本期封面所示为一条蝰蛇（Vipera latastei gaditana），其竖起的毒牙被毒牙鞘盖着。摄影：Ruben Schipper
对毒蛇所做的一项“演化-发育”研究，得出一个关于蛇的毒牙演化的新模型。蛇毒牙演化是一个有些争议的问题。毒牙的位置不是在上颚的前面就是在上颚的后面，人们所争议的是，这两种安排在演化上是否是相关的。通过观察来自8个物种的96个蛇胚胎的上颚中形成牙齿的上皮，研究人员发现，前毒牙类型和后毒牙类型在形态发生上惊人地相似。前毒牙是从上颚后面一部分发育形成的，这部分在发育过程中被前移；后毒牙是在位置保持不变的一个专门区域发育形成的。该新模型提出，形成牙齿的上皮中后面的一个子区域在发育上变得与剩余的牙齿形成过程脱离关系，从而使后牙能够独立演化，并且在不同毒蛇品种中发生很大改变（与毒腺密切相关）。这一发育事件有可能对新生代高级毒蛇的大规模辐射起到了推动作用，导致了我们今天所看到的极为可观的蛇类多样性。（生物谷Bioon.com）
生物谷推荐原始出处：
Nature 454, 630-633 (31 July 2008) | doi:10.1038/nature07178
Evolutionary origin and development of snake fangs
Freek J. Vonk1, Jeroen F. Admiraal1, Kate Jackson2, Ram Reshef3, Merijn A. G. de Bakker1, Kim Vanderschoot1, Iris van den Berge1, Marit van Atten1, Erik Burgerhout1, Andrew Beck4, Peter J. Mirtschin4,5, Elazar Kochva6, Frans Witte1, Bryan G. Fry7, Anthony E. Woods4 & Michael K. Richardson1
Institute of Biology, Leiden University, Kaiserstraat 63, PO Box 9516, 2300 RA, Leiden, The Netherlands
Department of Biology, Whitman College, Walla Walla, Washington 99362, USA
Faculty of Biology, Technion - Israel Institute of Technology, Haifa 32000, Israel
Sansom Institute, School of Pharmacy and Medical Sciences, University of South Australia, Adelaide, South Australia 5000, Australia
Venom Supplies Pty Ltd, Tanunda, South Australia 5352, Australia
Department of Zoology, Tel Aviv University, Tel Aviv 69978, Israel
Department of Biochemistry & Molecular Biology, Bio21 Institute, University of Melbourne, Parkville, Victoria 3010, Australia
Correspondence to: Michael K. Richardson1 Correspondence and requests for materials should be addressed to M.K.R. (Email: m.k.richardson@biology.leidenuniv.nl).
Many advanced snakes use fangs—specialized teeth associated with a venom gland1, 2—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 jaw3, 4, 5. A fundamental controversy in snake evolution is whether or not front and rear fangs share the same evolutionary and developmental origin3, 4, 5, 6, 7, 8, 9. 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 marker10, 11, 12, 13, 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 today6, 14, 15.
