Snails are not known for being fleet of foot, but they may hold at least one speed record. A report in today's Proceedings of the National Academy of Sciences shows that in one venomous snail, the evolution of poisons is occurring faster than protein evolution in any other known animal species.
Canus abbreviatus is a carnivorous snail living off the coast of Hawaii that primarily hunts marine worms, which it stuns with neurotoxins and then devours. Evolution has equipped C. abbreviatus with a cocktail of different toxins, some of which target specific prey species. How they acquired this diverse arsenal has been a mystery. Marine biologists Thomas Duda and Stephen Palumbi from Harvard University suspected the molecular mixer could be a mechanism called "gene duplication." This is a kind of risk-free molecular evolution: When multiple copies of a given gene are present in a species' genome, chance mutations in some copies can either ruin the proteins they encode or produce innovative versions; either way, the remaining intact copies still produce the original protein.
To test their hunch, Duda and Palumbi tracked down the different versions of a gene that encodes one type of neurotoxin. They found up to seven variations of the gene--some differing by as much as 25% of their base pairs--in a single snail. Next, they compared this variation to that among other genes in the snail's genome. They calculated that the neurotoxin genes had undergone some 17 to 48 changes per DNA "letter" per billion years--10 times more than other snail genes, and three times more than the fastest evolving proteins known in fruit flies.
Palumbi doesn't think the neurotoxin genes undergo more mutations than other snail genes. Instead, he argues, the variation may stick around in the neurotoxin genes because it is beneficial to produce more toxins for taking out more prey. However, Wen-Hsiung Li, a molecular evolutionist at the University of Chicago, says the Harvard team still has to prove that having a wide variety of poisons indeed helps the snails catch more food, and thus increases their fitness. Without that, he says, the team's explanation for the chemical cocktail is based on inference rather than evidence.