When genetic differences between two populations of a single species get to the point where the populations cannot interbreed, new species are born. This process (speciation) is a cornerstone of evolutionary theory, but any genetic change that makes it hard to interbreed also makes it hard for an animal to pass on its genes. How can genes for reproductive incompatibility spread from one generation to another? Masaki Hoso at Tokoku University and colleagues recently tested whether a single gene for speciation in land snails could be stabilized by snake predation. They published their work in a recent edition of Nature Communications.
The shells of most snails in Southeast Asia coil in a clockwise (right-handed) direction, but a few species have shells twisting the other way. Interestingly, a single gene controls which way shells twist. Mating between the two snail types is almost impossible because the opposite shaped shells get in the way. One group of snakes (Family: Pareatidae) in the region have co-evolved with these snails and have a taste for escargot. As right-handed shells are the more common of the two, pareatid snakes have evolved asymmetric tooth arrangements and hunting strategies that are optimized for extracting prey from right-handed shells. Hoso and colleagues postulated that right—left reversal of snail shell chirality could be responsible for instant speciation (it would block interbreeding immediately). Furthermore, they hypothesized that pareatid snake adaptations serve to maintain the genetic change that gives rise to left-handed shells.
First, the team tested how well pareatid snails can eat snails that twist to the left. The team presented rightward and leftward coiled snails to a panel of test snakes. The snakes had no problem eating their usual right-handed fare, but had real difficulty coping with left-handed shells. Size also mattered — the bigger the left-handed shell, the harder it was for snakes to extract their meal. These results suggest that a right—left reversal in shell chirality does indeed confer a survival advantage to snails.
Next, Hoso and co-workers examined the geographical distribution of snails with right-and left-handed shells and compared this with the geographical distribution of pareatid snakes. They found that the diversity of leftward coiled snails was indeed higher within the range of the snail-eating snakes. As a follow-up, the team examined the molecular phylogeny of one snail genus co-existing with pareatid snakes (Satsuma). They found that in the presence of snakes, leftward coiling snail species had arisen independently multiple times within the genus. Importantly, the frequency at which this has happened is unusually high, and difficult to explain without invoking snake predation as a stabilizing force.
Overall, the results of Hoso and colleagues support the hypothesis that right-handed snake predation is driving snail speciation by conferring a survival advantage to left-handed snails. This work is important because it shows that changing just a single gene can indeed lead to both adaptation and immediate reproductive isolation. This suggests that speciation could happen much quicker and entail much simpler genetic changes than previously thought.
