Scott, A., Penman, D.J., Skibinski, D.O.F., Beardmore, J.A
A male Oreochromis niloticus obtained by gynogenesis was found to sire 100% male progenies consistently when crossed with normal females. Evidence suggests that this fish is a ‘YY’ supermale and that its mother was spontaneously sex reversed (XY). Hormone treatment of normal fry followed by progeny testing was used to generate sex-reversed male and female O. niloticus. Intercrosses among these sex-reversed fish, normal males and females, and the supermale result in progeny sex ratios which are consistent with a monofactorial system of sex determination in O. niloticus where males are XY and females XX. However, some exceptional results were obtained suggesting the existence of minor sex-ratio modifying factors. It is shown experimentally that YY males can be used to study sex reversal to female in this species without the need for progeny testing. A programme for the commercial production and maintenance of YY broodstock and its use for the generation of monosex male progenies in aquaculture is discussed.