By Joseph Milton
Evolutionary advantage often makes for show-stopping stuff--a cheetah's speed, for example, or a moth's almost perfect mimicry of tree bark. In some snails, however, it's simply down to a poor fit with a snake's jaw.
Some species of Satsuma snail have shells that coil to the left, which probably evolved because the snakes that prey on them have jaws specialized for feeding on the molluscs' right-coiling ancestors, a study published December 7 in Nature Communications suggests.
Snail genera tend to be either dextral (right-coiling) or sinistral (left-coiling, but the genus Satsuma contains both dextral and sinistral species.
In most land snails, the switch between dextrality and sinistrality is controlled by a single gene, meaning that reversals are likely to occur frequently. However, sinistral Satsuma snails cannot mate with their dextral relatives, leading scientists to wonder how left-coiling individuals arising from random genetic mutation would be able to find sexual partners. Ordinarily, these snails would be expected to die out because the vast majority of potential mates would be dextral, and therefore incompatible.
The new report suggests that although sinistral individuals arising in a dextral species were disadvantaged when it came to mating, they had a distinct advantage over their right-coiling forebears, thanks to the fact that common snake predators that can easily eat dextral snails struggle to consume the sinistral ones.
This survival advantage outweighed the mating disadvantage, allowing sinistrality to spread throughout previously dextral populations. Sinistrality also prevented mating with dextral ancestors, leading to reproductive isolation and the evolution of
entirely new species.
"This could change the general view of evolutionary genetics," says Masaki Hoso, an ecologist at Tohoku University in Japan who led the study. "We've found that a single gene can have major effects on speciation and adaptation simultaneously."
To investigate the effect that living alongside snake predators might have had on the evolution of sinistral species, Hoso and his colleagues first looked at how effectively the snake Pareas iwasakii preys on Satsuma snails. They found that the snakes, which have more teeth on the right side of their jaws than the left, were able to eat all of the dextral snails fed to them, but only 12.5 percent of the sinistral snails.
Comparing the global distributions of both snakes and snails, the researchers found that sinistral snail species have evolved more often in areas in which predator and prey coexist. And a DNA-based family tree of the snail genus showed that sinistrality has arisen independently at least six times in Satsuma, more than would be expected were there not some driving force behind its evolution.
"We knew the snakes had trouble picking up sinistral snails," says Menno Schilthuizen, an evolutionary ecologist at the National Museum of Natural History of the Netherlands in Leiden, who specializes in snail evolution. "But Masaki has shown the snake might actually speed up the fixation of sinistrality, suggesting this is a very plausible speciation mechanism."