Concurrent temporal patterning of neural stem cells in the fly visual system.

In the Drosophila optic lobe, medulla neuroblasts (NBs) are patterned by temporal and spatial inputs, which contribute to the generation of the medulla's ~150 neuron types. Here, we describe a third patterning mechanism that further diversifies neuronal fates in the medulla. The neuroepithelium from which NBs are continuously produced is patterned by opposing temporal gradients of the Imp and Syp RNA-binding proteins and their downstream transcription factors. Imp/Syp patterning results in the generation of seven cell types in successive developmental windows from NBs at the Vsx1-Hth spatial-temporal address. Medulla NBs are thus patterned by two concurrent temporal mechanisms: (1) the Imp/Syp state of the neuroepithelium when they are generated; and (2) the transcription factor cascade that progresses within them as they age. We further find that the birth order of medulla neurons correlates with their position along the anterior-posterior axis of the adult cortex, resulting in unanticipated regionalization of the retinotopic map.