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It’s the ultimate form of solar power: eat a plant, become photosynthetic. Now researchers have found how one animal does just that.

Elysia chlorotica is a lurid green sea slug, with a gelatinous leaf-shaped body, that lives along the Atlantic seaboard of the US. What sets it apart from most other sea slugs is its ability to run on solar power.

Mary Rumpho of the University of Maine, is an expert on E. chlorotica and has now discovered how the sea slug gets this ability: it photosynthesises with genes “stolen” from the algae it eats.

She has known for some time that E. chlorotica acquires chloroplasts - the green cellular objects that allow plant cells to convert sunlight into energy - from the algae it eats, and stores them in the cells that line its gut.

Young E. chlorotica fed with algae for two weeks, could survive for the rest of their year-long lives without eating, Rumpho found in earlier work.

But a mystery remained. Chloroplasts only contain enough DNA to encode about 10% of the proteins needed to keep themselves running. The other necessary genes are found in the algae’s nuclear DNA. “So the question has always been, how do they continue to function in an animal cell missing all of these proteins,” says Rumpho.

Gene ‘theft’

In their latest experiments, Rumpho and colleagues sequenced the chloroplast genes of Vaucheria litorea, the alga that is the sea slug’s favourite snack. They confirmed that if the sea slug used the algal chloroplasts alone, it would not have all the genes needed to photosynthesise.

They then turned their attention to the sea slug’s own DNA and found one of the vital algal genes was present. Its sequence was identical to the algal version, indicating that the slug had probably stolen the gene from its food.

“We do not know how this is possible and can only postulate on it,” says Rumpho, who says that the phenomenon of stealing is known as kleptoplasty.

One possibility is that, as the algae are processed in the sea slug’s gut, the gene is taken into its cells as along with the chloroplasts. The genes are then incorporated into the sea slug’s own DNA, allowing the animal to produce the necessary proteins for the stolen chloroplasts to continue working.

Another explanation is that a virus found in the sea slug carries the DNA from the algal cells to the sea slug’s cells. However, Rumpho says her team does not have any evidence for this yet.

In another surprising development, the researchers found the algal gene in E. chlorotica’s sex cells, meaning the ability to maintain functional chloroplasts could be passed to the next generation.

The researchers believe many more photosynthesis genes are acquired by E. chlorotica from their food, but still need to understand how the plant genes are activated inside sea-slug cells.

Human photosynthesis?

Greg Hurst of Liverpool University in the UK says that DNA jumping from one species to another is not unheard of but that normally the DNA does not appear to function in the new species.

“Here we have something going across and working in an entirely different context, which is altogether more interesting,” he told New Scientist.

“There was an example recently of a whole bacterial genome that ended up in a fruit fly species, but no-one knows if it functions,” he says. “What is really unique here is the fact that the gene is transferred and appears to function.”

Other animals are able to harness sunlight after eating plants, says Rumpho, but this is only because they acquire entire plant cells, which is very different to transforming an animal cell into a solar-powered plant-animal hybrid.

It is unlikely humans could become photosynthetic in this way. “Our digestive tract just chews all that stuff up - the chloroplasts and the DNA,” she adds.

Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0804968105)