Drosophila embryonic neuroblasts give rise to larval-brain primary neurons,from which secondary adult neurons are subsequently derived. The gap gene empty spiracles (ems) has a well-known role in Drosophila embryonic brain development, whereas little is known about its function in adult secondary neurons. According to Lichtneckert et al., ems is expressed in a single neuroblast in each hemisphere of the adult brain and autonomously determines the number of progeny (as shown by MARCM clonal analysis) in the adult lineage(see ). Interestingly, ems expression persists throughout metamorphosis into adulthood. In ems mutant lineages, neurite projections are short and misdirected. This elegant study highlights how neuronal circuitry is established and maintained into adulthood during brain development, and reveals an unexpected role for a gap gene in translating lineage information into cell-number control and into the correct projection of a clonal unit of the adult brain. It also highlights conserved functions of orthologous ems/Emx homeobox genes in the early embryo and during later brain development.
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