Cocker, B. M. J.; Bostock, M. P.; Wei, H.; Fernandes, V. M.

Epithelial patterning is fundamental to organ development. Extensive work has focused on how neuroepithelia are patterned to generate diverse progenitors, yet how a single neuroepithelium is partitioned to produce distinct processing centres is poorly understood. Here, we focus on the Drosophila outer proliferation centre neuroepithelium, which generates the medulla and lamina visual processing centres. Medulla neuroblasts are produced by a proneural wave that initiates at the medial margin of the neuroepithelium and moves laterally propagated by EGFR-ERK signalling, whereas lamina precursors arise at the lateral neuroepithelial margin and have been proposed to require photoreceptor-derived Hedgehog. Here, we show that Hedgehog signalling is dispensable for lamina precursor specification but instead promotes their survival. In contrast, suppressing ERK and apoptosis together is sufficient to drive ectopic lamina precursor development. We find that cortex glia secrete the EGF antagonist Argos, which accumulates at the lateral neuroepithelium, thus repressing ERK activity locally. Together, our findings reveal a glia-mediated, extrinsic patterning mechanism that suppresses EGFR-ERK signalling in the lateral neuroepithelium, protecting these cells from the proneural wave and instructing lamina over medulla fate.