本期封面所示為一條蝰蛇（Vipera latastei gaditana）,，其豎起的毒牙被毒牙鞘蓋著,。攝影：Ruben Schipper
對毒蛇所做的一項“演化-發(fā)育”研究，得出一個關于蛇的毒牙演化的新模型,。蛇毒牙演化是一個有些爭議的問題,。毒牙的位置不是在上顎的前面就是在上顎的后面，人們所爭議的是,，這兩種安排在演化上是否是相關的,。通過觀察來自8個物種的96個蛇胚胎的上顎中形成牙齒的上皮，研究人員發(fā)現(xiàn),，前毒牙類型和后毒牙類型在形態(tài)發(fā)生上驚人地相似,。前毒牙是從上顎后面一部分發(fā)育形成的,，這部分在發(fā)育過程中被前移；后毒牙是在位置保持不變的一個專門區(qū)域發(fā)育形成的,。該新模型提出,，形成牙齒的上皮中后面的一個子區(qū)域在發(fā)育上變得與剩余的牙齒形成過程脫離關系，從而使后牙能夠獨立演化,，并且在不同毒蛇品種中發(fā)生很大改變（與毒腺密切相關）,。這一發(fā)育事件有可能對新生代高級毒蛇的大規(guī)模輻射起到了推動作用，導致了我們今天所看到的極為可觀的蛇類多樣性,。（生物谷Bioon.com）
生物谷推薦原始出處：
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: [email protected]).
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.
