Solar-Powered Sea Slugs Steal DNA from Algae

Elysia chlorotica, a photosynthetic sea slug. Patrick Krug

Several species of sea slugs can actually photosynthesize like plants, harnessing energy from the sun to produce food. Perhaps the most impressive example is a little green creature called Elysia chlorotica, which can eat algae when it's very young and then spend the rest of its 10-month life basking in the sun, no eating necessary.

Instead of just digesting the algae, the animals steal little components within them called chloroplasts that carry out photosynthesis, using sunlight to convert carbon dioxide and water into sugars. But these cellular mini-machines need constant upkeep and repair to stay running — scientists have long wondered how an animal would be able to provide for its continued function, considering it is devoid of the necessary plant genes for this process, of which there are hundreds if not thousands.

Sidney Pierce, at the University of South Florida, and colleagues have used an advanced imaging technique to show that one algal gene involved in photosynthesis is present in the genome of E. chlorotica — and is identical to a gene in the algae that the sea slug eats. These genes were present in larval and adult slugs who were not exposed to algae, meaning they must have gotten the genes from their parent(s) and not their environment, according to a study published in The Biological Bulletin. (Sea slugs are hermaphroditic and can self-fertilize, although this is not usually the case.) Known as prk, the gene encodes an enzyme used in photosynthesis that isn't found in animals.

This study lends support to previous work by Pierce and others that has found more than 50 algal genes in slug cells. It also suggests the gene moved from the algae to the slug via horizontal gene transfer, a process also used by humans to genetically modify organisms in lab conditions. And it stands to reason that there might be other genes in the slug that got there via the same mechanism, the study says.

Knowing more about how this process happens naturally could have potential applications for gene therapy, which seeks to use horizontal gene transfer to insert DNA into human cells. This has proved notoriously tricky for people, but the slug seems to have it taken care of.

For now, the mystery remains with the slug. "If I knew that, I'd have figured out how gene therapy works and I'd be a millionaire and retired," Pierce told me.