How to pattern an epithelium: lessons from achaete-scute regulation on the notum of Drosophila.

The notum of Drosophila is a good model system for the study of two-dimensional pattern formation. Attention has mainly focused on the regulation of the spatial expression of the genes of the achaete-scute complex (AS-C) that results in a stereotyped bristle pattern. Expression of AS-C genes has traditionally been viewed as a consequence of the activity of a group of factors that constitute a prepattern [Stern, 1954. Am. Sci. 42, 213]. The prepattern is thought to be composed of a mosaic of transcription factors that act in combination, through discrete cis-regulatory sequences, to activate expression of genes of the AS-C in small clusters of cells at the sites of each future bristle. Recent results challenge this view and suggest a hierarchy of activity amongst prepattern genes. It is suggested that in the medial notum, the selector-like gene pannier regulates the entire pattern, and is the only factor to directly activate AS-C genes. Other factors may play subsidiary roles. On the lateral notum genes of the iroquois complex appear to regulate the lateral pattern. Regulation of pannier and iroquois depends upon the signalling molecule Decapentaplegic. The majority of genes are expressed in either longitudinal or transverse domains on the notum and we discuss the possibility that pattern formation may rely on these two axial coordinates. We also discuss preliminary results suggesting that prepattern factors also regulate genes required for other, little studied, aspects of notal morphology, such as the muscle attachment sites and pigment distribution. Thus there may be a common prepattern for the entire structure.