The patterning of Drosophila occurs through a hierarchy of regulatory events which gradually refine spatial organization to the level of individual cell fate specification. Early acting genes establish the position-specific expression of late acting genes

The patterning of Drosophila occurs through a hierarchy of regulatory events which gradually refine spatial organization to the level of individual cell fate specification. Early acting genes establish the position-specific expression of late acting genes which generate diverse cell fates by creating unique local environments. The mechanisms which establish precise, spatially-restricted expression of locally acting genes, however, are not well understood. Here, we address this question using sensory organ development as a model system. In the Drosophila adult, each sensory structure is derived from a single precursor cell, the sensory mother cell (SMC), in a multi-step selection process. First, early patterning events establish expression of the proneural genes achaete (ac) and scute (sc), which act locally to confer neural competence to clusters of cells (reviewed by Campuzano and Modolell, 1992; Jan and Jan, 1993). Then, local regulatory events controlled by neurogenic genes select cells from within a proneural cluster to become SMCs and give rise to individual sensory organs (reviewed by Artavanis-Tsakonis and Simpson, 1991; CamposOrtega, 1993). Our focus is on the events that organize the microchaete sensory bristles of the Drosophila melanogaster leg. It has previously been shown that the longitudinal microchaete rows of the leg are patterned in part through the function of the hairy (h) gene (Orenic et al., 1993), which encodes a basic helix-loophelix (bHLH) protein (Rushlow et al., 1989) and is expressed in a spatially restricted manner in the leg imaginal disc (Fig. 1B) (Carroll and Whyte, 1989). In wing imaginal discs, h negatively regulates ac to pattern the bristle primordia (Cubas et al., 1991; Skeath and Carroll, 1991; Cubas and Modolell, 1992) by binding directly to DNA sequences that govern ac expression (Ohsako et al., 1994; Van Doren et al., 194). H also negatively regulates ac expression in the leg imaginal disc. In the everted leg, h is expressed in four longitudinal stripes (Carroll and Whyte, 1989), each of which is flanked by Ac stripes. In the absence of H function, ac expression expands into the domain normally occupied by H, resulting in disorganized bristle rows and ectopic bristle formation (Orenic et al., 1993). 2891 Development 126, 2891-2899 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 DEV5219