During neurogenesis in the Drosophila optic lobe, a wave of differentiation that converts neuroepithelial cells into neuroblasts sweeps across the neuroepithelial sheet in a medial to lateral direction. This differentiation wave is preceded by the ‘proneural wave’: the transient expression of the proneural gene lethal of scute [l(1)sc]. Now, Tetsuya Tabata and colleagues report that EGFR and Notch signalling play pivotal and coordinated roles in proneural wave progression in the Drosophila optic lobe (see p. ). They show that EGFR signalling is activated in neuroepithelial cells and induces l(1)sc expression. Transient, spatially restricted expression of Rhomboid regulates EGFR, they report, and Rhomboid expression is regulated by the EGFR signal, a feedback loop that moves the proneural wave laterally. The researchers also report that Notch signalling, which prolongs the proneural state, is regulated both by itself and by EGFR signalling. Based on these results, the researchers propose that coordinated sequential EGFR and Notch signalling regulates proneural wave progression, which, in turn, induces neuroblast formation in a precisely ordered manner.
