The evolutionary origin of the jaw remains one of the most enigmatic events in vertebrate evolution. The trigeminal nerve is a key component for understanding jaw evolution, as it plays a crucial role as a sensorimotor interface for the effective manipulation of the jaw. This nerve is also found in the lamprey, an extant jawless vertebrate. The trigeminal nerve has three major branches in both the lamprey and jawed vertebrates. Although each of these branches was classically thought to be homologous between these two taxa, this homology is now in doubt. In the present study, we compared expression patterns of Hmx, a candidate genetic marker of the mandibular nerve (rV3, the third branch of the trigeminal nerve in jawed vertebrates), and the distribution of neuronal somata of trigeminal nerve branches in the trigeminal ganglion in lamprey and shark. We first confirmed the conserved expression pattern of Hmx1 in the shark rV3 neuronal somata, which are distributed in the caudal part of the trigeminal ganglion. By contrast, lamprey Hmx genes showed peculiar expression patterns, with expression in the ventrocaudal part of the trigeminal ganglion similar to Hmx1 expression in jawed vertebrates, which labeled the neuronal somata of the second branch. Based on these results, we propose two alternative hypotheses regarding the homology of the trigeminal nerve branches, providing new insights into the evolutionary origin of the vertebrate jaw.

中文翻译：

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七鳃鳗和鲨鱼三叉神经节Hmx表达及神经元体体分布的比较分析：洞察三叉神经分支的同源性和脊椎动物颌骨的进化起源

颌骨的进化起源仍然是脊椎动物进化中最神秘的事件之一。三叉神经是了解颌进化的关键组成部分，因为它作为有效操纵颌的感觉运动接口发挥着至关重要的作用。这种神经也存在于七鳃鳗（一种现存的无颌脊椎动物）中。七鳃鳗和有颌脊椎动物的三叉神经都有三个主要分支。尽管传统上认为这些分支中的每一个在这两个分类单元之间是同源的，但这种同源性现在受到怀疑。在本研究中，我们比较了下颌神经（rV3，有颌脊椎动物中三叉神经的第三分支）的候选遗传标记 Hmx 的表达模式，以及三叉神经节中三叉神经分支的神经元体的分布​​。七鳃鳗和鲨鱼。我们首先证实了Hmx1在鲨鱼rV3神经元体中的保守表达模式，该体体分布在三叉神经节的尾部。相比之下，七鳃鳗Hmx基因表现出独特的表达模式，在三叉神经节腹尾部的表达与有颌脊椎动物中的Hmx1表达相似，后者标记了第二分支的神经元体体。基于这些结果，我们提出了关于三叉神经分支同源性的两种替代假设，为脊椎动物颌骨的进化起源提供了新的见解。