Micro Farmers

By Cristina Luiggi Micro Farmers Dustin Rubenstein discusses how the discovery of amoebas that farm their own food links the development of agriculture with the evolution of social behavior. Although agriculture is often touted as a pivotal human invention, it is not unique to us. It turns out that even slime molds with a penchant for sociality can farm.

By | May 1, 2011

Micro Farmers

Dustin Rubenstein discusses how the discovery of amoebas that farm their own food links the development of agriculture with the evolution of social behavior.

Although agriculture is often touted as a pivotal human invention, it is not unique to us. It turns out that even slime molds with a penchant for sociality can farm. For Dustin Rubenstein, an evolutionary ecologist at Columbia University, this unexpected finding points to an evolutionary link between the ability to cultivate food and the development of complex social behavior (Nature, 469:393-96, 2011).

The Scientist: In which species, besides humans, do we see agriculture?

Dustin Rubenstein: We see it in a variety of organisms, most commonly in insects. Ants are the most well-known species outside of humans. But there are also beetles that farm their own food, and there are various subtypes of agriculture in some species of fishes. It’s certainly not widespread in the animal kingdom, but it’s more common than just in humans.

The fruiting body of Dictyostelium discoideum.
© M.J. Grimson & R.L. Blanton, Biological Sciences Electron Microscopy Laboratory, Texas Tech University

TS: Earlier this year, researchers led by Rice University evolutionary biologists David Queller and Joan Strassmann discovered a primitive form of agriculture performed by the social amoeba Dictyostelium discoideum. Is this just another species that farms, or is there a deeper significance to this finding?

DR: When you look at where agriculture has been found, again, it has been found in insects and in some vertebrates. Here you’ve got this group of slime molds, or social amoebas, that is ancestral to fungi and vertebrates. It’s way out there in the evolutionary tree. So the fact that we find agriculture in such a basal group of organisms is quite interesting. Until this paper came out, I don’t think anyone would have thought we would see farming in amoebas.

TS: What exactly do these slime molds farm?

DR: Slime molds feed on bacteria. They inhabit a local area and eat all the bacteria. When they eat it all down, they cooperate to form a fruiting body that then sends spores off so they can repopulate new areas and find new food. What the authors of this paper discovered was that some of the individuals in this population actually carry bacteria with them [on spores]. And it looks like they will use the bacteria that they carry to inoculate a new area where they land. If the slime molds were to land in an area that didn’t have any bacteria, then they would die. It’s sort of an insurance mechanism. They take it with them, so when they move they definitely have food.

TS: It seems that in any given slime mold population, only about a third of individuals are farmers, suggesting that while there are benefits to farming, there are also some costs that keep all individuals from employing the strategy.

DR: Slime molds that have the bacteria move more slowly, and if they land in an area that does have good bacteria, then the ones that carry their own bacteria don’t do as well as the ones without them. What’s interesting, too, is that they farm multiple species of bacteria; they don’t just farm a single one. So a lot of the farmers will specialize in a single species of bacteria and then they’ll coevolve together. But because they can feed on multiple species and some are better than others, it doesn’t always pay to farm.

TS: Do the bacteria derive some kind of benefit from being picked up and dispersed to other places?

DR: That could be. It could be a way for bacteria to move long distances. If they’re going to be spread and then be eaten, then it’s not beneficial. But since the slime molds don’t seem to eat it all, the bacteria will persist for long periods of time. So it could be a mutualism that’s benefiting both slime molds and the bacteria.

TS: This amoeba species is well known for its social behavior. How does this finding change what we know about its social life?

DR: If you think about other species that farm, they’re often highly social. For example, the leaf-cutter ants have some of the largest societies of any species. When humans developed agriculture, societies took off, grew, and were able to be more cooperative and more social. So there could be an evolutionary link between farming, agriculture, and complex social societies. That sort of makes a prediction that in invertebrates that are cooperative and live in societies, we should go look for evidence of farming.

How do slime molds farm?

Dustin Rubenstein, an evolutionary ecologist at Columbia University, chats about the discovery of social amoebas that farm.

F1000 member Dustin Rubenstein studies the evolution of sociality in African starlings and Caribbean sponge-dwelling snapping shrimp. You can access his review of the paper here.


Avatar of: Rivkah Rubinstein

Rivkah Rubinstein

Posts: 11

May 17, 2011

Often the evolutionary narrative relies on establishing a purposeful consciousness that drives the need for change, which in turn gives a reason for sustaining the species, or for adaptation, mutation, etc. TV-naturalists can be positively giddy when presenting behaviors of some species, especially when they appear to be taught, or when animals readily accept training. This is often presented as cognition on behalf of the animal for change requirements. With invertebrates, these types of arguments become muted, since they do not correspond to the common understanding of brain size and functionality in the majority of invertebrates. Rather the argument becomes one of a group consciousness, typically guided by chemical stimuli, etc. Some natural wonders, such as the multi-generational migration of monarch butterflies across vast continental expanses, fail to attract any evolution commentary. So it is truly surprising to witness the same tired evolutionary rubric being applied to habits of one-celled animals. No argument; these creatures have many complex behaviors. Some appear to farm, some hunt, and some actively protect themselves by coating their outer membranes with tiny inorganic particles. However, far from supporting evolutionary stool, adaptive behaviors of single-celled organisms knock out its legs, tipping the whole decrepit theory into the cesspool of failed notions. The rigidity of modern investigative perspectives continues to sadden me. When will we be expected to submit our unbiased observations without the color commentary?

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