| Summary: | Production of all-male tilapia for aquaculture is assuming an increasingly important role. An important pre-requisite to repeated obtainment of monosex tilapia is a clear understanding of the mechanisms underlying sex differentiation. Histological observations on gonadal morphorgonesis and sex differentiation provided basic data for hormonal sex manipulation in four commercially important species. Results indicate that gonadal morphogenesis starts at different times ranging from eight days after hatching in 0. mossambicus to 17 days in 0. niloticus. Sex differentiation followed a similar pattern, and ranged from 22 days in O. mossambicus to 36 days in 0. niloticus. The effects of subjecting fish to different rearing temperatures was assessed. No significant influence was found on sex ratio of treated fish. Observations on offspring sex ratio in intraspecific breeding and interspecific hybridization demonstrated that significant differences between batches are a common occurrance and their regularity cannot be adequately explained on the basis of sex chromosome theory alone. Treating fish with synthetic androgen (17 alpha methyltestosterone) and synthetic oestrogen, (17 alpha ethenylestradiol) resulted in species specific and dosage dependant differences in sex ratios. Results also revealed significant differences in sex ratios of different batches of fish subjected to the same treatment, thus demonstrating that success rate in sex inversion varies not only between species and between stocks, but in sib groups as well. Results of intraspecific and interspecific breeding suggest that sex determination in tilapia is under the influence of multiple factors. Results of hormone treatments indicate variations in inversion rate at batch level, thus demonstrating presence of individual differences in lability. On the basis of results from these four experiments, it is hypothesized that sex in tilapia is influenced by multiple genes and the fishes' propencity to change sex varies in individual fish. Progeny testing oestrogen sex inversed fish indicates that on the basis of the chromosome theory of sex determination, S. galileaus and O. niloticus are female homogametic, while O, macrochir is female heterogametic. The implications of the results obtained in this study for production of all-male tilapia are briefly discussed.
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