Scientists have discovered a gene that determines whether Japanese snails develop shells that are left or right-coiling – with implications for understanding the underlying handedness of our own bodies.
Researchers from the University of Nottingham and Tohoku University studied wild Euhadra snails in Japan to understand what makes one snail a mirror-image of another.
They found that the coiling direction, or chirality, of a snail shell is due to natural differences in a gene that makes a protein called myosin. Myosin helps cells move and change shape by working with other proteins to make the cell scaffold.
The findings are published in in Current Biology.
The researchers believe that a few small changes in the myosin protein affect how cells twist – either clockwise or anti-clockwise – in the earliest stages of development. This sets the direction for the chiral development of the adult snail's shell and body organs.
In ways that we don't fully understand, the chirality of the basic building blocks of life ends up making bodies that are themselves also handed, or chiral. This is the foundation for our hearts being on left hand side of our bodies and snail shells that usually coil to the right. But while the majority of snails have shells that are right-coiling, a good number are left-coiling and few species have both coiling types. Until now, we had no idea how they do this. It turns out that small changes in a myosin gene produces mirror-image snails, while still producing healthy, viable animals.
The finding is important because it helps understand the common factors by which diverse animals develop their left and right sides. It is also timely because solving the mystery of how chirality comes about received a Nobel prize last week.
Unlike snails, nearly all other animals show the same fixed asymmetry, with rare reversals of body organs are often associated with problems during the development of the embryo.
In comparison, the team found that the left- and right-coiling versions of the Japanese snail develop normally as embryos, without any evidence of disease or pathology. Both the left and right-coiling versions are ancient, but the "lefty" version shows evidence for natural selection.
In other snails, left- and right coiling versions are not usually able to mate, because the genitals are on opposite sides. The key to the new work was finding a valley deep in the mountains of Japan, where both left and right types are able to mate and exchange genes.
The researchers also compared their findings with a pond snail, in which lefties are rare. In pond snails, a mutation in a different gene makes snails with left-coiling shells, but also causes them to develop abnormally.
Since other animals ordinarily have a fixed left-right body plan, the work with snails can help to understand why other animals don't vary. There is still a deep mystery in understanding how snails use molecules of the same chirality to make bodies and shells that have a variable chirality.
