The achaete-scute complex: Generation of cellular pattern and fate within the Drosophila nervous system

In developing embryos, cells receive and interpret positional information as they become organized into discrete patterns and structures. One excellent model for understanding the genetic regulatory mechanisms that pattern cellular fields is the regulation and function of the achaete-scute complex (AS-C) in the developing nervous system of the fruit fly, Drosophila melanogaster. Three structurally homologous proneural genes-achaete (ac), scute (sc), and lethal of scute (l'sc)-are required for neural stem cell formation. In Drosophila, the AS-C genes are initially expressed in patterns of cell clusters at reproducible anteroposterior (AP) and dorsoventral (DV) coordinates that foreshadow where neural precursors arise. In the embryonic central nervous system (CNS), the gene products of AP and DV axis-patterning genes act combinatorially via a large array of cis-regulatory regions scattered throughout the AS-C to generate a segmentally repeated pattern of proneural clusters. Within each cluster (an equivalence group), one cell then retains proneural gene expression and is singled out as the neural stem cell (neuroblast). The neuroblast inhibits the surrounding cells from adopting neural fates (lateral inhibition) through a signaling pathway that is mediated via the action of the proneural and neurogenic genes. The proneural genes therefore represent a nodal point in the patterning of the nervous system. They receive global positional information, transduce it to discrete sets of cells, and trigger local cell interactions that mediate cell fate decisions.