How do dimorphisms in gene expression shape nervous system anatomy and physiology, explaining dimorphisms in behaviour? 
We have dissected the motor neuron control system for male song and its multifunctional use in flight control (O’Sullivan et al. 2018). Motor neurons are present in both sexes. In contrast, interneurons for motor patterning and action selection develop sex-specific cell fates, morphologies and physiological characteristics under the control of the transcription factors Fruitless and/or Doublesex. So far, the circuits for courtship song have been studied under the assumption that only male flies sing. The discovery of female song redefines the functional interpretation of dimorphic circuit development and provides a starting point for identifying new genetic and neuronal motifs underlying acoustic communication. We aim at investigating to which extent the neuronal substrate for acoustic signalling overlaps in both sexes and how differences in male and female singing behaviour can be explained on the level of gene expression, neuromodulation and circuit architecture.