Neuroepithelial cells are converted into neural progenitors during an elongated interphase in Drosophila larval optic lobe.

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The first step in the development of the Drosophila optic medullar primordia is the expansion of symmetrically dividing neuroepithelial cells (NEs) ; this step is then followed by the appearance of asymmetrically dividing neuroblasts (NBs). Here, we performed detailed analyses demonstrating that individual NEs are converted into NBs. We also showed that this transition occurs during an elongated G1 phase. We also found that Notch signaling pathway was activated just before the transition and was rapidly downregulated. Furthermore, the clonal loss of the Notch wild copy in the NE region near the medial edge caused the ectopic accumulation of Delta, leading to the precocious onset of transition. Taken together, these findings indicate that the activation of Notch signaling during a finite window coordinates the proper timing of the NE-to-NB transition.