Maturation of a central brain flight circuit in Drosophila requires Fz2/Ca2+ signaling

The fruit fly Drosophila melanogaster is an aerial acrobat. These insects can suddenly change direction in less than one hundredth of a second, explaining why a moving fly can be so difficult to swat. To perform their aerial manoeuvres, the flies continually combine information from multiple senses, including vision, hearing and smell, and use these data to control the activity of the neural circuits that support flight. These flight circuits are established during the pupal stage of fly development, during which the fly transforms from a larva into its adult form. In 2013, researchers showed that a protein called dFrizzled2 must be present in pupae for flight circuits to mature correctly. This protein forms part of a pathway that ultimately controls which specific chemicals—called neurotransmitters—are released by neurons to communicate with other cells. Agrawal and Hasan—who worked on the 2013 study—now extend their findings to investigate the role of dFrizzled2 in more detail. The new experiments show that for the flight circuits to mature, dFrizzled2 must be active in a cluster of neurons known collectively as PAM. Specifically, dFrizzled2 is needed to make an enzyme that helps to produce a neurotransmitter called dopamine. This in turn enables the PAM neurons to communicate with a region of the fruit fly brain called the mushroom body, which it thought to play an important role in complex behaviors such as reward-based learning. The absence of dFrizzled2 results in adult flies that rarely remain airborne for more than 20 s at a time, whereas normal flies can typically fly for over 700 s. Given that dopamine is known to signal reward, one possibility is that the dopamine signals from the PAM neurons to the mushroom body serve as a reward to encourage continuous flight. Mutant flies that lack dFrizzled2—and thus these dopamine signals—lose their motivation to fly after only a few seconds. Overall, Agrawal and Hasan's findings suggest that the mushroom body has an important role in coordinating a fly's movements with information from it senses. Future research will be needed to determine exactly how the mushroom body performs this role.