidence a sex specificity of the steroidogenic potentialities.
Hence, estrogens are essentially synthesised in ovaries while 11-oxygenated androgens are
produced in testis. Experiments on hormonal treatments promoting sex-inversion have
shown that 11hydroxy-androstenedione is the most potent masculinising hormone among
the androgens identified, while estradiol is the most potent feminising hormone. These
treatments are mostly efficient when applied before 15 DPF. Thus, we measured the gonadal
endogenous levels of estradiol and androgens at this critical period and during sex
differentiation. In addition, expression studies of genes coding for two key enzymes
involved in the synthesis of these steroids (11hydroxylase for 11hydroxyandrostenedione and aromatase for estradiol) were performed. The aromatase gene is
strongly expressed during female differentiation in correlation with the determination of high
estradiol levels in the ovaries, whereas such gene expression and levels of hormone are lower
in the developing testes. On the other hand, the expression of the 11hydroxylase gene is
weak and does not appear to be sex-specific. Furthermore, masculinising genetic females by
treatment with high temperatures, we also studied aromatase and 11hydroxylase
expressions in the gonads of genetic all-female and all-male populations reared either at the
standard (27ºC) or at a masculinising temperature (35ºC) during sex differentiation. Gonadal
expression of 11hydroxylase was low and was not modified in genetic females when
masculinised by a high temperature exposure. In contrast, aromatase expression was strongly
repressed in these fishes. The potential role of both androgens and estrogens during natural
or temperature induced sex differentiation is discussed.