The Drosophila optic lobe shares many characteristics with mammalian visual systems and might provide a powerful model for investigating the formation of visual processing circuits. Little is known, however, about the mechanisms that create neuronal diversity and organise neuronal circuits in the medulla, the optic lobe's primary region. Now, on p. , Makoto Sato and colleagues describe the key features of the developing fly medulla. They show that, during larval development, the medulla is subdivided into concentric zones that are characterised by the expression of the transcription factors Drifter, Runt, Homothorax and Brain-specific homeobox. The birth order of the medulla neurons correlates with the expression pattern of these factors, they report, and each neuronal type exhibits an extensive but defined pattern of migration that disrupts the concentric zones during early pupal development. These results, and those of clonal analyses, lead the researchers to suggest that the concentric zone genes may form a genetic hierarchy that specifies neuronal identity and establishes neuronal circuits in the developing medulla.
