How Snakes Picked Their Poison

Take a bite. Venom researcher Bryan Fry milks a king cobra

To craft their deadly poisons, venomous snakes recruited proteins that played relatively mundane roles and shaped them into potent killers, a new genetic analysis shows.

Venomous snakes first evolved some 60 million to 80 million years ago, and since then they've acquired more than 24 kinds of poisonous proteins. Bryan Fry of the University of Melbourne in Australia has found how they did it. His analysis is based on the idea that organisms can evolve new proteins when a gene becomes duplicated in the genome: One copy of the gene retains its original function, while the other mutates into a new one--in this case, a poison.

By comparing the venom gene sequences with other known sequences, Fry was able, in three-quarters of the cases, to infer which gene had duplicated. He found that the various toxins traced their way back to proteins that play diverse roles in an assortment of organs, from the brain to the testes. For example, proteins in the acetylcholinesterase group are common neurotransmitters for muscle control, but the snake's venomous form induces paralysis and difficulty breathing; similarly, kallikrein proteins regulate blood pressure, but the toxic versions induce a fatal drop in blood pressure. "It's an elegantly evil strategy that they've taken," Fry says.

Fry's findings, which appear in this month's issue of Genome Research, also indicate that the snake's venom gland may not have evolved from the pancreas, as had long been hypothesized: None of the venom precursor genes are related to genes expressed in the pancreas.

The work is an important contribution to the field, says Elazar Kochva of Tel Aviv University in Israel. It shows "how economic evolution can be." But Kochva believes the origin of the venom gland itself remains open to debate, and he expects there are relationships still to be found between pancreatic enzymes and some venom proteins.

Related sites
Study abstract
Bryan Fry's homepage