Glia Relay Differentiation Cues to Coordinate Neuronal Development in Drosophila

Neuronal birth and specification must be coordinated across the developing brain to generate the neurons that constitute neural circuits. This is particularly true for sensory systems where topographic maps are built by connecting neurons at different levels of circuitry. I use the ordered Drosophila visual system to investigate how neuronal development is coordinated across different brain regions to establish retinotopy, afeature of all visual systems. Photoreceptors were believed to achieve retinotopy by inducing their target field in the optic lobe, the lamina composed of five neuron types, by a secreted differentiation cue (Epidermal Growth Factor; EGF). I will discuss my recent finding that communication between photoreceptors and lamina precursor cells is indirect, and requires a signaling relay through wrapping glia. Specifically, I have found that in response to EGF from photoreceptors, wrapping glia produce insulin-like peptides, which in turn induce lamina neuronal differentiation. I propose that the signaling relay through glia is a strategy to translate an otherwise homogenous photoreceptor cue into one with spatio-temporal stratification. Additionally, incorporating a relay via glia delays the differentiation cue to the lamina, thus ensuring that lamina ‘unit’ assembly is completed prior to differentiation.