Aromatase plays a key role during normal and temperature-induced sex differentiation of Tilapia Oreochromis niloticus

In the tilapia Oreochromis niloticus, sex is determined genetically (GS D), by temperature (TSD) or by temperature/genotype interactions. Functional m asculinization can be achieved by applying high rearing temperatures during a critical period of sex differentiation. Estrogens play an important role in female differentiation of non-mammalian vertebrates. The involvement of aromatase, was assessed during the natural (genetic all-females and all-ma les at 27 degreesC) and temperature-induced sex differentiation of tilapia (genetic all-females at 35 degreesC). Gonads were dissected between 486-702 degree x days. Aromatase gene expression was analyzed by virtual northern and semi-quantitative RT-PCR revealing a strong expression during normal ov arian differentiation concomitant with high levels (465 +/- 137 fg/g) of oe stradiol-17 beta (E2-17 beta). This was encountered in gonads after the ons et of ovarian differentiation (proliferation of both stromal and germ cells prior to ovarian meiosis). Genetic males exhibited lower levels of aromata se gene expression and E2-17 beta quantities (71 +/- 23 fg/g). Aromatase en zyme activity in fly heads established a sexual dimorphism in the brain, wi th high activity in females (377.9 pmol/head/hr) and low activity in males (221.53 pmol/head/hr). Temperature induced the masculinization of genetic f emales to a different degree in each progeny, but in all cases repression o f aromatase expression was encountered. Genetic males at 35 degreesC also e xhibited a repression of aromatase expression. Aromatase brain activity dec reased by nearly three-fold in the temperature-masculinized females with al so a reduction observed in genetic males at 35 degreesC. This suggests that aromatase repression is required in the gonad land perhaps in the brain) i n order to drive differentiation towards testis development. (C) 2001 Wiley -Liss, Inc.