A challenge of numbers and diversity: neurogenesis in the Drosophila optic lobe.

The brain areas that endow insects with the ability to see consist of remarkably complex neural circuits. Reiterated arrays of many diverse neuron subtypes are assembled into modular yet coherent functional retinotopic maps. Tremendous progress in developing genetic tools and cellular markers over the past years advanced our understanding of the mechanisms that control the stepwise production and differentiation of neurons in the visual system of Drosophila melanogaster. The postembryonic optic lobe utilizes at least two modes of neurogenesis that are distinct from other parts of the fly central nervous system. In the first optic ganglion, the lamina, neuroepithelial cells give rise to precursor cells, whose proliferation and differentiation depend on anterograde signals from photoreceptor axons. In the second optic ganglion, the medulla, the coordinated activity of four signaling pathways orchestrates the gradual conversion of neuroepithelial cells into neuroblasts, while a specific cascade of temporal identity transcription factors controls subtype diversification of their progeny.