The bereft Gene, a Potential Target of the Neural Selector Gene cut, Contributes to Bristle Morphogenesis

The neural selector gene cut, a homeobox transcription factor, is required for the specification of the correct identity of external (bristle-type) sensory organs in Drosophila. Targets of cut function, however, have not been described. Here, we study bereft (bft) mutants, which exhibit loss or malformation of a majority of the interommatidial bristles of the eye and cause defects in other external sensory organs. These mutants were generated by excising a P element located at chromosomal location 33AB, the enhancer trap line E8-2-46, indicating that a gene near the insertion site is responsible for this phenotype. Similar to the transcripts of the gene nearest to the insertion, reporter gene expression of E8-2-46 coincides with Cut in the support cells of external sensory organs, which secrete the bristle shaft and socket. Although bft transcripts do not obviously code for a protein product, its expression is abolished in bft deletion mutants, and the integrity of the bft locus is required for (interommatidial) bristle morphogenesis. This suggests that disruption of the bft gene is the cause of the observed bristle phenotype. We also sought to determine what factors regulate the expression of bft and the enhancer trap line. The correct specification of individual external sensory organ cells involves not only cut, but also the lineage genes numb and tramtrack. We demonstrate that mutations of these three genes affect the expression levels at the bft locus. Furthermore, cut overexpression is sufficient to induce ectopic bft expression in the PNS and in nonneuronal epidermis. On the basis of these results, we propose that bft acts downstream of cut and tramtrack to implement correct bristle morphogenesis.