Variation in sex determination and the application of the YY male technology for the production of all-male populations of the tilapia Oreochromis mossambicus

Tilapia is presumed to have a well-defined genetic mechanism of sex determination, but not all sex ratios are compatible with a monofactorial sex determination model.A theory of autosomal gene influence, as well as temperature sex determination (TSD) has been proposed in order to explain large variations in sex ratios.This study assessed the variation in progeny sex ratio in O. mossambicus as a basis for the application of YY male technology in the production of all-male progeny groups.Three populations of O. mossambicus were sampled that are representative of the geographical distribution of the species in Southern Africa.Progeny groups were produced from randomly selected parents and maintained at constant temperature during the labile period of sex differentiation.Variation in sex ratios between different families of the same strain as well as between different strains was calculated.No significant differences were observed in sex ratio between strains, though a significant intra-group variation was identified.This study identified both male and female-biased sex ratios.The data in general conform to a monofactorial sex determination model.Male-biased sex ratio observed in one strain can possibly be ascribed to temperature sex determination (TSD).This strain of O. mossambicus originated from an area with different annual temperature patterns and the possibility of TSD having an adaptive advantage is discussed.This thesis also presents the results of a program to produce monosex male tilapia through the application of the YY male technology in O. mossambicus.Viable XY female and YY male genotypes were produced.XY females sired progenies ranging from 68-94% male ofspring, while YY males sired a mean progeny of 94% male phenotypes.From these results it is concluded that YY male technology provide a viable method for the production of all-male progeny in O. mossambicus.Once available on a commercial scale, the technology can be made more reliable through the application of the appropriate selection methods.v ACKNOWLEGDEMENTS My family and friends for their support and patience during the write-up of this project.I would like to thank the following people for their contributions: Dr Danie Brink