Specific expansion of motor cortical projections in a singing mouse

Elucidating how modifications in neural circuit architecture drive behavioural innovation remains a key challenge in neuroscience and evolutionary biology. In mammals, the neocortex is posited to play a crucial part in facilitating rapid behavioural innovations^1-3. Although changes in long-range connectivity have been proposed to underlie such innovations^4,5, these hypotheses remain largely untested quantitatively, which is partly due to the lack of high-throughput neuronal projection data at single-neuron resolution across species. Here we studied the Alston's singing mouse (Scotinomys teguina), which exhibits a striking vocal behaviour absent in the laboratory mouse (Mus musculus), to quantitatively determine species-specific changes in motor cortical projections throughout the brain. We used bulk tracing, serial two-photon tomography and high-throughput DNA sequencing of more than 76,000 barcoded neurons to discover a specific and substantial expansion of orofacial motor cortical projections to an auditory cortical region and the midbrain periaqueductal grey, regions that are implicated in vocal behaviours^6-9. Moreover, analyses of projection motifs of individual orofacial motor cortical neurons revealed preferential expansion of exclusive projections to the auditory cortical region in the singing mouse. Our results suggest that selective expansion of ancestral motor cortical projections may lead to behavioural divergence over short timescales. Furthermore, the results facilitate mechanistic investigations of enhanced cortical control over vocalizations-a crucial preadaptation for human language^10,11. This approach of comparing recently diverged species with substantial behavioural divergences can be readily generalized across other model clades to discover quantitative rules of neural circuit evolution.