THE VENTRAL NERVOUS-SYSTEM DEFECTIVE GENE CONTROLS PRONEURAL GENE-EXPRESSION AT 2 DISTINCT STEPS DURING NEUROBLAST FORMATION IN DROSOPHILA

Within the Drosophila embryo, the formation of many neuroblasts depend s on the functions of the proneural genes of the achaete-scute complex (AS-C): achaete (ac), scute (sc) and lethal of scute (l'sc), and the gene ventral nervous system defective (vnd). Here, we show that vnd co ntrols neuroblast formation, in part, through its regulation of the pr oneural genes of the AS-C. vnd is absolutely required to activate ac, sc and l'sc gene expression in proneural clusters in specific domains along the medial column of the earliest arising neuroblasts. Using ac- lacZ reporter constructs, we determined that vnd controls proneural ge ne expression at two distinct steps during neuroblast formation throug h separable regulatory regions. First, vnd is required to activate pro neural cluster formation within the medial column of every other neuro blast row through regulatory elements located 3' to ac; second, throug h a 5' regulatory region, vnd functions to increase or maintain proneu ral gene expression in the cell within the proneural cluster that norm ally becomes the neuroblast. By following neuroblast segregation in va d mutant embryos, we show that the neuroectoderm forms normally and th at the defects in neuroblast formation are specific to particular pron eural clusters.