“If you can pack your food source with you, it’s a serious advantage,” said molecular biologist Debra Brock of Rice University, co-author of the slime mold study, published Jan. 19 in Nature.

Dictyostelium discoideum, the best-known of a group of creatures called slime molds, spend part of their lives as single-celled amoebae feeding on bacteria that grow in decomposing leaves on forest floors.

When food is short, hundreds of thousands of amoebae come together, fusing into a single entity. It may crawl off as a slug in search of richer pastures, then form a stalk topped by a “fruiting body” that bursts to disperse a few lucky amoebae-turned-spores; or form the stalk right away, without crawling.

It’s been thought that slime molds simply scavenged, eating bacteria they like and oozing out the rest. In laboratories, researchers “cure” slime molds of their bacteria by allowing them to purge themselves on Petri dishes. But Brock, who studies how slime mold cells communicate and self-organize, found that one of her strains kept contaminating its dishes with bacteria.

“The typical response to finding two species in a culture is, ‘Ick, I don’t want this!’” said molecular biologist Kevin Foster of Oxford University, who wasn’t involved in the study. “[Brock's team] had the insight to realize this was more than a simple contamination, that something else was going on here.”

Brock’s team took new samples of different slime molds in the wild, growing them with careful attention to their dietary and excretory habits. They found that some strains didn’t gorge themselves and “lick the plate clean” of bacteria, but instead saved some inside of the colony. They were farmers, and fared better in soil than their non-farming counterparts.

In another experiment, the researchers gave antibiotics to their slime molds, killing off the colonies’ bacteria. When Brock’s team reintroduced  bacteria, the farmers absorbed multiple strains, keeping some but not eating all of them. Non-farmers simply consumed bacteria or left them behind.

Follow-up experiments are underway to see what genes may differ, if any, between farmers and non-farmers.

Foster said he’d like to know where farmed bacteria hide when slime molds form spores. “If they’re taken inside spores, that’s even stronger evidence of an adaptation for farming,” he said.

In an accompanying commentary, University of Copenhagen biologist Jacobus Boomsma noted that “the ancestors of these slime moulds were among the earliest colonizers of terrestrial habitats, so the history of this bacterial husbandry symbiosis may go back further than any other farming system.”

“They may well possess unknown adaptations that, if revealed, would illuminate fundamental questions of conflict and cooperation across species boundaries,” Boomsma wrote.

“As humans, we have very intimate relationships with microorganisms,” Brock said. “[Slime molds] have amazing similarities to humans, with all kinds of developmental genes similar to ours, and even have immune systems. We can use them to attack basic questions about ourselves.”

Images: 1) Scanning electron microscope image of Dictyostelium discoideum in several developmental stages. Shown in this image are slime molds growing stalks topped with spore-filled balls (top, left to right), as well a slug (bottom left) and mounds (bottom, center)./M.J. Grimson & R.L. Blanton via Dictybase.org. 2) A light microscope photograph of Dictyostelium discoideum fruiting bodies./Scott Solomon.

Video: A yellow slime mold (note: not Dictyostelium discoideum) grows over 5 hours on a log./Vimeo/sesotek.

See Also:

• Slime Mold Grows Network Just Like Tokyo Rail System
• Searching for Network Laws in Slime
• Complexity Theory in Icky Action: Meet the Slime Mold
• Microbe May Answer Mystery of Multicellular Life
• Farmer Ants Fertilize Their Gardens With Bacteria
• Brain Amoebas. Organ Transplants. Brrr.

Citations: “Primitive agriculture in a social amoeba.” By Debra A. Brock, Tracy E. Douglas, David C. Queller & Joan E. Strassmann. Nature, Volume 469 Number 7330, January 20, 2010.

“Farming writ small.” By Jacobus Boomsma. Nature, Volume 469 Number 7330, January 20, 2010.

Authors: Dave Mosher

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