In heterozygous flies, a large number of bristles on the head and thorax are lost, and the wing veins , mostly the fourth and fifth longitudinals, are shortened. Hairless acts as an antagonist of Notch-signaling activity. Experiments were performed to determine if Hairless is essential during embryonic development or only later, during imaginal disc development. H transcripts are present in the developing CNS at the time of neuroblast segregation. To eliminate the maternal contribution of H to the embryo, germ-line clones homozygous for a mutant H allele were generated.
Mutant embryos derived from null oocytes survive to the larval stage.
To distinguish between these two alternatives, the ability of H mutant cells to inhibit neighboring wild-type cells was measured. The prediction that H acts cell-autonomously cannot be experimentally tested in the pupal notum during bristle development, since presumptive neural cells failing to differentiate cannot be identified from epidermal cells. However, the cell-autonomous behaviour of H mutant cells may be studied in the wing. H mutant clones that interrupt vein differentiation were analyzed at the cell level. Mutant cells do not participate in the formation of veins, but instead appear to interrupt vein differentiation in a cell-autonomous manner.