The EMBO Journal vol. 1 1 no.9 pp.3385 - 3393, 1992 The extramacrochaetae gene provides information for sensory organ patterning Pilar Cubas and Juan Modolell Centro de Biologfa Molecular, Consejo Superior de Investigaciones Cientificas and Universidad Aut6noma de Madrid, Canto Blanco, 28049 Madrid, Spain Communicated by J.Modolell The Drosophila adult epidermis displays a stereotyped pattern of bristles and other types of sensory organs (SOs). Its generation requires the proneural achaete (ac) and scute (sc) genes. In the imaginal wing disc, the anlage for most of the thoracic and wing epidermis, their products accumulate in groups of cells, the proneural clusters, whose distribution prefigures the adult pattern of SOs. These proteins then induce the emergence of SO mother cells (SMCs). Here, we show that the extra- macrochaetae (emc) gene, an antagonist of the proneural function, is another agent that contributes to SO positioning. In the wing disc, emc is expressed in a complex and evolving pattern. SMCs appear not only within proneural clusters but also within minima of emc expression. When one of these spatial restrictions is eliminated, by ubiquitously expressing ac-sc, SMCs still emerge within minima of emc. When in addition, the other spatial restriction is reduced by decreasing emc expression, many ectopic SMCs emerge in a relatively even spaced and less constant pattern. Thus, the hetero- geneous distribution of the emc product is one of the elements that define the positions where SMCs arise. emc probably refines SMC (and SO) positioning by reducing both the size of proneural clusters and the number of cells within clusters that can become SMCs. Key words: achaete/extramacrochaetae/pattern formation/ scute/sensory organ determination Introduction In Drosophila, the set of bristles and other types of cuticular sensory organs (SOs) constitutes a classical model to study pattern formation. On the notum of the fly, two types of bristles appear: large bristles (macrochaetae), which appear in fixed number at constant positions, and small bristles (microchaetae), which are evenly distributed but whose number and position vary slightly. Each SO is generated by a sensory mother cell (SMC) that undergoes two differen- tial divisions (Bodmer et al., 1989; Hartenstein and Posakony, 1989). The four progeny cells differentiate into the components of the SO. In the case of the notum macrochaetae, their SMCs appear during the third instar larva and early pupa stages in precisely defined positions of the wing imaginal disc (Cubas et al., 1991; Huang et al., 1991; Skeath and Carroll, 1991). Their distribution prefigures the adult macrochaetae pattern. Thus, the generation of SMCs at specific sites largely explains macrochaetae positioning. How, within the imaginal discs, are the positions of the SMCs specified? The achaete (ac) and scute (sc) proneural genes, whose protein products confer on cells the ability to become SMCs, are instrumental to this process (reviewed in Ghysen and Dambly-Chaudiere, 1988, 1989; Campuzano and Modolell, 1992). In wild type imaginal discs, ac and sc are co-expressed in groups of cells (the proneural clusters), which define the areas where SMCs arise (Romani et al., 1989; Cubas et al., 1991; Skeath and Carroll, 1991). ac/sc expression in these clusters is thought to be controlled by a complex set of cis-regulatory sequences, which respond to local combinations of transcriptional regulators (pre- pattern) present in imaginal discs (Ruiz-Gomez and Modolell, 1987; Leyns et al., 1989) and by self- and cross- stimulatory interactions between these genes (Martinez and Modolell, 1991; Skeath and Carroll, 1991). A fixed number of cells from each cluster (one or a few) become SMCs and prevent, by cellular interactions (reviewed in Simpson, 1990), similar determination of neighbouring cells. In mutants with expanded proneural clusters, extra SMCs appear in new positions (Cubas et al., 1991; Skeath and Carroll, 1991; our results). Thus, although the primary function of ac and sc is to promote neural determination, their spatially restricted expression helps define SMC posi- tioning. However, there appear to be additional topographical cues for positioning SMCs. In the absence of the ac-sc endogenous genes, homogeneous but limited expression of sc promotes development of a small number of notum macrochaetae (Rodriguez et al., 1990). Although this generalized expression should not restrict SMC determina- tion to specific sites, these SOs still appear in wild type positions. This indicates that cells at the sites where SMCs emerge have an increased ability to respond to the proneural effects of ac-sc and become SMCs. Such capacity to respond is unevenly distributed in the disc and may help SMC positioning in the wild type. This conclusion is supported by the facts that in wild type discs, SMCs arise in extremely reproducible positions within proneural clusters and that some clusters of ac - sc expressing cells do not give rise to SMCs (Cubas et al., 1991). A candidate for modulating the responsiveness of cells to proneural function is the extramacrochaetae (emc) gene (Botas et al., 1982), a negative trans-regulator of ac and sc (Moscoso del Prado and Garcia-Bellido, 1984a). The EMC, AC and SC proteins have a helix-loop-helix (HLH) dimerizing domain, but only AC and SC contain an adjacent basic region necessary for DNA binding and transcriptional activation (Villares and Cabrera, 1987; Ellis et al., 1990; Garrell and Modolell, 1990; for a review see Garrell and Campuzano, 1991). AC and SC would activate genes implementing the neural developmental pathway. EMC would interfere with this AC-SC proneural function by 3385