Conditional female lethal genetic systems evaluated in Drosophila melanogaster. (Top) Heat inducible female lethal system. The hid open reading frame was interrupted with the female-specific intron from the dsx gene, which contains a weak splice acceptor site. Immediately 3' of hid was the dsxRE, which includes several TRA/TRA2 binding sites. Binding of TRA/TRA2 to the dsxRE enhances (+++) splicing of the dsx intron in females. Further downstream was the intron-exon from the Dror2 gene, which has an optimal splice acceptor site. Males were predicted to use the stronger Dror2 splice site whereas females would use the dsx acceptor site. Thus only the female hid transcript was predicted to encode full-length protein. (Bottom) Tetracycline-repressible female lethal system. On normal diet, expression of tTA in the female fat body of third instar larvae led to activation (+++) of hid gene expression and female-specific lethality. Lethality was repressed by the addition of tetracycline to the diet.