Snakes reveal deadly fang secret
Thursday, 31 July 2008 Dani Cooper
ABC
First bite: unravelling the mystery of snake fang evolution (Source: iStockphoto)
New research could settle a long-standing controversy over the evolution of deadly snake fangs.
In today's issue of the journal Nature an international team of scientists argue that different types of fangs, thought by some to have evolved independently, actually have a common evolutionary pathway.
Syringe-like teeth connected to venom glands are used by many snakes to inject poison into their prey.
But such fangs can be in the front of the jaw - as in cobras and vipers - or in the back and for years scientists have argued over the origin of such different fangs.
Dr Bryan Fry, of the University of Melbourne's Bio21 Institute, who contributed to the new research, says advanced snakes share a common ancestor that had venom glands before fangs developed.
"Logically venom preceded fangs as there cannot be a selection pressure for the evolution of intricate delivery systems [for venom] in the absence of something worth delivering," he says.
Fry says early in the evolutionary pathway, snakes used regular teeth to chew the venom in with some lineages later developing enlarged rear teeth.
"The true innovation in venom delivery came with the development of hollow-syringe fangs to deliver the venom in a high-pressure manner," he says.
This allowed snakes to envenomate new prey because they could "efficiently get past the thick fur of mammals or puncture the hard scales of other snakes".
Fry says the new research has now found that this major development in snake evolution, involving the back teeth and venom gland, ocurred independently from the rest of the teeth.
Snake embryo study
Fry and colleagues studied fang development in 96 snake embryos covering eight species, including those with rear fangs and those with front fangs.
They tracked the movement of tooth-forming cells in the upper jaw of snake embryos, by monitoring the expression of the sonic hedgehog gene (shh).
Different levels of the gene's expression cause different types of cells to be formed in the developing embryo.
They found all fangs originated at the back of the upper jaw and the front fangs migrated forwards as the snake embryo developed.
'Uncoupling'
The researchers say the findings show an apparent "uncoupling" of the back region of the tooth-forming layer from the rest of the teeth during evolution.
They say this evolutionary event could have contributed to the massive increase in snake diversity and numbers in the Cenozoic era, which started about 65 million years ago.
Reptile expert, Professor Rick Shine at the School of Biological Sciences at the University of Sydney agrees.
"Snakes are a relatively young group," says Shine. "But they are one of the great success stories in recent times in an evolutionary sense."
Shine says the "elegant" paper is "a huge leap forward in understanding how modern snakes have perfected the killing system that they have".
And he says the team has shown that snakes are "much more similar than we would have thought".
"Scientists have been puzzling over the evolutionary relationships between snake lineages for a long time," says Shine.
"What we are seeing [in this paper] are variations on a theme" with a bit of genetic modification along the way.
http://tinyurl.com/5r9nec
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Nature Abstract
http://www.nature.com/nature/journal...ature07178.pdf
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).
Top of pageAbstractMany 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.
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