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From salamanders to little green men?

by Rowan Hooper

When I was a child, I remember wondering why there were no animals that could photosynthesize. Maybe that’s a bit odd, but it’s not that I was especially geeky; I just felt almost indignant that there weren’t animals with green skin. It seemed to make so much sense.

Can’t find food for dinner? No problem! Just make your own from sunlight, air and water.

Maybe I’d got the idea from “Swamp Thing” or “Black Orchid” or some other comic-book character that was half human, half plant.

It was only years later, deferring reluctantly to academic texts rather than comics, that I learned how plants and animals have been evolving separately for an incredibly long time. They’ve been apart for billions of years, and that means there is just no way you could get both ways of making a living — feeding like an animal and fixing the energy of the sun — in the same organism.

But not so fast. My younger self would have been thrilled to know (as I am now) that a number of animals have been discovered that have plant cells growing in their bodies.

There’s a solar-powered sea slug as well as some flatworms and lots of species of sponges, anemones and jellyfish that all host algal cells in their bodies — and from them derive food fixed from the sun. And, of course, there are the corals — animals that form a partnership with algae to the mutual benefit of both parties.

The sea slug is pretty cool: It has, effectively, its own solar panels. Unlike sponges and corals, it also roams around freely under its own power. But it is, after all’s said and done, only a slug.

Until recently, there had been no vertebrate known to have, or have had, such an intimate partnership with a plant. Now, though, a species of algae has been discovered to live inside the cells of the developing embryos of spotted salamanders.

This is the first known example of an algae living stably inside the cells of any vertebrate, during any part of its life cycle.

“It raises the possibility that more animal/algae symbioses exist that we are not aware of,” said Indiana University at Bloomington biologist Roger Hangarter. “Since other salamanders and some frog species have similar algae/egg symbioses, it is possible that some of those will also have the type of endosymbioses we have seen in the spotted salamander.”

So why is it that only a relatively few animals have taken advantage of what is, it seemed to my naive young self, such an obvious partnership?

First, if you’re an animal and want to derive energy from the sun, you need plenty of sunlight; you need to stay in that light; and you need, ideally, to have some nice broad areas to soak it up.

For a jellyfish, it’s not a problem — they spend a lot of their time bobbing about near the surface of the ocean, where a lot of light penetrates. And they have transparent bodies.

Imagine a photosynthetic mammal — say a rabbit. Our green bunny would have to spend much of its time exposed to the sun, which would also expose it to predators. But although serious, that’s a practical problem that could be worked around.

There’s a more important reason why we haven’t seen photosynthetic vertebrates. How do they get the chloroplasts — the green, light-harvesting molecules in plants — into their bodies? And even if they could get them in there, like sea slugs, how would they pass them on to their offspring?

Plants and animals have evolved separately for too long. It would need humans to intervene, with major genetic engineering, to create an animal with chloroplasts in its cells.

It’s actually been known for a long time that there is some sort of relationship between spotted salamander eggs and green algae. The algae that associates with the eggs was named Oophilia, which means “egg-loving,” and experiments in the 1980s showed that salamander embryos do not develop as quickly or as fully in the absence of the green algae. Likewise, algae grown separately from the embryos, but in the presence of water exposed to the embryos, also grew more robustly.

It seems they both need each other. Each benefits from the relationship. The algae are nourished by nitrogen from the waste products of the embryos, and the embryos are refreshed by the oxygen generated by the algae.

However, it was difficult to see algal cells until the arrival of modern fluorescent microscopes, and an RNA-tagging technique was developed.

Hangarter and colleagues used a molecule that sticks to a length of RNA — the sister molecule to DNA — that is particular to the Oophilia algae. When they looked at the salamander embryo cells that had been treated so as to be tagged, they found that the algae RNA showed up throughout.

The fact that the algae was found in the cells of the embryos, and also in the reproductive tract of some adult females, suggests that the algae may in fact be retained in adults, too, though the researchers need to investigate this further. An account of their work to date is published in the Proceedings of the National Academy of Sciences.

“I think it is important for people to realize that you do not need to go to exotic locations to make interesting scientific discoveries,” said Hangarter. “The vernal ponds that the salamanders mate in are also essential for many other amphibians and other organisms, but such ponds are often among the first things destroyed when humans develop wooded areas.”

Failing green-skinned photosynthesizing animals, I’ll have to settle for green eggs. But you never know what other symbioses are going on out there.

Rowan Hooper is the news editor of New Scientist magazine. Follow him on Twitter @rowhoop. The second volume of Natural Selections columns translated into Japanese is published by Shinchosha at ¥1,500. The title is “Hito wa Ima mo Shinka Shiteru” (“The Evolving Human”).