Single gene controls snail shell coiling

Left-right asymmetry, at least in snails, is governed by a single gene, Japanese researchers have found.

In a paper published in the journal Development, Masanori Abe and Reiko Kuroda from the Tokyo University of Science report using CRISPR gene-editing technology to reverse the direction in which the shells of a common snail species curl.

Under normal circumstances, the shells of the freshwater snail (Lymnaea stagnalis) curl exclusively towards the right.

Abe and Kuroda discovered that by manipulating a gene known as Lsdia1 they were able to produce ones that curled in the opposite, or sinistral, direction.

The “left-handed” shells remained constant in subsequent generations, confirming that the genetic instruction had been permanently altered.

The finding is significant. Across the biological realm, asymmetry – sometimes known as “chirality” – is commonplace. In humans, for instance, some essential organs, such as the heart or the liver, are positioned only on one side of the body, while others, such as the lungs and kidneys, grow symmetrically. 

On a broader scale, chirality plays an important role in trait development. Many species show evidence of asymmetry. Recent research, for instance, found that individual bees demonstrate left or right handedness during flight.

Just how or where genetic instructions for this type of structure or behaviour are encoded, however, has long be a mystery.

Now, in one aspect at least, an answer has been found.

“This is the gene sought for over a century,” Abe and Kuroda write.

Asymmetry, the researchers discovered, is not a property that develops late in embryo development.

“We also show that the gene sets the chirality at the one-cell stage, the earliest observed symmetry breaking event linked directly to body handedness in the animal kingdom,” they report.

The discovery of the function of Lsdia1 therefore suggests something fundamental. 

Abe and Kuroda write that it “may provide new insight on unifying mechanisms in eukaryotes”. 

It may also, they add, carry implications for a condition called situs inversus, which affects an estimated 0.01% of the human population and manifests in the heart growing on the “wrong” side of the torso.