Based on its name alone, the slime mold is a contender for the world's grossest organism. But don't let its grubby name fool you—slime molds have accomplished a feat that only a handful of other organisms on earth can claim—they have developed agriculture.
Slime molds—or "social amoebae," as researchers prefer to dignify them—are relatively simple creatures that spend most of their lives as single-celled individuals living in the soil, feasting on bacteria as they travel. Fascinatingly, these solitary cells have the ability to coalesce into what amounts to a superorganism that acts as a single unit despite consisting of separate individuals.
But new research, announced today in the journal Nature by my Rice University colleagues Joan Strassmann and David Queller, suggests that some social amoebae have taken this partnership to the next level. (I was not involved in the study.) Instead of slurping up every last bacterial cell they encounter as they ooze their way through the soil, certain genetic strains of Dictyostelium discoideum—aka "Dicty"—have evolved the ability to store bacteria and carry it with them in what amounts to a microscopic grain silo. Should they happen to find themselves in an area lacking in suitable bacteria, they just plant a new crop using their stored reserves.
It's not the first known case of nonhuman agriculture, but it is the first by an organism outside the animal kingdom. Several types of insects, including certain species of ants and termites as well as some beetles, cultivate fungi that they use for food. In these cases—especially the fungus-growing ants that I study—some of the cultivated fungi have become truly domesticated, having evolved to become better food for their hosts while losing the ability to survive on their own.
As is so often the case in science, the discovery that some strains of Dicty are farmers happened by accident. As a beginning graduate student in Strassmann and Queller's lab, Debra Brock, the lead author on the new study, was interested in studying competition between Dicty strains. As she examined spores with a high-magnification microscope, she noticed that several Dicty clones had something unusual in their sori, a globular mass elevated off the ground by a long, thin stalk that together looks like a tiny mushroom. All Dicty use this structure, called a fruiting body, to elevate cells producing spores for reproduction. But Brock also thought she saw something else in the sorus. To figure out what it was, she tried to cultivate it.
It turned out to be bacteria—the same type Dicty normally eat. Brock even tried "curing" the Dicty of their bacteria by treating them with antibiotics, thinking that they might have some sort of disease. It worked—the clones that housed bacteria lost the contents of their picnic baskets after the treatment. But as soon as she exposed the antibiotic-treated Dicty to a fresh plate of bacteria, the Dicty clones that contained bacteria prior to antibiotic treatment—which Brock dubbed "farmers"—readily took up the bacteria and stored them away. Genetically similar clones that had not previously housed bacteria did not. In other words, certain Dicty clones can farm and others simply cannot. Further stacking the odds in favor of the agriculturists, Brock's farmers produced more spores than nonfarmers when grown in an environment lacking bacteria, suggesting that when food is scare, those able to farm can produce more offspring.
Humans underwent nearly the same transition nearly 10,000 years ago, shifting from a nomadic, hunter-gatherer lifestyle to one in which a reliable food source is stored for later consumption. Planting crops caused people to settle in one place, and a steady food supply allowed populations to grow and people to specialize in tasks other than finding food. The result was civilization as we know it.
Are slime molds in the process of forming advanced civilizations? Probably not. Dicty lack the sophisticated brains of humans and even ants—or anything resembling a nervous system. Although previous research has shown that simple microbes are capable of performing many seemingly complex tasks, including communication, recognizing relatives, even navigating mazes.
Despite our obvious differences, there is something we farmers have in common: We're social. We live in groups that typically consist of close relatives, which may be important for developing agriculture. This isn't necessarily obvious. If growing your own food is such an advantage, why don't solitary organisms cultivate fungi or bacteria the way we grow corn and raise cattle? Is it simply too much work for any one animal—or amoeba—to handle?
Not necessarily. It may be that farming has benefits that extend beyond the lives of the individual by providing a reliable food source for future generations. If planting crops will benefit your children and grandchildren as well—think of an apple orchard—then farming becomes a good strategy for propagating your genes. That only works when groups are made up of family units—as is the case for slime molds, fungus-growing ants, termites, and us.
There are downsides to being a farmer. Being loaded down with bacteria makes Dicty farmers travel more slowly. Although farmers thrive when bacteria are absent, nonfarmers produce more spores than farmers when food is readily available. In fact, social amoebae may seem an unlikely candidate for agriculturists precisely because they eat bacteria, which, as far as food sources go, are a pretty reliable one. Even a population of slime molds that numbered in the trillions could theoretically be hard-pressed ever to go hungry. So why bother storing any for later?
It may be a matter of taste. Brock collected soil from a Houston park, which was teeming with bacteria but apparently inferior to what the Dicty farmers had already stashed away. Farmers were more successful than nonfarmers when grown on the park soil, presumably because they had access to better grub.
Admittedly, agriculture as performed by Dicty is still pretty basic, at least as far is currently known. It's still premature to say whether Dicty meet the complete definition of farmers. It remains to be seen, for example, whether the bacteria Dicty carry are dependent on their farmers for propagation as our crops depend on us. But perhaps we are glimpsing the first step in a major evolutionary transition. After all, the first human to plant a seed instead of munching on it could not have foreseen the implications of his or her actions.
And if not—if what we are observing today is the most sophisticated behavior that Dicty ever develop—it's still not bad for a blob.