A RETINAL AXON FASCICLE USES SPITZ, AN EGF RECEPTOR-LIGAND, TO CONSTRUCT A SYNAPTIC CARTRIDGE IN THE BRAIN OF DROSOPHILA

Photoreceptor axons arriving in the Drosophila brain organize their po stsynaptic target field into a precise array of five neuron ''cartridg e'' ensembles. Here we show that Hedgehog, an initial inductive; signa l transported along retinal axons from the developing eye, induces pos tsynaptic precursor cells to express the Drosophila homolog of the epi dermal growth factor receptor (EGFR). The EGFR ligand Spitz, a signal for ommatidial assembly in the compound eye, is transported to retinal axon termini in the brain where it acts as a local cue for the recrui tment of five cells into a cartridge ensemble. Hedgehog and Spitz thus bring about the concerted assembly of ommatidial and synaptic cartrid ge units, imposing the ''neurocrystalline'' order of the compound eye on the postsynaptic target field.