Suppression of the facet-strawberry position effect in Drosophila by lesions adjacent to notch.

The recessive visible rough eye mutant effect of fa( swb), a small deletion at the 5' end of the Notch locus, is suppressed when fa(swb) is coupled to five different closely linked deficiencies distal to salivary band 3C7. In addition, an inversion with a proximal breakpoint between 3C3 and 3C5 similarly suppresses the mutant effect. The data support the position effect interpretation of fa( swb): The small deletion allows functions distal to Notch to interfere with functions at Notch, and when the interference is eliminated, the fa(swb)-mutant effect disappears.-The fa(swb) deletion also interacts with another recessive visible rough-eye mutant at Notch called fa(g). In the cis condition, fa(swb) fa( g) double mutants have a mutant-eye phenotype like fa( g) (similar to the mutant effect of fa(swb)) and, in addition, express an accessory phenotype (thickened wing veins). Although the mutant-eye effect of fa(swb) can be suppressed by lesions adjacent to Notch, the accessory phenotype of the coupled mutants is not suppressed. It is suggested that the fa(swb) deletion has two observable effects: One is a modifiable position effect causing the fa(swb) rough-eye phenotype; the other is a stable effect exerted upon a 5-kb insertion that is the probable cause of the fa(g) mutant expression, thus resulting in a wing effect that accompanies the eye effect of fa( g).