Control of early neurogenesis of the Drosophila brain by the head gap genes tll, otd, ems, and btd.

The progenitors of the Drosophila central nervous system (CNS), called neuroblasts, segregate from the neurectoderm of the early embryo in a stereotyped pattern. The neuroblasts that give rise to the brain segregate from the procephalic neurectoderm and form three neuromeres, called protocerebrum, deuterocerebrum, and tritocerebrum. The expression of the proneural genes of the achaete-scute complex (AS-C) is required for neurectodermal cells to acquire the competency to form neuroblasts. We show here that the expression of the proneural gene lethal of scute (l'sc) is required for the development of the majority of the procephalic neuroblasts. l'sc expression in the procephalic neurectoderm is controlled by the head gap genes tailless (tll), orthodenticle (otd), buttonhead (btd), and empty spiracles (ems), which are expressed in partially overlapping domains of the head neurectoderm. Loss of function of a given head gap gene results in the absence of l'sc expression in its domain, followed by the absence of neuroblasts that would normally segregate from this domain. Loss of tll function results in the absence of all protocerebral neuroblasts, otd functions in a domain that includes a large part of the protocerebrum and a smaller part of the adjacent deuterocerebrum. Both ems and btd are required in partially overlapping subsets of neuroblasts of the deuterocerebrum and tritocerebrum.