Identification of enhancers that drive the spatially restricted expression of and in the outer proliferation center of the developing optic lobe.

Combinatorial spatial and temporal patterning of stem cells is a powerful mechanism for the generation of neural diversity in insect and vertebrate nervous systems. In the developing medulla, the neural stem cells of the outer proliferation center (OPC) are spatially patterned by the mutually exclusive expression of three homeobox transcription factors: Vsx1 in the center of the OPC crescent (cOPC), Optix in the main arms (mOPC), and Rx in the posterior tips (pOPC). These spatial factors act together with a temporal cascade of transcription factors in OPC neuroblasts to specify the greater than 80 medulla cell types. Here, we identify the enhancers that are sufficient to drive the spatially restricted expression of the and genes in the OPC. We show that removal of the cOPC enhancer in the inversion mutant leads to the loss of Vsx1 expression in the cOPC. Analysis of the evolutionarily conserved sequences within these enhancers suggests that direct repression by Optix may restrict the expression of and to the cOPC and pOPC, respectively.