LAUSR

Spatial patterning of neural stem cell populations is a powerful mechanism by which to generate neuronal diversity. In the developing Drosophila medulla, the symmetrically dividing neuroepithelial cells of the outer proliferation center (OPC) crescent are spatially patterned by the non-overlapping expression of three transcription factors: Vsx1 in the center, Optix in the adjacent arms, and Rx in the tips. These spatial genes compartmentalize the OPC and, together with the temporal patterning of neuroblasts, act to diversify medulla neuronal fates. The observation that the dorsal and ventral halves of the OPC also grow as distinct compartments, together with the fact that a subset of neuronal types are generated from only one half of the crescent, suggests that additional transcription factors spatially pattern the OPC along the dorsal-ventral (D-V) axis. Here, we identify the spalt (salm and salr) and disco (disco and disco-r) genes as the D-V patterning transcription factors of the OPC. Spalt and Disco are differentially expressed in the dorsal and ventral OPC from the embryo through to the third instar larva, where they cross-repress each other to form a sharp D-V boundary. We show that hedgehog is necessary for Disco expression in the embryonic optic placode and that disco is subsequently required for the development of the ventral OPC and its neuronal progeny. We further demonstrate that this D-V patterning axis acts independently of Vsx1-Optix-Rx and thus propose that Spalt and Disco represent a third OPC patterning axis that may act to further diversify medulla fates.