SIGNALS TRANSMITTED ALONG RETINAL AXONS IN DROSOPHILA - HEDGEHOG SIGNAL RECEPTION AND THE CELL CIRCUITRY OF LAMINA CARTRIDGE ASSEMBLY

The arrival of retinal axons in the brain of Drosophila triggers the a ssembly of glial and neuronal precursors into a 'neurocrystalline' arr ay of lamina synaptic 'cartridges'. Hedgehog, a secreted protein, is a n inductive signal delivered by retinal axons for the initial steps of lamina differentiation. In the development of many tissues, Hedgehog acts in a signal relay cascade via the induction of secondary secreted factors. Here we show that lamina neuronal precursors respond directl y to Hedgehog signal reception by entering S-phase, a step that is con trolled by the Hedgehog-dependent transcriptional regulator Cubitus in terruptus. The terminal differentiation of neuronal precursors and the migration and differentiation of glia appear to be controlled by othe r retinal ason-mediated signals. Thus retinal axons impose a program o f developmental events on their postsynaptic held utilizing distinct s ignals for different precursor populations.