Social Amoeba Farmer Uses Two Different Bacteria for 'Crop' Raising

An amoeba may seem like a primitive organism--boring and not worth our time. Yet scientists have discovered something interesting about this single-celled creature, revealing that they're a bit more complex than we may have once thought. The social amoeba Dictyostelium discoideum is actually a type of farmer, picking up edible bacteria and moving them to new locations before harvesting them like crops.

Farming in the animal kingdom isn't anything that's unusual. Leafcutter ants, for example, cultivate fungus for food by feeding the organism cut leaves. Damselfish also keep "lawns" of algae, carefully cropping and fertilizing their patch of food and encouraging species diversity. Yet it turns out that this kind of farming can also occur with more primitive species.

Scientists took a closer look at the world's smallest farmer in this latest study. It turns out that the amoeba carries not just one, but two strains of bacteria in its cultivating activities. One strain of bacteria acts as the "seed corn" for a crop of edible bacteria, and the other is a weapon that produces defensive chemicals.

The plot thickened as the researchers examined the bacteria a bit further. It turned out that the edible bacteria evolved from the toxic one. In fact, the two strains differ by many mutations but a single key mutation, which hit an important controller in the genome of the nonfood strain, alters expression of 10 percent of its genome. This alteration, in turn, increases the expression of some genes and decreases the expression of others.

So why were the non-edible bacteria unable to be eaten? It turns out that they produce two different chemicals: chromene and pyrrolnitrin. This in and of itself wouldn't be so exciting if it weren't for the fact that chromene is a new compound.

"We determined chromene increases spore production in the farmer strain and suppresses spore formation in the nonfarmer strain," said Joan Strassmann, one of the researchers, in a news release. "We saw the same increases in the farmer and decreases in the non-farmer with pyrrolnitrin. A known antibiotic and antifungal, pyrrolnitrin probably also suppresses other organisms in the soil that might compete with the farmer strain."

The absence of these chemicals, though, didn't make the food bacterium edible. Something else was going on. In the end, the scientists found that the expression change of 10 percent of the bacteria's genome was the root cause. By examining a bacteria family tree, they were also able to find out a few other things.

"The tree also tells us that edibility is a derived trait," said David Queller, one of the researchers, in a news release. "That's just a weird thing to evolve: to be able to be eaten."

The findings reveal a little bit more about the lifestyle of this amoeba farmer and show exactly how it cultivates these two different strains of bacteria. It shows that, contrary to popular belief, single-celled organisms can be far more complex than anyone imagined.

The findings are published in the journal Proceedings of the National Academy of Sciences.