WHAT'S FOR DINNER: Humans are not the only farmers cultivating their preferred foods. Several organisms, from fish down to humble slime mold, have been found to engage in primitive agriculture behavior. Image: SCOTT SOLOMON
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Slime mold might not seem like a terribly advanced life form, but these single-celled social amoebae are adept at joining forces to increase their chances of passing on their genes. Loners for most of their lives, thousands of these organisms will together to form a multicellular, sluglike unit to seek out and populate new habitat when resources become scarce.
Some populations of these eukaryotes have upped their survival odds even further by bringing their favorite bacterial food along with them to create a fresh crop that is seeded and consumed in the new habitat. In doing this the tiny Dictyostelium discoideum are practicing a primitive version of agriculture, a group of researchers from Rice University now claims.
"Instead of consuming all bacteria in their patch, [the amoebae will] stop feeding early and incorporate bacteria into their fruiting bodies," wrote the researchers in a new paper, published in the January 20 issue of Nature. (Scientific American is part of Nature Publishing Group.) "They then carry bacteria during spore dispersal and can seed a new food crop, which is a major advantage if edible bacteria are lacking at the new site."
Farming as humans practice it—with tools and complex planning—has only been around for some 10,000 years, but basic elements of agriculture include strategic crop harvesting and dispersal. And the amoebae are doing just this with the bacteria. "They take it to new nutrient sources for the bacteria to grow, and they also prudently harvest—they don't eat all of it—and save some of it for the dispersal," explains Debra Brock, a graduate researcher at Rice's Department of Ecology and Evolutionary Biology and co-author of the new study.
Like many in the biology field, Jacobus Boomsma of the University of Copenhagen's Center for Social Evolution was surprised to learn of the bacteria-farming amoebae. But he says that "it made good sense once you started to think about the results." Boomsma, who also wrote an essay on the amoebae in the same issue of Nature, noted that it is "a fascinating finding because the Dictyostelium-bacterial symbiosis is evidently driven by mutualistic advantages, despite the obvious risks of bacterial exploitation of the dispersal opportunities provided by the hosts."
Even when the amoebae ended up in a new site already replete with resident bacteria, the farming amoebae "could still gain by bringing preferred bacteria…just as humans seed preferred plants in an already green world," Brock and her colleagues wrote in the paper.
But bringing bacteria along for the ride is not without its drawbacks. By refraining from consuming all of the bacteria in their habitat, farming amoebae might be depriving themselves of the total nourishment that a nonfarming colony could ingest. The researchers also found that, on average, the bacteria-carrying slug units traveled a shorter distance before fruiting than did the nonfarmers.
Only about a third of the wild-collected Dictyostelium clones (sampled from Mountain Lake Biological Station in Virginia and Lake Itasca Biological Station in Minnesota) were bacteria farmers. The researchers suggested that when the amoebae happen to release spores "at sites already containing appropriate bacteria, the costs of early feeding cessation are not compensated for, which may account for the dichotomous nature of this farming symbiosis."
Brock notes that the organisms' social nature and life cycle help to explain why farming might be an advantage for some amoeba. In the wild the fruiting bodies comprise a closely related clonal isolate, so even if roughly one fifth of the individuals die in the course of becoming the structure's stem, plenty of genetically similar material will be passed along to future generations via the spores.
"It's really cool that they can do this," Brock says of the Dictyostelium. "But in a bigger picture I think that this possibly offers a lot of new ways to ask questions." For instance, she sees the amoeba–bacteria relationship as an interesting model for studying the complex interactions humans have with bacteria. Because the genome for Dictyostelium has been sequenced, researchers are already deeply immersed in teasing apart different genes and traits that might help explain the differences between farmers and nonfarmers and how the symbiosis might have evolved in the first place.
The amoebae are not the only animals other than humans to engage in quasi-agricultural behavior—although they are the tiniest. Ants, termites, beetles, snails and some fish have been shown to also plant, harvest—and in some cases tend—their preferred food sources.
View slide show of nonhuman cultivators.