Data from: Distinct neural and neuromuscular strategies underlie independent evolution of simplified advertisement calls

Independent or convergent evolution can underlie phenotypic similarity of derived behavioural characters. Determining the underlying neural and neuromuscular mechanisms sheds light on how these characters arose. One example of evolutionarily derived characters is a temporally simple advertisement call of male African clawed frogs (Xenopus) that arose at least twice independently from a more complex ancestral pattern. How did simplification occur in the vocal circuit? To distinguish shared from divergent mechanisms, we examined activity from the calling brain and vocal organ (larynx) in two species that independently evolved simplified calls. We find that each species uses distinct neural and neuromuscular strategies to produce the simplified calls. Isolated Xenopus borealis brains produce fictive vocal patterns that match temporal patterns of actual male calls; the larynx converts nerve activity faithfully into muscle contractions and single clicks. In contrast, fictive patterns from isolated Xenopus boumbaensis brains are short bursts of nerve activity; the isolated larynx requires stimulus bursts to produce a single click of sound. Thus, unlike X. borealis, the output of the X. boumbaensis hindbrain vocal pattern generator is an ancestral burst-type pattern, transformed by the larynx into single clicks. Temporally simple advertisement calls in genetically distant species of Xenopus have thus arisen independently via reconfigurations of central and peripheral vocal neuroeffectors.

独立演化或趋同演化可解释衍生行为性状的表型相似性。阐明其背后的神经与神经肌肉机制，有助于揭示这类性状的起源路径。演化衍生性状的典型案例之一，是非洲爪蟾属（Xenopus）雄性的时间结构简化型求偶鸣叫，该性状至少从更为复杂的祖先鸣叫模式独立演化出现了两次。发声回路的简化是如何发生的？为区分共享机制与分化机制，我们对两类独立演化出简化型鸣叫的爪蟾物种的发声脑与发声器官（喉，larynx）的活动展开了检测。研究发现，两个物种分别采用不同的神经与神经肌肉策略生成简化型鸣叫：离体的北井爪蟾（Xenopus borealis）脑可产生与雄性真实鸣叫时间模式匹配的虚构发声模式（fictive vocal patterns），其喉可将神经活动精准转化为肌肉收缩与单次声脉冲。与之相反，离体的邦巴爪蟾（Xenopus boumbaensis）脑产生的虚构活动为短时长神经爆发，其离体喉需要受到爆发式刺激才能产生单次声脉冲。由此可见，与北井爪蟾不同，邦巴爪蟾的后脑发声模式发生器（hindbrain vocal pattern generator）输出的是祖先型爆发式活动模式，再经由喉将其转化为单次声脉冲。遗传距离较远的爪蟾物种中，时间结构简化的求偶鸣叫正是通过中枢与外周发声神经效应器（central and peripheral vocal neuroeffectors）的重配置独立演化而来的。