If you step on a snail, you’ll know it. The crunch of its shell or the feel of their squishy, slimy bodies underfoot. Despite their slow speeds, and simple bodies, apple snails (Pomacea canaliculata) have eyes that are anatomically similar to human eyes. Both species have complex camera-like eyes with a lens, cornea, and retina that visually capture the world around them.
Unlike humans, apple snails can regrow their peepers if they are injured or amputated. To do this, the snails have developed some unusual tools to alter its genome that could inform future ways for humans to do the same. These findings are detailed in a study published August 6 in the journal Nature Communications.
“Regeneration isn’t a magic trick, it’s a process encoded by genes, which means, one day, the ability to recover from injury could be fundamentally improved for everyone,” Alejandro Sánchez Alvarado, a study co-author and biologist at Stowers Institute for Medical Research, tells Popular Science.
Meet the apple snail and their amazing eyes
In parts of the United States, apple snails are considered an invasive species. These two to three-inch-long snails are native to South America, particularly to parts of Brazil and Argentina. They have made it to North America, Europe, and Asia through the aquarium trade and accidental exposure in recent years, where they can be a threat to the local ecosystem. According to the Texas Invasive Species Institute, snail damage in uncontrolled fields can be as high as 100 percent for rice seedlings in the germinating stage.
“Interestingly, the same traits that make them invasive pests also make them excellent laboratory animals,” Alice Accorsi, a study co-author and developmental biologist currently at the University of California, Davis, tells Popular Science. “They are resilient, grow quickly, reproduce abundantly. They also have direct development without larval phase or metamorphosis, which simplifies their life cycle for experimental work.”
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The researchers also looked at a gene known to play a crucial role in vertebrate and fruit fly eye development (called pax6) that is also present in apple snails.
“With the advent of CRISPR technology, we can now manipulate genes in this species. This included targeted disruption of the pax6 gene, an essential regulator in eye development and regeneration,” explains Sánchez Alvarado.
It takes apple snails about 28 days to regenerate their eyes, from the initial amputation to full restoration. This process includes four separate stages: wound healing, formation of a special cell mass, emergence of a lens and retina, and the maturation of all eye components.
All vertebrates (including us humans) can only perform that first wound healing stage. Now, researchers are looking to see exactly where that regeneration and development diverge and trying to pinpoint what genetic switch the snails use to trigger new eye development.
Editing eye genes
In the new study, the team took to the lab and used CRISPR gene editing to disrupt pax6 gene function. The new line of snails created with the edited genes was healthy, but had missing eyes.
During each stage of eye regeneration, the team collected and analyzed gene activity present. This important data about the timing of gene expression can now help narrow down which genes are likely most promising for eye regeneration. With this list of candidate genes for eye regeneration, the team can further use CRISPR to disrupt the genes to see if they are required for an eye to regenerate.
“What surprised me most during this research was how fast, precise, and reproducible eye regeneration is in these snails,” says Accorsi. “After the entire eye is removed, the first signs of regrowth appear in less than two weeks, and a new eye, with all its components, is restored in under a month.”
Their molecular studies also showed that many of the same genes are involved in building both snail eyes and human eyes. While they evolved independently, Accorsi says that it demonstrates that there may be many ways in nature to build an eye.
“The fundamental genetic building blocks are shared between very different species (humans and snails),” she adds.
Nature’s ‘experiments through evolution’
According to the team, understanding how this process works at a molecular level has applications in several fields. One of the most immediate results is for a better understanding why humans and vertebrates struggle to regenerate complex structures like eyeballs. Beyond just eyes, apple snails could also serve as a model organism for studying tissue regeneration.
It also has some potential therapeutic applications. Genes like pax6 may play similar roles across many animals, so the findings in this study could help develop treatments that protect eye health and one day promote repair or regeneration in human eyes.
“Understanding how some animals can regenerate complex eyes (and what genes are involved) could one day inform therapeutic approaches for human eye injuries or degenerative conditions like corneal dystrophies or macular degeneration,” says Sánchez Alvarado.
Work like this also highlights why it’s important to keep studying a wide range of organisms to make new discoveries.
“Nature has already carried out several ‘experiments’ through evolution, and by exploring how different species solve similar biological challenges, we often find that there is more than one way to achieve the same outcome,” says Accorsi. “Uncovering these diverse strategies can open up entirely new perspectives on how we understand — and ultimately treat — our own bodies.”
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