Social amoebae travel with a posse, have amazingly complicated social lives

In 2011, Nature announced that scientists had discovered a single-celled organism that is a primitive farmer. The organism, a social amoeba called Dictyostelium discoideum, picks up edible bacteria, carries them to new locations and harvests them like crops.

D. discoideum enjoyed a brief spell in the media spotlight, billed as the world's smallest farmer. Now a collaboration of scientists at Washington University in St. Louis and Harvard University has taken a closer look at one lineage, or clone, of a D. discoideum farmer.

This farmer carries not one but two strains of bacteria. One strain is the "seed corn" for a crop of edible bacteria, and the other strain is a weapon that produces defensive chemicals.

The edible bacteria, the scientists found, evolved from the toxic one. 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 increases the expression of some genes and decreases the expression of others.

A mutation that affects this much of a genome could be lethal, but in this case it had the surprising effect of making the bacterium edible by changing its chemical profile. Scientists sent the two bacteria out to be identified genetically and both came back as Pseudomonas fluorescens: the same species, even though they were morphologically so different.