Temporal patterning of Drosophila medulla neuroblasts controls neural fates

In the Drosophila optic lobes, the medulla processes visual information coming from inner photoreceptors R7 and R8 and from lamina neurons. It contains ~40,000 neurons belonging to over 70 different types. We describe how precise temporal patterning of neural progenitors generates these different neural types. Five transcription factors--Homothorax, Eyeless, Sloppy-paired, Dichaete and Tailless--are sequentially expressed in a temporal cascade in each of the medulla neuroblasts as they age. Loss of either Eyeless, Sloppy-paired or Dichaete blocks further progression of the temporal sequence. We provide evidence that this temporal sequence in neuroblasts, together with Notch-dependent binary fate choice, controls the diversification of the neuronal progeny. Although a temporal sequence of transcription factors had been identified in Drosophila embryonic neuroblasts, our work illustrates the generality of this strategy, with different sequences of transcription factors being used in different contexts.