Your spiral or mine? Snail gene reverses coil, makes new species.

A snail with a shell spiraling to the right can't mate readily with a lefty. So, changes in the single gene that controls shell direction have created new snail species, say researchers.

Among the 20 species of Euhadra snails, an abundant Japanese genus, the shell-spiraling direction has changed from left to right at least three times, forming new species in each case, according to Rei Ueshima of the University of Tokyo. That conclusion comes from analysis of a family tree based on the snails' DNA, say Ueshima and Takahiro Asami of Shinshu University in Matsumoto, Japan. In the Oct. 16 Nature, the two scientists describe a case in which different forms of a single gene have driven the formation of species. Asami says this marks the first reported example of single-gene speciation.

The notion that coil reversal in snail shells could have such a dramatic effect has intrigued theorists for some 15 years, comments Menno Schilthuizen of the University of Malaysia Sabah in Kota Kinabalu. "We've seen quite a bit of work, ranging from creative speculation to computer simulation, showing that in theory it's possible," he says. "Ueshima and Asami have done the field of snail speciation a great favor."

For most species of snails, shells and body plans curl in only one direction. The Nature article presents photos that illustrate the reason for that uniformity: A rare lefty in a right-spiraling species closely related to Euhadra writhes heroically but fails to mate with a partner with the majority spiral.

Single-gene speciation doesn't, in theory, take place among just any group of snails, Asami says. Although researchers have shown that for the three other snail groups tested so far, a single gene determines spiral direction, those groups' righties can mate with lefties.

When the Japanese researchers analyzed a DNA family tree of Euhadra, however, they found a single lineage that gave rise to three right-shelled and four left-shelled species. The tree indicated that the common ancestor of the seven species was left shelled.

In a particularly dramatic example, the genome of the lefty Euhadra quaesita is almost identical to that of the righty Euhadra aomoriensis, the researchers report. However, close examination of the snails reveals subtle morphological differences in shell characteristics.

Ueshima and Asami suggest that because the right-coiling gene is dominant, a speciation shift from a left-coiling E. quaesita ancestor to the right-coiling E. aomoriensis might have happened multiple times in this lineage. This contrasts with other known animal speciations, in which the emergence of the new species occurs only once.