GENETIC MANIPULATION OF SEX-RATIO FOR THE LARGE-SCALE PRODUCTION OF ALL-MALE TILAPIA, OREOCHROMIS-NILOTICUS

We describe the results of a breeding programme for Nile tilapia, Oreo chromis niloticus, incorporating estrogen-induced sex reversal of male to female and progeny testing to generate novel 'YY' male genotypes. 'YY' male genotypes proved to be as viable and fertile as normal XY ma les and to sire a mean progeny sex ratio of 95.6% male. While these re sults confirm the hypothesis of predominantly monofactorial sex determ ination, there appears to be a genetic basis for the occurrence of occ asional females in the progeny of 'YY' males. It is likely that these arise through the action of several autosomal sex modifying genes. We report the first instance of feminization of YY genotypes in tilapia a nd the mass production of 'YY' males through YY x YY matings, obviatin g the need for time-consuming progeny testing to discriminate XY and Y Y male genotypes. This enables the production of 'YY' males and their all-male progeny, known as genetically male tilapia, to be mass produc ed on a commercial scale. We conclude that the YY male technology prov ides a robust and reliable solution to the serious and widespread prob lem of early sexual maturation, unwanted reproduction, and overpopulat ion in tilapia culture.