THE FUNCTION OF THE PRONEURAL GENES ACHAETE AND SCUTE IN THE SPATIOTEMPORAL PATTERNING OF THE ADULT LABELLAR BRISTLES OF DROSOPHILA-MELANOGASTER

The sensory precursors for labellar taste bristles develop from the la bial disc in three distinct temporal waves occurring at 0 h, 8 h and 1 4 h of pupal development. In each temporal wave, transcripts for the a chaete (ac) and scute (sc) genes are expressed in overlapping patterns in cells of the disc epithelium prior to the appearance of sensory mo ther cells (SMCs). No bristles form in mutant flies in which the ac an d sc genes are absent. When the sc gene alone is deleted, a set of sev en bristles fail to form. Pulses of ubiquitous sc(+) expression during pupal development, in a strain mutant for both ac and sc, can result in flies with all the labellar bristles at their correct positions. sc (+) pulses at times corresponding to the initiation of each of the wav es of SMC specification in the disc was sufficient to restore bristle pattern. Bristles were not induced at ectopic positions and times as a results of the ubiquitous expression of sc(+). These results suggest that the proneural genes ne and sc do not themselves set the pattern o f the labellar bristles. Instead, they are required for the elaboratio n of the pattern set by other gene products. We also show that the for mation and positioning of the later waves of bristles can take place e ven in the absence of bristles normally specified earlier.