Patterning of the adult peripheral nervous system of Drosophila.

The peripheral nervous system (PNS) of the adult Drosophila melanogaster comprises over one thousand sensory organs (bristles and other types of sensilla) displayed in stereotyped positions of the epidermis. This two-dimensional pattern of sensory organs is generated by the emergence of the sensillum mother cells at specific positions of the imaginal discs, the precursors of the adult epidermis. These positions are largely specified by the interplay of three sets of genes: the proneural genes, their antagonists, and the neurogenic genes. The proneural genes confer upon cells the ability to become neural precursors. Among them, achaete and scute, two genes that encode transcriptional activators of the basic region-helix-loop-helix (bHLH) family, are most important for generating the adult PNS. Their expression is restricted to groups of cells, the proneural clusters, which appear at specific positions of the imaginal discs. Sensory organ precursor cells are born within these clusters. The known proneural antagonists either titrate these proteins by forming inactive complexes (extramacrochaetae) or repress achaete/scute expression at specific sites (i.e., hairy). In both cases, they refine sensory organ positioning by reducing the number of cells competent to become sensory organs. The neurogenic genes mediate cell-cell interactions that prevent most competent cells of a proneural cluster from becoming sensory organ mother cells. Depending on the size and shape of the proneural clusters and on their overlaps with regions of maxima or minima of expression of antagonists, sensory organs are generated either as single elements at unique positions, or as linear arrays containing many elements, or as characteristically shaped, two-dimensional arrangements covering specific regions of the fly's body.