Hormonal sex reversal in red tilapia (Oreochromis niloticus and Oreochromis mossambicus) and inheritance of body colour in O. mossambicus and red tilapia: implications for commercial farming

The present investigation was conducted with a view to produce a female-free population and to test the heredity of body colour in tilapia. In this study, 6-day-old red tilapia were fed diets incorporated with 0, 25, 50, 75 and 100 mg/kg diet of a synthetic androgen, 17α-methyltestosterone (17α-MT), for 30 days. Administration was followed by 120 days of rearing in nylon hapas where hormone-free diet was given. At the end of the 120-day rearing, 17α-MT at 50 ppm induced 100% sex reversal, whereas 25, 75 and 100 mg/kg resulted in incomplete sex reversal. Immersion of newly hatched red tilapia fry in an aqueous solution of 17α-MT at 200 μg/L for 1 h produced 73% males; however, when given to 2-day-old fry, it yielded 78% males. The results indicate that oral administration of 17α-MT is more effective than treatment by immersion to produce male dominant red tilapia populations. Treatment of red tilapia with the androgen did not indicate any definite trend in the gonado-somatic index of both males and females. The results of progeny testing indicate that red tilapia does not belong to either XX-XY or WZ-ZZ sex determination systems, the two most common sex determination systems that exist in fish. When mating, Oreochromis mossambica (black tilapia) and red hybrid tilapia produced F1 progeny with a brown colour, the intermediate colour between the two parental body colours. When F1 hybrids were interbred, the three phenotypes got separated in the F2 generation with a 1 black/2 brown/1 red phenotypic ratio as expected of a single autosomal gene with incomplete dominant gene action. The importance of monosex male red tilapia farming and body colour inheritance is also discussed.