Affiliations: 1) Department of Cell and Systems Biology, University of Toronto, Toronto, ON; 2) Department of Biology, University of Toronto (Mississauga), Mississauga, ON
Keywords: o. stem cells; d. neuronal specification
The Drosophila optic lobe serves as an excellent model system in which to study the mechanisms that regulate neurogenesis. The largest neuropil of the optic lobe, the medulla, is comprised of 40 000 neurons belonging to over 90 neuronal types. These neurons are generated from a neuroepithelial crescent called the outer proliferation center (OPC). At the beginning of the 3rd larval instar, a proneural wave converts each neuroepithelial cell into a neuroblast (NB), which subsequently divides asymmetrically to generate two neuronal or glial cells. It has been previously shown that OPC NBs generate unique sets of neurons based on their spatial origin and temporal state. In the spatial axis, the OPC is subdivided by the expression of Vsx1,Optix, Rx and Hh to generate eight distinct compartments. In the temporal axis, each NB sequentially expresses a cascade of five temporal transcription factors (tTFs) - Hth, Ey, Slp1/2, D and Tll - as it ages. Therefore, as the NBs in each OPC spatial domain divide, they rapidly cycle through the tTFs to generate a set of unique neurons at each spatio-temporal birth address. Here, we describe a third patterning axis that further diversifies medulla neuronal fates. We show that the OPC neuroepithelium is patterned by the expression of five long-range temporal factors; Imp, Syp, Chinmo, Mamo and E93. Over the course of the two days of OPC neurogenesis, we find that Imp and Chinmo are expressed in descending gradients in the OPC neuroepithelium, whereas Syp is reciprocally expressed in an ascending gradient. We show that these genes are required for the expression of the transcription factors Mamo and E93 in early and late temporal windows, respectively. To determine whether these long-range temporal patterning genes are required for neuronal fate specification, we focused on the neurons that are generated from the Vsx1-Hth spatio-temporal window. Surprisingly, we find that four (not one) neuronal types – Tm23, TmY15, Pm3 and TmY12 - are sequentially generated by Vsx1+Hth+ NBs during the two days of neurogenesis. We further show that the long-range temporal genes are required for this unexpected diversity; mamo promotes early neuronal fates (Tm23 and TmY15), whereas syp is required for late fates (Pm3 and TmY12). Taken together, our findings show that medulla NBs integrate three inputs – one spatial and two concurrent temporal (rapidly changing tTFs and long-range) – to generate the extensive neuronal diversity observed in the medulla.