Snakes were originally rear-fanged
Some species of snakes have fangs at the front of the mouth, others at the back. The origin and evolution of fangs which form in different positions is the subject of long dispute among evolutionary biologists. A team led by Leiden researchers sheds light on the topic in a cover story in Nature of 31 July.
Image: Freek Vonk shows the fangs of a jumping viper (Atropoides nummifer).
© Sumardi Moentiah.
A problem with the dispute about evolution is that the most common assumption that serpents with fangs at the front of the mouth have a different origin from snakes with fangs at the back of the mouth is difficult to account for. Evolutionary biologists find it hard to accept that such an advanced weapon as the fang could have evolved several times independently. Nonetheless, cobras and vipers are far apart in their genealogy, while both have fangs at the front of the mouth. The big question was therefore whether all fangs are related in evolutionary terms, and if so, how? The researchers have now discovered that all venomous snakes stem from one snake with fangs at the back of the mouth.
The primary author of the article in Nature is the Leiden researcher Freek Vonk. He graduated just one month ago and this is already his third publication in this authoritative journal. Last year, together with Professor Michael Richardson, he was one of the teams which competed for the Academic Year Prize and in April pof this year he was awarded a Top Talent subsidy by the Netherlands Organisation for Scientific Research (NWO). This subsidy of 180,000 euro is intended to allow highly talented scientists to develop their own doctoral programme. Vonk has been referred to as the new Steve Irwin, but as well as being able to impart his knowledge with great enthusiasm, he is first and foremost a good researcher.
'We studied 96 embryos from eight types of snake,' explains Vonk. ‘This was the first time this had been done, because it's no easy job gathering snake embryos.' For each embryo the researchers tracked one gene which is involved in the development of the jaw, the so-called sonic hedgehog gene. Vonk: ‘We saw that in all the embryos the fangs develop at the back of the mouth. For some of the snakes, this fang imigrates forward, as it were, during the period in the egg. The reason for this is that one part of the jaw grows faster than the rest.' The assumption is then that the fangs which are at the front of the mouth originated at the back.
The researchers also discovered that the fangs at the back of the mouth are uncoupled from the rest of the dentition. 'The fangs develop from separate embryonic tissue,' says Vonk. 'Using three-dimensional reconstructions, we have demonstrated a remarkable similarity in the development between the fangs at the back and those at the front of the mouth.' In the first Nature article of 17 November 2005, on which Vonk worked as a third-year student, it was shown that a common ancestor of a number of lizard families and all snakes had developed venom glands in both the upper and lower jaw. Lizards developed into species with venom glands in the lower jaw, while snakes developed the venom mechanism in the upper jaw.
Image: the family tree of the snake genus. At the bottom, in black, are the primitive, non-venomous constrictor snakes. Above them, the advanced venomous snakes: purple denotes those with fangs at the front of the mouth, blue those with fangs both at the front and the back, and green with fangs at the rear.
Vonk: ‘The fangs of snakes probably developed because the back teeth were disconnected from the rest of the teeth. The teeth were then able to evolve separately, without constraint from the rest of the teeth. This was closely associated with the venom gland, finally forming the fang-gland complex.' Based on the development observed in the embryos, the researchers established that there is hardly any difference in the way in which the fang and the venom gland developed in the various groups of snakes.
The difference between cobras and vipers, both of which have fangs at the front of the mouth, can also be explained by this uncoupling from the rest of the dentition. 'Cobras and adders probably pushed their fang forwards during evolution,' says Vonk. 'The front row of teeth - or rather the relevant embryonic tissue which developed into this row of teeth - was lost. We think it often happens that a particular evolution takes place because tissues uncouple from one another during embryonic development. This allows the uncoupled tissue to evolve separately and without constraint.'
Image: biologist Freek Vonk in Indonesia eye-to-eye with a female King Cobra.
© Freek Vonk.
There are some three thousand types of snakes spread over almost the whole world, from treetop to ocean, from rain forest to desert. There are also snakes in the Netherlands. The fact that they are so successful is at least partly due to the decoupling of the rear teeth from the rest of the dentition early in evolution, is Vonk's hypothesis. 'It enabled the snakes to develop one of the most powerful weapons in the natural world. Snakes with fangs at the back of the mouth prey on smaller and less dangerous animals. They generally catch their prey in a similar way to the non-venomous constrictor snakes: securing their prey and biting repeatedly to inject the venom. Snakes with fangs at the front of the mouth, attack suddenly with a single bite and then withdraw to wait for the venom to take effect.' The advanced venom system has allowed the snakes to develop over 65 million years into one of nature's most successful groups of animals.
The publication in Nature is the result of an international partnership betgween researchers at Leiden University and institutes in the USA, Australia and Israel.
Evolution origin and development of snake fangs, by Freek J. Vonk, Jeroen F. Admiraal, Kate Jackson, Ram Reshef, Merijn A.G. de Bakker, Kim Vanderschoot, Iris van den Berge, Marit van Atten, Erik Burgerhout, Andrew Beck, Peter J. Mirtschin, Elazar Kochva, Frans Witte, Bryan G. Fry, Anthony E. Woods and Michael K. Richardson.
(30 July 2008/SH)
Website of Freek Vonk
(30 July 2008/SH)