Neuroepithelial depletion schedules cessation of neurogenesis in the Drosophila optic lobes

The brain is consisted of diverse neurons arising from a limited number of neural stem cells. Drosophila neural stem cells called neuroblasts (NBs) produces specific neural lineages of various lineage sizes depending on their location in the brain. In the Drosophila visual processing centre - the optic lobes (OLs), medulla NBs derived from the neuroepithelium (NE) give rise to neurons and glia cells of the medulla cortex. The timing and the mechanisms responsible for the cessation of medulla NBs are so far not known. In this study, we show that the termination of medulla NBs during pupal development is determined by the exhaustion of the NE stem cell pool. Altering NE-NB transition during larval neurogenesis disrupts the timely termination of medulla NBs. Medulla NBs terminate neurogenesis via a combination of cell death, terminal symmetric division, and a switch to gliogenesis. We show that temporal progression is not required for the termination of medulla NBs. The timing of NB cessation can be altered through the acquisition of a glial cell fate via Glial cells missing, or through conversion to type II NB cell fate via Tailless, or by inhibition of differentiation via Prospero knockdown. As the Drosophila OL shares a similar mode of division with mammalian neurogenesis, determining how and when these progenitors cease proliferation during development can have important implications for mammalian brain size determination and regulation of its overall function.