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。解析其背后的神经与神经肌肉机制，有助于阐明此类性状的起源过程。演化衍生性状的典型案例之一，便是雄性非洲爪蟾（Xenopus）的时间结构简化求偶鸣唱——该鸣唱至少两次独立起源于结构更为复杂的祖先鸣唱模式。那么发声回路的简化机制是如何实现的？为区分共享机制与趋异机制，本研究对两类独立演化出简化鸣唱的爪蟾物种的鸣唱相关脑区与发声器官（larynx）的活动开展了检测。研究结果表明，两个物种分别采用截然不同的神经与神经肌肉策略来生成简化鸣唱。离体的北方爪蟾（Xenopus borealis）脑组织可产生与雄性真实鸣唱时间模式相匹配的虚构发声模式（fictive vocal patterns）；其喉组织可将神经活动精准转换为肌肉收缩与单声脉冲。与之形成鲜明对比的是，离体的布姆巴爪蟾（Xenopus boumbaensis）脑组织所产生的虚构发声模式为短时长的神经活动爆发；其离体喉组织需要接受刺激爆发才能生成单声脉冲。因此，与北方爪蟾不同，布姆巴爪蟾的后脑发声模式发生器所输出的为祖先型爆发式神经模式，该模式经喉组织转换为单声脉冲。由此可见，遗传关系疏远的两类爪蟾物种，其时间结构简化的求偶鸣唱均通过中枢与外周发声神经效应器的重构实现了独立演化。