Structure, Activity and Function of a Singing CPG Interneuron Controlling Cricket Species-Specific Acoustic Signaling.

The evolution of species-specific song patterns is a driving force in the speciation of acoustic communicating insects. It must be closely linked to adaptations of the neuronal network controlling the underlying singing motor activity. What are the cellular and network properties that allow generating different songs? In five cricket species, we analyzed the structure and activity of the identified abdominal ascending opener interneuron, a homologous key component of the singing central pattern generator. The structure of the interneuron, based on the position of the cell body, ascending axon, dendritic arborization pattern, and dye coupling, is highly similar across species. The neuron's spike activity shows a tight coupling to the singing motor activity. In all species, current injection into the interneuron drives artificial song patterns, highlighting the key functional role of this neuron. However, the pattern of the membrane depolarization during singing, the fine dendritic and axonal ramifications, and the number of dye-coupled neurons indicate species-specific adaptations of the neuronal network that might be closely linked to the evolution of species-specific singing. A fundamental question in evolutionary neuroscience is how species-specific behaviors arise in closely related species. We demonstrate behavioral, neurophysiological, and morphological evidence for homology of one key identified interneuron of the singing central pattern generator in five cricket species. Across-species differences of this interneuron are also observed, which might be important to the generation of the species-specific song patterns. This work offers a comprehensive and detailed comparative analysis addressing the neuronal basis of species-specific behavior.