The Drosophila gene Hairless encodes a novel basic protein that controls alternative cell fates in adult sensory organ development.

The mechanosensory bristles of adult Drosophila are composed of four cells that, in most cases, are progeny of a single sensory organ precursor (SOP) cell. Two sister cells in this lineage, the trichogen and tormogen, produce the external shaft and socket of the bristle, respectively. Loss-of-function mutations of Hairless (H) confer two distinct mutant phenotypes on adult bristles. The bristle loss phenotype results from the failure to specify and/or execute the SOP cell fate; the double socket phenotype results from the transformation of the trichogen (shaft) cell into a second tormogen (socket) cell. We have found that the H gene encodes a novel basic protein with a predicted molecular mass of 109 kD. Basal levels of expression of a transgene (P[Hs-H]) in which the H protein-coding region is under the control of the Hsp70 promoter are sufficient to provide full rescue of H mutant phenotypes. Heat shock treatment of P[Hs-H] transgenic animals as late larvae and early pupae produces a tormogen-to-trichogen (double shaft) cell fate transformation, as well as bristle multiplication and loss phenotypes very similar to those caused by loss-of-function mutations in the neurogenic gene Notch. Our results indicate that the SOP cell fate requires H to antagonize the activity of the neurogenic group of genes and that the expression of distinct cell fates by the trichogen/tormogen sister cell pair depends on an asymmetry in their levels of H+ activity or in their thresholds for response to H.