Agriculture clearly revolutionized human society, but the practice seems to have occurred on a simpler scale among a broad range of species. Termites, ants, fish, and snails all practice simple forms of "farming," in which a food source is organized and encouraged to proliferate in order to ensure a steady food supply. Today's issue of Nature identifies a similar case, this one involving a very unexpected species: the slime mold, which spends a big part of its life cycle operating as a single-celled amoeba.

One species of slime mold, Dictyostelium discoideum, has been used extensively for research, but no one had observed anything resembling farming within the lab populations. A team at Rice University, however, spotted it while working with samples freshly obtained from wild populations. At two sites, the farming strain was only about a third of the total population. So it's not clear whether this is a case of nobody having looked carefully or the lab strains not exhibiting any farming behavior.

To understand the farming behavior, you have to know a bit about the Dicty life cycle. When food is plentiful, the organisms will proliferate in a single-celled, amoeba-like state. But, when nutrients become scarce, these cells start to talk among themselves, and gather into a single mass. That mass eventually forms a multicellular organism, which undergoes a startling transition to a slug stage, at which point the ensemble is mobile and able to move at least some distance away from the starvation-inducing environment.

But the escape isn't complete yet. After some time, the slug stops and its cells form a broad disk, which transitions to what's called the "Mexican hat" stage. The peak of the hat eventually forms a thin filament topped by a pod; spores form within the pod, and can spread even further away once the pod opens. Thus, the spores get the chance to start over in a fresh environment, hopefully one with better nutrient conditions. All the other cells, which were once free-living organisms, die, sacrificing themselves to ensure the continued proliferation of their neighbors (which are, in all probability, close genetic relatives—their genes live on, and Darwin is satisfied).

So, where does farming come in? In the strains that perform the feat, the cells in the spore do not travel alone. Instead, they are accompanied by bacteria from the environment in which they formed. When the spores break open, these bacteria do what bacteria normally do: they start dividing, and rapidly form a food source for the recently arrived slime mold.

This generally works out well for the farming slime mold. When put on plates with no bacteria, the non-farmers couldn't grow at all, but the farmers managed to proliferate. When placed on plates containing soil from outdoor sites, the farmers outcompeted their non-farming kin. Only when given a rich, bacterial lawn could the non-farmers outdo the agriculturalists, although the advantage is slight compared to the edge held by the farmers. Still, these advantages come at a cost. The non-farmers were able to pick their environment clean before giving up and forming a slug, while the farmers left plenty of food behind in order to ensure that some was available for packing into spores—in some cases, about half the available food was left uneaten. In addition, the farmers didn't move as far during the slug stage, although the authors speculate that this may be a result of the fact that they really don't have to try as hard to escape starvation conditions.

Those of you who have worked in labs will undoubtedly be wondering whether this might be a case of contamination that happens to work out well for the contaminee, since bacteria will grow just about anywhere if given half a chance. But the authors show that the behavior is stably inherited. You can take the farmers and non-farmers, and grow them on plates with nothing but dead bacteria and antibiotics, eliminating any chance they have to pack food in spores. Switch them back to live bacteria, however, and old behaviors reemerge. Farmers start farming again, and the non-farmers don't show any tendency to pack bacteria into their spores.

The fact that these two different approaches to sporulation exist in a single species suggest that farming is under what evolutionary biologists would call balanced selection. There are good and bad aspects to both approaches to sporulation, so neither one is able to sweep the other out of a population. Part of this is that the agriculture practiced by Dicty is pretty primitive. They can't discriminate between different types of bacteria, so they'll happily pack in species they don't feed on. It's possible to conceive of a system in which Dicty could evolve a way to farm a single species, much as leaf cutter ants work with only a single species of fungus, which might provide a more clear-cut advantage.

The neat thing about this paper is the number of other experiments it suggests. Environmental samples from a variety of conditions can be tested to see whether farmers or non-farmers prevail in some circumstances. Mixed cultures of the two types can be grown to see whether they segregate into different slugs, or determine what fraction of Dicty needs to farm for the whole spore to go along. Finally, Dicty has an extensive collection of genetic tools, so it should be simple to identify the genes that allow the organism to farm. (I'd place a bet on the ones that register nutrient levels).

There isn't any level of conscious decision-making involved here, so this farming is nothing like the development of human agriculture. But the authors conclude by noting that, in some senses, Dicty is a social species, one where individuals cooperate and make sacrifices for their genetic relatives. And they posit that some form of social behavior may help predispose a species towards a state where some form of agriculture becomes favorable.

Nature, 2011. DOI: 10.1038/nature09668 (About DOIs).

Listing image by Scott Solomon