46 J.Y. KWON ET AL. JOURNAL OF EXPERIMENTAL ZOOLOGY 287:46–53 (2000) © 2000 WILEY-LISS, INC. JEZ 0836 Masculinization of Genetic Female Nile Tilapia (Oreochromis niloticus) by Dietary Administration of an Aromatase Inhibitor During Sexual Differentiation JOON YEONG KWON, VAHID HAGHPANAH, LUZ MARY KOGSON-HURTADO, BRENDAN J. MCANDREW, AND DAVID J. PENMAN* The Institute of Aquaculture, University of Stirling, Stirling FK9 4LA, Scotland, United Kingdom ABSTRACT A series of experiments was carried out in which genetically female Nile tilapia (Oreochromis niloticus) fry were treated with Fadrozole, a nonsteroidal aromatase inhibitor (AI), in the diet during the period of sexual differentiation. Batches of tilapia fry treated with AI during the first 30 days following yolk-sac resorption (7–37 days post hatch, dph) showed a dose-depen- dent increase in the percentage of males from 0 to 200 mg · kg –1 . The percentage of males re- mained approximately constant (92.5–96.0%) from 200 to 500 mg · kg –1 . Any continuous 2- or 3-week treatment with 500 mg · kg –1 AI in this 4-week period successfully masculinized the major- ity of the treated fish (>80%). Treatments of 1 week duration revealed that the most sensitive time to AI lies in the first week (between 7 and 14 dph). Progeny testing of males from AI-treated groups gave results indicating that these were XX males, as expected. These experiments strongly implicate aromatase activity as a key factor in sexual differentiation in the Nile tilapia. J. Exp. Zool. 287:46–53, 2000. © 2000 Wiley-Liss, Inc. Grant sponsor: Overseas Research Student Award. Vahid Haghpanah is deceased. Luz Mary Kogson-Hurtado’s current address: Calle 66A, Nº11-138 Barrio La Sultana, Manizales-Caldas, Colombia. *Correspondence to: Dr. David J. Penman, Institute of Aquacul- ture, University of Stirling, Stirling FK9 4LA, Scotland. E-mail: djp1@stir.ac.uk Received 7 July 1999; Accepted 17 November 1999 Treating fish with exogenous sex steroid hor- mones during early developmental stages can cause sex-reversal against genotypic sex, or ste- rility. It has been proved in many fish species that exogenous androgens generally cause masculin- izing effects, while exogenous estrogens cause feminizing effects (Yamazaki, ’83). However, the mechanism of action of these steroids on sex dif- ferentiation is, so far, not well understood. The exact nature of the role of the endogenous sex ste- roids in sex differentiation is not known (Rothbard et al., ’87; Feist and Schreck, ’96), nor is it clear to what extent exogenous sex steroids mimic the role of endogenous sex steroids (Piferrer et al., ’94). A detailed understanding of steroid metabolism during sexual differentiation would be advanta- geous in furthering our knowledge of the role of steroids in this process. Recently, the function of cytochrome P450 aromatase in sex determination has been exam- ined, because it is the enzyme responsible for the aromatization of androstenedione into estrone and testosterone into estradiol-17β (Jeyasuria et al., ’96). In some species, inhibiting the action of this enzyme caused masculinizing effects similar to those caused by androgens (e.g., in the bullfrog Rana catesbeiana, Yu et al., ’93; in the chicken Gallus domesticus, Elbrecht and Smith, ’92; Wartenburg et al., ’92; and in chinook salmon Oncorhynchus tshawytscha, Piferrer et al., ’94). Aromatization in genetic females during early de- velopmental stages appears to be a key step in ovarian ontogeny. Thus, the investigation of aromatase function in early developmental stages should increase our understanding of sex deter- mination in fish. The Nile tilapia, Oreochromis niloticus, is of major importance in aquaculture (FAO, ’97). As the production of all male populations can elimi- nate unwanted reproduction during culture, the sex determination system of this species has been investigated and various sex control techniques have been developed. These include treatment