Home - The Big Switcheroo – March/April 2009
Sea slugs living as both animals and plants could provide clues to innate immunity
It’s been said that you are what you eat. If that truly were the case, some of us would resemble hamburgers or greasy slices of pizza, while others would look more like granola bars or glasses of soy milk.
For one tiny creature, however, the idea of becoming what you eat isn’t that far off base.
Referred to as the “solar-powered” sea slug, Elysia chlorotica has fascinated scientists for years because of its ability to retain “stolen” chloroplasts and carry out photosynthesis as if it were a plant.
Although they are slugs, these small green creatures aren’t the yellowish-brown slimy garden variety. Rather, they are emerald green marine molluscs that look like a plant leaf, and only need to eat early in their life cycle.
Since 1987, University of Maine biochemistry professor Mary Rumpho-Kennedy has been studying Elysia chlorotica found in saltwater marshes along the East Coast from Nova Scotia to North Carolina, and sometimes as far south as Florida.
Rumpho-Kennedy’s recent groundbreaking research offers insight into the potential for evolution of photosynthesis in an animal through symbiosis and gene transfer.
What makes this sea slug different is that it acts more like a plant than an animal. It even looks like a leaf and reacts to sunlight in much the same way as a plant, opening up when exposed to sunlight.
But how do sea slugs get that way?
As their first meal, sea slugs suck out the cellular contents of their algal prey and retain the green chloroplasts in cells lining their digestive gut. This transforms the molluscs from a reddish-brown to a green color. Rumpho-Kennedy hypothesizes that the algal nuclei also go through the sea slug’s gut and are most likely broken open, releasing the algal DNA.
This DNA, if not digested, may be either taken up freely floating by cells lining the gut or transferred by some type of vector, possibly a virus. The foreign DNA then becomes part of the animal nuclear DNA, transferring genetic information from the algal nucleus to the sea slug.
This DNA contains the genetic information to make chloroplast proteins essential for photosynthesis to continue. Animal DNA does not contain these genes and, thus, cannot support photosynthesis.
With this special type of symbiosis, sea slugs never need to eat again. Instead, they survive for months on sunlight and air —just like a plant — by carrying out photosynthesis.
“When you eat lettuce, chloroplasts go through your gut, but the enzymes chew them up and digest them,” Rumpho-Kennedy says. “With the sea slug, the chloroplasts aren’t digested and the animal turns green. They must acquire these chloroplasts early in development or they die.”
Animal cells don’t have chloroplasts, so the sea slug has to get them from the algae in order to photosynthesize to produce enough energy to survive.
Scientists have long studied a phenomenon called vertical gene transfer, in which genetic material (a copy of one’s DNA) is passed on from an organism’s ancestor to the next generation.
They’ve also studied horizontal gene transfer between prokaryotes (typically a single-cell organism that lacks a nucleus that contains its genetic material), or from a prokaryote to a eukaryote (that has a nucleus that contains its DNA), or, more rarely, between two closely related eukaryotes. But the idea of horizontal gene transfer between two unrelated multicellular eukaryotes — from an alga to a mollusc, in the case of the sea slug — is something new.
“Your immune system should kill it,” Rumpho-Kennedy says. “We can eat all kinds of plants and we don’t become plants.”
Humans have more sophisticated immune systems than slugs, she says, but the tiny molluscs still should try to attack the foreign chloroplasts and DNA in their bodies.
Rumpho-Kennedy ultimately hopes to discover how the sea slug is able to get the algal DNA into its system and make it work, determine the minimal requirements for photosynthesis, and understand how the foreign material avoids destruction in the sea slug.
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