PATTERNING OF THE ADULT PERIPHERAL NERVOUS-SYSTEM OF DROSOPHILA

The peripheral nervous system (PNS) of the adult Drosophila melanogast er comprises over one thousand sensory organs (bristles and other type s of sensilla) displayed in stereotyped positions of the epidermis. Th is two-dimensional pattern of sensory organs is generated by the emerg ence of the sensillum mother cells at specific positions of the imagin al discs, the precursors of the adult epidermis. These positions are l argely specified by the interplay of three sets of genes: the proneura l genes, their antagonists, and the neurogenic genes. The proneural ge nes confer upon cells the ability to become neural precursors. Among t hem, achaete and scute, two genes that encode transcriptional activato rs of the basic region-helix-loop-helix (bHLH) family, are most import ant for generating the adult PNS. Their expression is restricted to gr oups of cells, the proneural clusters, which appear at specific positi ons of the imaginal discs. Sensory organ precursor cells are born with in these clusters. The known proneural antagonists either titrate thes e proteins by forming inactive complexes (extramacrochaetae) or repres s achaete/scute expression at specific sites (i.e., hairy). In both ca ses, they refine sensory organ positioning by reducing the number of c ells competent to become sensory organs. The neurogenic genes mediate cell-cell interactions that prevent most competent cells of a proneura l cluster from becoming sensory organ mother cells. Depending on the s ize and shape of the proneural clusters and on their overlaps with reg ions of maxima or minima of expression of antagonists, sensory organs are generated either as single elements at unique positions, or as lin ear arrays containing many elements, or as characteristically shaped, two-dimensional arrangements covering specific regions of the fly's bo dy.