Mobile element 297 in the Abd-B gene of Drosophila melanogaster, not Delta 88, is responsible for the tuh-3 mutation.

The tumorous-head-3 (tuh-3) mutation has been associated with the insertion of mobile element Delta 88 at +200 on the bithorax complex (BX-C) DNA map, 5' of all Abdominal-B (Abd-B) transcripts. Different phenotypes of tuh-3 are regulated by the tumorous-head-1 (tuh-1) maternal effect locus. In the presence of the recessive tuh-1h maternal effect, tuh-3 offspring produce homeotic abdominal and genital tissue in the head. In the presence of the dominant tuh-1g maternal effect, tuh-3 offspring have normal heads but now show genital defects. One other mutant, I127B, produces flies with identical defects to that of tuh-3 in the presence of both maternal effects. Molecular analysis of I127B revealed the insertion of mobile element 297 in the Abd-B gene, approximately 25 kb downstream of the Delta 88 insertion in tuh-3. No other abnormalities were detected. Reexamination of our tuh-3 strain revealed a 297 insertion in an identical region to that of I127B, in addition to the Delta 88 insertion. Recombinants of tuh-3, carrying 297 only, produced homeotic head defects and genital defects in the presence of the tuh-1h and tuh-1g maternal effects, respectively. Recombinants of tuh-3, carrying Delta 88 only, failed to produce any defects in the presence of either maternal effect. Based upon these results, we propose that it is the 297 insertion in the Abd-B gene, not Delta 88, that is responsible for the tuh-3 mutation.