HEDGEHOG, TRANSMITTED ALONG RETINAL AXONS, TRIGGERS NEUROGENESIS IN THE DEVELOPING VISUAL CENTERS OF THE DROSOPHILA BRAIN

The development of the visual centers of the Drosophila brain is tight ly regulated by the ingrowth of retinal axons from the developing eye. In the first optic ganglion, the lamina, arriving retinal axons trigg er the precursors of their synaptic partners to complete a final cell division and commence neural differentiation. The secreted product of the hedgehog gene regulates the temporal assembly of photoreceptor pre cursor cells into ommatidial clusters in the compound eye. Here, we sh ow that Hedgehog is transmitted along the retinal axons to serve as th e inductive signal in the brain. Hedgehog acts in the first of two ret inal axon-mediated steps in the assembly of lamina synaptic cartridges . These observations provide a novel insight into the molecular intera ctions that orchestrate the assembly of neural precursor cells into pr ecise synaptic circuits.