Gene regulatory co-option drives birdsong neural circuit specialization

As animals evolve complex motor skills, they acquire more diverse supporting motor circuits in their nervous systems. Yet the molecular mechanisms driving motor circuit evolution remain poorly understood. Birdsong, a learned complex motor skill with parallels to human speech, is controlled by a dedicated neural circuit - the song system - that is distinguished from nearby sensorimotor regions by molecular, physiological, and connectivity specializations. By profiling gene expression and chromatin accessibility in the songbird brain, we found that each projection neuron type in the song system has a molecularly similar sister neuron type in adjacent non-song regions; these sister neurons lack specialized gene expression and are transcriptionally similar to neurons in the chicken brain. The gene regulatory networks (GRNs) controlled by transcription factors MAFB and EMX2, typically active in fast-spiking interneurons and astrocytes, are specifically active in song-dedicated extratelencephalic projection neurons. Furthermore, the heterologous expression of MAFB or EMX2 in chicken projection neurons was sufficient to drive expression programs characteristic of song neurons. These results support a model in which song-dedicated neurons emerged from ancestral neural types in part through the co-option of GRNs active in other cellular contexts, providing a genetic mechanism underlying the evolution of birdsong.