Comparative effects of a natural androgen 11 Beta-hydroxyandrostenedio ne, and a synthétic androgen, 17 Alpha-methyltesterone, on the sex ratios of Oreochromis niloticus

Synthetic androgens are often considered to be more potent than natural androgens for sex reversal treatments in fish. Preliminary experiments on the natural androgen, 11 [bêta]-hydroxyandrostenedione (11 [bêta]-OH[delta]4), recently identified in the gonads of fry of O. niloticus in the early stages of testicular ontogenesis, showed a high masculinizing potential. Fry of 10-14 days post-fertilization (dPF) were produced from single pair matings using untreated females and either normal or sex reversed males, to assess any significant deviation in sex ratio compared to the controls. The fry were then treated during a minimum period of 21 days either with the natural or synthetic androgen added to the feed at different concentrations. Complete sex reversal was achieved with both steroids. All-male populations were produced in eight groups treated with 10-35 µg 11 [bêta]-OH[delta]4.g-1 of feed and in two groups treated with 17 [alpha]-methyltestosterone (17 [alpha]-MT) at 5 and 20 µg.g-1 of feed, respectively. There was no significant difference in the potency of the two androgens for concentrations higher or equal to 10 µg-g-1. However, at 5 µg.g-1, only 17 [alpha]-MT produced 100% male populations whereas 11 [bêta]-OH[delta]4 produced 88.9%. In contrast, the lowest concentration of 11 [bêta]-OH[delta]4 (1 µg.g-1) significantly deviated sex ratios whereas 17[alpha]-MT had no effect compared to the controls. The natural androgen 11 [bêta]-OH[alpha]4 can therefore constitute an alternative treatment to synthetic steroids. Low concentration treatments can be optimized by increasing the duration of treatment. (Résumé d'auteur