Going Green

By Carl Zimmer | November 14, 2008 3:50 pm

green-slug.jpgMemo to Sci-Fi Movie Development Dept.

Re: Plant People Concept

Here are the specifics for the aliens in our next movie, VOYAGE TO THE PLANET OF THE PLANT PEOPLE. The people there are vegetarians, eating salads, seaweed drinks, etc. They are green, because when they digest their meals, their bodies move some of the plant matter to their skin. They use it to capture energy from the sun through photosynthesis. So the aliens sit outside in the sun a lot, getting lots of energy and growing to huge sizes, which is why they’re so hard to kill when Earthlings show up. I see the people from Blue Man Group–just add some yellow and we’re set. Or maybe William Dafoe?

Behold, Hollywood, the sea slug Elysia chlorotica. This emerald green slug grazes on algae. As it breaks down the algae, it preserves their photosynthetic structures, called plastids. The plastids are shipped to the surface of its body, where they can continue to photosynthesize. The slug thus makes a living as plants do. And it turns a beautiful shade of green along the way.

Recently, some scientists discovered that the sea slug is even more plantlike than previously thought thought. They wondered if some genes from the algae the slug ate had become incorporated into their own DNA. This movement of genes is called horizontal gene transfer. It’s common among bacteria, which swap genes for antibiotic resistance and such. It’s not as common among multicellular creatures, but it has happened a number of times. For example, our ancestors swallowed up bacteria that eventually became our mitochondria, the structures in our cells that use oxygen to generate energy. Mitochondria still have some of their own bacterial genes, and other bacterial genes have moved into our own DNA. The ancestors of green algae and plants swallowed up photosynthetic bacteria and harnessed their ability to photosynthesize. Those bacteria became plastids. The p of the genes from the plastids are now part of the DNA of plants.

Mary Rumpho of the University of Maine and her colleagues suspected that something like this had happened to the sea slugs. They were struck by the fact that the plastids continue to function in the slugs for months after they’ve been extracted from algae. But plastids normally can’t function on their own. They need help from proteins encoded by genes that are now carried in the algae’s DNA. It was possible that the slugs were making the proteins for their plastids.

So Rumpho’s group gathered up some sea slugs off the coast of Martha’s Vineyard and took a look at their DNA. They also took a look at the DNA of the one species of algae that the slugs put in their skin. As they suspected, the plasmids don’t have all the genes necessary for photosynthesis. The scientists discovered a crucial photosynthesis gene, called psbO, in the DNA of the slug. In fact, the sequence of the slug’s psbO gene is identical to the one in the species of algae that supplies them with their plastids.

Now that scientists can peer into genomes without too much difficulty, they’re probably going to find a lot of these gene transfers, and some of them are going to turn out to be big leaps–like the jump from the plant kingdom to the animal kingdom.  What I want to know is how a photosynthesis gene from the nucleus of algae got integrated into the DNA of the slug that eats it. I also want to know why other plant-eating animals didn’t merge with their food as well. Why aren’t sheep green? And finally, when will Voyage to the Planet of the Plant People wrap up?

Until then, you can enjoy Rumpho’s great web site dedicated to this symbiosis–complete with slug videos.


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The Loom

A blog about life, past and future. Written by DISCOVER contributing editor and columnist Carl Zimmer.

About Carl Zimmer

Carl Zimmer writes about science regularly for The New York Times and magazines such as DISCOVER, which also hosts his blog, The LoomHe is the author of 12 books, the most recent of which is Science Ink: Tattoos of the Science Obsessed.


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