Two conserved vocal central pattern generators broadly tuned for fast and slow rates generate species-specific vocalizations in Xenopus clawed frogs

Across phyla, species-specific vocalizations are used by males to attract females. Functional analyses of the neural circuitry underlying behavior have been difficult, particularly in vertebrates. However, using an ex vivo brain preparation that produces fictive vocalizations, we previously identified anatomically distinct fast and slow central pattern generators (CPGs) that drive the fast and slow clicks of male courtship calls in male African clawed frogs, Xenopus laevis. To gain insight into the evolution of neural circuits underlying courtship calls, we extended this approach to four additional species. Here, we show that although the exact rate and duration of the clicks are unique to each species, fast and slow CPGs identified in male X. laevis are conserved across species. Further, we show that the development of fast CPGs depends on testosterone in a species-specific manner: testosterone facilitates the development of fast CPGs in a species with a courtship call containing fast clicks, but not in a species with a courtship call made entirely of slow clicks. Finally, we showed that, unlike other vestigial neural circuits that remain latent, the fast CPGs are not inherited by all species; rather, they are possessed only by the species that produce fast clicks. The results suggest that species-specific calls of the genus Xenopus have evolved by utilizing conserved fast or slow CPGs that are broadly tuned to generate fast or slow trains of clicks, the development of which appear to be regulated by a strategic expression of testosterone receptors in the brain of each species.