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Snakes reveal deadly fang secret

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 venom 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.