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, Oreochromis 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 males and to sire a mean progeny sex ratio of 95.6% male. While these results confirm the hypothesis of predominantly monofactorial sex determination, there appears to be a genetic basis for the occurrence of occasional 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 and the mass production of 'YY' males through YY x YY matings, obviating the need for time-consuming progeny testing to discriminate XY and YY male genotypes. This enables the production of 'YY' males and their all-male progeny, known as genetically male tilapia, to be mass produced on a commercial scale. We conclude that the YY male technology provides a robust and reliable solution to the serious and widespread problem of early sexual maturation, unwanted reproduction, and overpopulation in tilapia culture.