Even single-celled farmers have to protect their crop from hungry mouths. That's why slime molds carry certain toxic bacteria inside their bodies on their way to farming others in the soil. Like living Roundup, these bacteria harm competitors while helping their farmer hosts to survive and even thrive.
Slime molds start out life as one-celled amoebae, living in soil or mulch and munching their way through the bacteria there. Once food becomes scarce, they seek each other out and glom together into big, gooey colonies. Some species form blobs that are big enough to see with your naked eye as they ooze across a forest floor in search of greener pastures. Individuals in the blob may use their bodies to build stalks, lifting other individuals as spores. These travelers will be carried away by the wind to start over someplace new.
Dictyostelium discoideum amoebae, when they set out into the world as spores, don't go unprepared. Debra Brock, a PhD student at Rice University at the time, announced in 2011 that some slime molds are farmers. Before the last of the food runs out, they tuck a few edible bacteria into their bodies. Then they launch themselves into the wind, and when they land they seed the soil with the crop they've been carrying.
Brock found that about a third of wild Dictyostelium discoideum slime molds are farmers. But if this is true, how do they deal with moochers? Farmers pay a price for their pastime—they have to set aside the last bit of available bacteria instead of eating it. If their neighboring amoebae can eat all the food they want, then live off the bacteria crop that farmers plant in a new location, why aren't farmers at a major disadvantage?
Brock, who's now a research scientist at Washington University in St. Louis, may have found the answer by digging deeper into the pockets of the amoebae. Farmers don't only carry food with them when they disperse, she saw: they also carry inedible bacteria. Once everyone makes their way to a new plot of land, could these bacteria somehow be protecting their hosts from moochers?
To find out, she mixed farmers and non-farmers together in dishes with plenty of food bacteria—and added a little sprinkle of the inedible pocket bacteria. Farmers stayed perfectly healthy. But non-farmers suffered, producing only half as many spores as usual.
Next Brock spun down samples of the inedible bacteria to extract a liquid from them, which would contain any chemicals the bacteria give off. When she gave doses of this liquid to both farmer and non-farmer amoebae, the non-farmers suffered again. Not only were the farmers fine, but they actually benefited, producing more spores than when they were dosed with a control liquid.
Finally, farmers and non-farmers were grown together and allowed to compete, along with a few species of bacteria that the farmers had carried with them. The more farmers there were, the worse non-farmers fared.
The Dictyostelium discoideum individuals that opt for a farming lifestyle seem to keep deadbeats away by bringing other bacteria with them. Brock doesn't know what weapon these bacteria give off that's bad for non-farmers. But she speculates that the system may have evolved because certain slime molds were genetically predisposed to gather up bacteria before traveling. These farmers would have been likely to vacuum up toxic bacteria along with the edible ones. By developing a resistance to the toxin—and even a way to benefit from it—the slime molds found a way to make farming profitable.
Image: Dictyostelium aggregation, by Bruno in Columbus (via Wikimedia Commons)
Debra A. Brock, Silven Read, Alona Bozhchenko, David C. Queller, & Joan E. Strassmann (2013). Social amoeba farmers carry defensive symbionts to protect and privatize their crops. Nature Communications DOI: 10.1038/ncomms3385