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Evolutionary origin and development of snake fangs

机译：毒牙的进化起源与发展

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Many advanced snakes use fangs-specialized teeth associated with a venom gland-to introduce venom into prey or attacker. Various front- and rear-fanged groups are recognized, according to whether their fangs are positioned anterior (for example cobras and vipers) or posterior (for example grass snakes) in the upper jaw. A fundamental controversy in snake evolution is whether or not front and rear fangs share the same evolutionary and developmental origin. Resolving this controversy could identify a major evolutionary transition underlying the massive radiation of advanced snakes, and the associated developmental events. Here we examine this issue by visualizing the tooth-forming epithelium in the upper jaw of 96 snake embryos, covering eight species. We use the sonic hedgehog gene as a marker, and three-dimensionally reconstruct the development in 41 of the embryos. We show that front fangs develop from the posterior end of the upper jaw, and are strikingly similar in morphogenesis to rear fangs. This is consistent with their being homologous. In front-fanged snakes, the anterior part of the upper jaw lacks sonic hedgehog expression, and ontogenetic allometry displaces the fang from its posterior developmental origin to its adult front position-consistent with an ancestral posterior position of the front fang. In rear-fanged snakes, the fangs develop from an independent posterior dental lamina and retain their posterior position. In light of our findings, we put forward a new model for the evolution of snake fangs: a posterior subregion of the tooth-forming epithelium became developmentally uncoupled from the remaining dentition, which allowed the posterior teeth to evolve independently and in close association with the venom gland, becoming highly modified in different lineages. This developmental event could have facilitated the massive radiation of advanced snakes in the Cenozoic era, resulting in the spectacular diversity of snakes seen today.
机译：许多高级蛇使用与毒液腺相关的fang牙专用牙齿，将毒液引入猎物或攻击者。根据前牙和后牙的毒牙是位于上颚的前部（例如眼镜蛇和毒蛇）还是后部（例如草蛇）而识别出的。蛇进化中的一个基本争议是前牙和后牙是否具有相同的进化和发育起源。解决该争议可以确定高级蛇的大量辐射以及相关的发育事件的主要进化过程。在这里，我们通过可视化覆盖96个蛇胚胎的上颚中形成牙齿的上皮（涵盖8种物种）来研究此问题。我们使用声波刺猬基因作为标记，并三维重建41个胚胎的发育。我们显示前牙从上颚的后端发展，并且在形态学上与后牙非常相似。这与他们的同源性是一致的。在前fang蛇中，上颚的前部缺乏声波刺猬的表达，并且个体遗传学上的异位使该牙从其后发展原点移动到其成年前部位置，与该前fang牙的后部位置一致。在后牙蛇中，牙牙从独立的后牙板发育而来，并保持其后牙位置。根据我们的发现，我们提出了一种新的蛇牙演化模型：牙齿形成上皮的后部子区域与剩余的牙列在发育上脱开，从而使后牙独立地进化并与牙齿的紧密结合。毒腺，在不同血统中都高度修饰。这一发展事件本来可以促进新生代时代高级蛇的大量辐射，从而导致今天看到的蛇的壮观多样性。

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