Production of a high percentage of male offspring in growth-enhanced transgenic tilapia using Oreochromis aureus ZZ selected pseudofemales

Abstract Gene transfer has offered a new tool for the development of improved fish strains for aquaculture. Monosex fish populations could minimize the impact of genetically modified organisms in the environment. In Oreochromis aureus , the use of pseudofemale spawners (sex-reversed male with a female phenotype) is an alternative technique for producing genetically male tilapia offspring. O. aureus fry were treated with 17β-estradiol at 100 mg/kg of food for 45 days. We obtained 77.1% females and 45.9% in the control group. Females randomly taken from the treated group were crossed with normal males. Fry from pseudofemales producing more than 90% male progenies were submitted to 17 β-estradiol treatment to obtain F 2 pseudofemales. The results of the sex-reversal were low and variable ranging between 66.0 and 84.3% females. F 2 pseudofemales were crossed with transgenic males from the F70 line ( O. aureus  ×  O. urolepis hornorum ) and non-transgenic ( O. aureus ) males. The sex ratio of progeny of F 2 pseudofemale deviated significantly ( P 2 /day) was similar to the normal females in the crosses with transgenic and non-transgenic males. To our knowledge this is the first report on the production of a near monosex population in genetically modified fish.