Thesis (MPhil)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: 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. / AFRIKAANSE OPSOMMING: Tilapia spesies is oorwegend onderworpe aan ‘n goed gedefinieerde enkelfaktor model van genetiese geslagsbepaling, hoewel afwykings van sodanige geslagsbepalingsmodel soms waargeneem word. Die invloed van outosomale gene, sowel as omgewings temperatuur word voorgehou as verklaring van die waargenome variasies in geslagsverhoudings. Hierdie studie behels die evaluasie van variasie in geslagsdifferensiasie van O. mossambicus as basis vir die implementering van die YY manlike tegnologie vir die produksie van slegs-manlike nageslag groepe. Monsters is bekom van drie populasies van O. mossambicus verteenwoordigend van die geografiese verspreiding van die spesie in Suider Afrika. Nageslag groepe is geproduseer vanaf ewekansig gekose ouerpare en by ‘n konstante temperatuur gehuisves tydens die sensitiewe tydperk van geslagsdifferentiasie. Variasie in geslagsverhouding tussen verskillende families binne dieselfde subpopulasie sowel as binne verskillende populasies is bepaal. Geen betekenisvolle verskille is waargeneem in die geslagsverhouding tussen die verskillende populasies nie terwyl betekenisvolle verskille tussen families binne ‘n populasie waargeneem is. Die data voldoen aan ‘n enkelfaktor genetiese geslagsbepalingsmodel, met die uitsondering van een populasies waar ‘n moontlike omgewingsverwante geslagsbepalingseffek waargeneem is. Die tesis sluit ook in die resultate in van ’n program om YY-tegnologie in O. mossambicus te vestig ten einde manlike nageslag groepe te produseer. Funksionele vroulike XY- en manlike YY genotipes is ontwikkel. Die vroulike XY-genotipes het manlike nageslag groepe opgelewer wat wissel van 68%-94% manlik, terwyl manlike YY-genotipes nageslag groepe gelewer het wat gemiddeld 94% manlik was. Die resultate bevestig die lewensvatbaarheid van YY-tegnologie vir die kommersiële produksie van manlike nageslaggroepe in O. mossambicus.