A small genetic difference can make one snail a mirror image of another. Now researchers studying Japanese land snails have identified a gene that helps determine whether their shells twist clockwise or counterclockwise, providing new clues about how animals develop their left and right sides.
The study, led by researchers at the University of Nottingham and Tohoku University in Japan, examined wild Euhadra snails to understand the biological mechanisms behind this phenomenon, known as chirality. Their results suggest that subtle variations in a gene responsible for producing a protein called myosin can result in healthy snails with opposite body plans.
The discovery could also help scientists investigate a broader biological puzzle: how the same basic cellular machinery creates a consistent left-right asymmetry in most animals, while allowing some species to evolve as mirror images of one another.
How a gene determines the handedness of a snail
The study, published in Current Biology under the title “An ancient polymorphism in myosin I a/b determines chiral variation in Japanese Euhadra snails,” identifies naturally occurring variants of a myosin gene as the basis for the difference.
Myosin proteins help cells move, change shape, and interact with the internal structures that support them. The researchers believe that small changes in the protein influence how cells twist in the earliest stages of embryonic development. This initial movement determines the direction in which the snail’s shell unwinds and determines the development of its internal anatomy.
Most snails roll their shells to the right, a condition called dextrorotation. Left-handed snails are called sinistral. Although these shapes look like mirror images, the difference is not just cosmetic: their internal organs and reproductive anatomy may also be arranged on opposite sides.
The team found that both forms of Japanese Euhadra snails develop normally, without the abnormalities seen in other species with reversed body asymmetry. The two genetic variants are also ancient, suggesting that the difference has persisted across evolutionary timescales. The researchers found evidence that natural selection acted on the left-handed shape.
Why it was important to find both species in one valley
One challenge in studying mirror-image snails is that they may have difficulty reproducing among themselves. Their reproductive organs may be on opposite sides, making mating difficult.
The researchers found a population deep in a Japanese mountain valley where left- and right-wing snails can mate and exchange genes. This gave the team the opportunity to study how the two forms persist within the same population, rather than studying them as isolated groups.
The results also differ from what researchers have observed in pond snails. In these animals, a mutation in another gene can cause the animals to twist to the left, but it is also associated with abnormal development. In Japanese Euhadra, the myosin variants produce opposite forms without the same obvious developmental problems.
What this might tell us about human development
The research offers a useful way to examine a long-standing question in biology: how organisms establish a consistent left-right body arrangement.
In humans, for example, the heart is typically located on the left side, while other internal organs occupy characteristic positions. Reversals can occur, but these are far rarer than the natural variation observed in these snails, and some cases are associated with developmental complications.
Understanding how myosin variants produce healthy, mirror-image snails could help scientists identify the cellular processes that cause body asymmetry in different animals. However, the discovery does not mean that the same genetic variation determines organ placement in humans.
The researchers say an important question still remains. How do cells with the same underlying molecular handedness produce bodies that can develop in opposite directions? Japanese Euhadra snails provide a rare opportunity to investigate this mystery.
