THE ACHAETE-SCUTE COMPLEX - GENERATION OF CELLULAR-PATTERN AND FATE WITHIN THE DROSOPHILA NERVOUS-SYSTEM

In developing embryos, cells receive and interpret positional informat ion as they become organized into discrete patterns and structures. On e excellent model for understanding the genetic regulatory mechanisms that pattern cellular fields is the regulation and function of the ach aete-scute complex (AS-C) in the developing nervous system of the frui t fly, Drosophila melanogaster. Three structurally homologous proneura l genes - achaete (ac), scute (sc), and lethal of scute (l'sc) - are r equired for neural stem cell formation. In Drosophila, the AS-C genes are initially expressed in patterns of cell clusters at reproducible a nteroposterior (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 combi natorially via a large array of cis-regulatory regions scattered throu ghout the AS-C to generate a segmentally repeated pattern of proneural clusters. Within each cluster (an equivalence group), one cell then r etains 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 pathwa y that is mediated via the action of the proneural and neurogenic gene s. The proneural genes therefore represent a nodal point in the patter ning of the nervous system. They receive global positional information , transduce it to discrete sets of cells, and trigger local cell inter actions that mediate cell fate decisions.