Transcriptomic profiling reveals neurophysiological gene candidates underlying vocal evolution in African clawed frogs

Neurophysiologists have discovered many mechanisms underlying the production of animal behaviors in specific species; these involve a collection of neuromuscular systems, neuronal membrane and neural network properties, as well as the hormones and neuromodulators known to modify them. However, the mechanistic basis of behavioral evolution is less well-studied, and causal links between differences in gene expression, cellular mechanisms and species-typical behaviors are rare. Vertebrate vocal behaviors are an excellent system for studying the evolution of behaviors because they are ancient, diverse and readily quantifiable. Xenopus frogs are particularly well-suited to the study of vocal evolution due to the temporal diversity of male advertisement calls between closely related species and the well-described vocal pattern generating circuitry. Here we focus on two species, X. laevis and X. petersii, that diverged 8.5 million years ago and produce advertisement calls with distinct timing. To begin bridging the gap between behavioral and mechanistic diversity in Xenopus vocal behaviors, we performed RNA sequencing of the parabrachial nucleus, a vocal premotor hindbrain area known to encode species-typical temporal patterns in X. laevis and X. petersii. We identified hundreds of differentially expressed genes between the two species, including many genes related to hormone signaling, neuromodulation, neuronal and synaptic functions, ion channels and neurotransmitter receptors. We explore several testable hypotheses emerging from these results that may explain mechanisms by which candidate genes and gene families may contribute to vocal pattern differences between X. laevis and X. petersii.