Netrins and receptors control Drosophila optic lobe organization and transmedullary neuron axon targeting

During development, integration of temporal patterning and spatial patterning of neural progenitors as well as Notch-dependent binary fate choice between sister neurons contribute to generation of neural diversity. How these upstream neural fate specification programs regulate downstream effector genes to control axon targeting and neuropil assembly remains less well-understood. Here we show that Notch-dependent binary fate choice in Drosophila medulla neurons regulates the expression of Netrin, and that Netrin pathway controls axon guidance of transmedullary (Tm) neurons and contributes to the organization of optic lobe neuropils. Netrins are enriched in the lobula where Tm axons target, and the attractive receptor Frazzled is expressed broadly in medulla neurons, while the repulsive receptor Unc-5 is excluded from Tm neurons and this is necessary for their correct targeting to the lobula. Frazzled is required collectively in a group of early-born Tm neurons to establish the inner optic chiasm (IOC) through which Tm axons target lobula. In addition, Frazzled acts in the layer-specific targeting step of Tm3 and Tm4 cell-autonomously, and is also required for the formation of the lobula branch of TmY3. Moreover, we show that the diffusibility of Netrins is necessary for Netrin enrichment in the lobula, the IOC formation and layer-specific targeting of Tm3 and Tm4. Netrin enrichment in the lobula is promoted by Frazzled expressed in Tm neurons, while Unc-5 appears to have an opposite role in Netrin distribution. Furthermore, we show that Netrin B is expressed in the Notch-on hemilineage of medulla neurons including most Tm and TmY neurons that target lobula, and loss of Su(H) abolished NetB expression in the medulla. Without medulla-originated NetB, Tm axons from late-born medulla columns cannot join the IOC. Therefore, the Notch-dependent binary fate choice regulates the assembly of the optic lobe neuropils by controlling the expression of Netrin.