• Helen Fields is a freelance science writer based in Washington, D.C.

The World's Smallest Farmers

19 January 2011 1:27 pm

Scott Solomon

Tiny farmer. D. discoideum forms a stalk with a fruiting body on top; inside are spores and some bacteria.

It's too bad they don't make microscopic overalls. The amoeba Dictyostelium discoideum could use a pair. A new study finds that the single-celled organism is a farmer of sorts. It picks up bacteria, carries them to new locations, and harvests them like crops.

D. discoideum, or "Dicty," is a so-called social amoeba, formerly classified as a slime mold. An individual amoeba cell can live independently, slurping up bacteria in the soil. When the food is gone, it joins with its comrades, forming a sluglike organism about half a centimeter long that can wriggle to greener pastures (see video). Once there, the slug becomes a stalk with a fruiting body—a tiny globe on top that releases spores, each spawning a single amoeba—to start the cycle all over again.

Debra Brock, a graduate student in ecology and evolutionary biology at Rice University in Houston, Texas, spent a lot of time looking at Dicty under a microscope before she started graduate school; she worked as a technician in a lab that studies the organism's development. Most labs that work on Dicty order strains from a catalog, almost all of which are descended from one clone collected decades ago. But the lab where Brock is getting her Ph.D. has a large collection of clones from the wild. It was Brock's first time looking at spores from wild amoebae, and she saw something she'd never seen before: bacteria hanging out in the fruiting body. "I was going, 'This is really odd,' " she says.

Coming together. Single-celled amoebas come together into an aggregate and form slugs over about 10 to 15 hours..
Credit: Owen Gilbert

At least Brock thought the specks she saw were bacteria. But she couldn't be sure. So she embarked on a series of experiments, described online today in Nature. First, she stuck a very thin pipette tip into a Dicty fruiting body, sucked out the contents, and spread them on a plate for growing bacteria. After 2 days, some of the plates had patches of bacteria, suggesting that some Dicty clones harbor the bugs within their spores.

But were the bacteria just an infection? To find out, Brock killed them by giving antibiotics to the Dicty clones. She then placed the amoeba on a fresh patch of bacteria. The clones that had originally harbored bacteria picked up the bugs again, indicating that they were collecting bacteria. "That was like Nirvana," Brock says. "I was going around in the lab going, 'Yay! Yay!' "

Other experiments showed that the amoebas "planted" their new environments with food. "They can carry [the bacteria], they disperse them, and they seed them in new places, and they actually harvest," Brock says. "We felt that was sufficient to be designated a farmer."

Only about a third of the clones that Brock and her colleagues tested were farmers. That means farming must not give a consistent advantage, or it would have taken over. Indeed, she found that farming wasn't great in all environments. Farmers thrived if their spores were placed on a plate with no bacteria, because they could plant their own. But on a plate that already had bacteria, nonfarmers did well, whereas farmers struggled—possibly because they stop eating before the food is gone.

Researchers have already discovered several animals that farm: ants and termites that grow fungus, damselfishes that tend algae, and intertidal snails that tend fungus. Dicty, the first microbe shown to farm, is less sophisticated. The fungus-farming ants, for example, carefully tend their crops, fertilizing them and killing pests. "It's really amazing the amount of care they give to their crop. An amoeba cannot do that," Brock says.

Koos Boomsma, an evolutionary biologist at the University of Copenhagen who did not work on the study, is not surprised that farming is scattered through the tree of life. "But if I would've had to predict where I would have next expected farming to be discovered, I would never have predicted a slime mold," he says.

"It's a wonderful paper," says evolutionary biologist Bernard Crespi of Simon Fraser University in Burnaby, Canada. Still, he says, "It really is the first data that's out there, so there's always going to be questions." For example, he'd like to see more data about the environments where farmers and nonfarmers are found in the wild—whether farmers are in places where the food is patchier. Such research might even allow comparisons to early human agriculture. "You're always looking for convergences" in evolutionary biology, Crespi says. "Slime molds and humans is one of the more unusual comparisons."

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