Activity regulates brain development in the fly.

It has long been appreciated that activity sculpts the formation of neuronal circuits in the mammalian brain. By contrast, in Drosophila it was generally thought that genetically hardwired mechanisms of development, mediated by cell recognition molecules, are sufficient to specify the connectome-the complex and stereotyped pattern of connections between neurons. Here, we consider recent findings that activity during development also contributes to circuit assembly in the fly. These observations suggest that activity is a fundamental aspect of brain development in general. They also highlight the opportunities to learn about the role of this phenomenon through the extensive Drosophila toolkit for examining circuit development, structure, and function.