CRISPR snails: Tweaking a single gene in snails created mirror-image shell swirls

Left-swirling snails have trouble hatching & finding mates, which isn't great news for their kind.

Turns out, the spin doctor that prescribes which way a snail's shell swirls — clockwise or counterclockwise — is genetic. What's more, this twist in the story of snail shells manifests right at the moment of its birth, when the embryos of the snail are still just a single cell.

Most pond snails in the world have shells that turn clockwise. However, a few have been seen taking a different turn — a counterclockwise one.

Researchers think a single DNA sequence — the lsdia1 gene — decides the fate of this spiral-spin counter-evolution in snail shells. A very similar gene, lsdia2, was also suspect since it has 89.4 percent identical DNA to lsdia1.

Teasing them apart and finding out which of the two was really in charge of turning a snail's shells was tricky, but scientists have finally done it using gene editing technology CRISPR.

CRISPR snails: Tweaking a single gene in snails created mirror-image shell swirls

Snail swirl and twirl both ways.

Researchers cut the lsdia1 gene and made it non-functional using gene editor CRISPR/Cas9. The snipped form of the gene was passed on to some of the snail's young ones. If both snail parents pass on non-functional copies of lsdia1 gene to their kids, the snail will grow to have counterclockwise (left-coiling) also known as sinistral shells. These lefty snails from the gene-editing study are the first time scientists have edited the genes in a snail and seen the changes manifest.

Snails aren't the only species with asymmetry. Humans have organs placed asymmetrically and so do many other species. This asymmetry is critical to the anatomy of species.

For instance, in mammals, the asymmetry is a must for the intestines to properly fold half a dozen times into a relatively small nook, Martin Blum, a developmental biologist from the University of Hohenheim, told ScienceNews.

Lefty snails are a result of mutations that make then less fit for survival than researchers thought.

Lefty snails are a result of mutations that make then less fit for survival than researchers thought.

While lsdia1 is the only known gene to decide the fate of snail shells, there are two known genes responsible for asymmetry in snails: nodal and Pitx. While still a single-cell embryo, the genetically-tweaked snail turned these genes on and in a pattern that is a mirror image to that of right-coiling snails (creating lefties), the study found.

Unlike evolutionary mutations that make species more fit to survive, the swirl in snails probably isn't helping snails in the wild, researchers think. But it does indirectly affect their survival. Snails that have left-swirling shells have trouble hatching and finding mates, according to Angus Davison, an evolutionary geneticist and lead author of the study from the University of Nottingham.

"If you're a snail out in the wild, it's game over for that mutation," Davidson added.

The study's findings were published in the journal Development.

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