Abstract
Two consecutive experiments were carried out to investigate the effects of phytoestrogens on sex reversal of Nile tilapia (Oreochromis niloticus) larvae. Experiment 1 investigated the effects of phytoestrogen contents of soybean meal (SBM) on sex ratio. Nile tilapia larvae (0.01g/fish) were fed isonitrogenous (35% crude protein) and isocaloric (19MJ/kg) diets containing fishmeal (FM) or SBM as protein sources, each with or without supplementation of 17α-methyltestosterone (MT) at 60mg/kg diet, for 28days. The control, SBM-based diet produced significantly higher percentage of females (77%) than the FM-based diet (52%). The addition of MT to SBM resulted in a significant reduction in percentage of females (31%), however, this percentage was significantly higher than that produced by FM-based diet treated with MT (only 3% females).The second experiment was carried out to verify the results of the first experiment by investigating the effects of different concentrations of pure, crystalline genistein and daidzein (the major phytoestrogens present in soybean) on sex reversal of Nile tilapia larvae fed on a FM-based diet treated with MT. The same FM-based diet used in experiment 1 was used in experiment 2, and treated with MT, and then divided into equal portions treated with genistein or daidzein at 0, 10, 20 and 30mg/kg diet. The diets were fed to Nile tilapia larvae for 28days. The control, hormone-free diet produced almost equal ratio of females and males (0.96:1). The addition of MT significantly increased the percentage of males. The percent of males was sharply reduced with increasing daidzein and genistein concentrations in the diets. These results revealed that phytoestrogen contents of SBM have estrogenic effects on Nile tilapia larvae. Therefore, soybean meal and other plant protein sources containing high levels of phytoestrogens should be avoided as protein sources for Nile tilapia larvae during sex reversal treatments.
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