Scientists say the discovery is much more than a mere curiosity, because the colony consists of what are known as social amoebas. Only an apparent oxymoron, social amoebas are able to gather in organized groups and behave cooperatively, some even committing suicide to help fellow amoebas reproduce. The discovery of such a huge colony of genetically identical amoebas provides insight into how such cooperation and sociality might have evolved and may help to explain why microbes are being found to show social behaviors more often than was expected.

Image An amoeba growing on a fish tank in California became a giant among microbes, reaching more than an inch across. Credit... Manfred Schliwa

“It is of significant scientific interest,” said Kevin Foster, an evolutionary biologist at Harvard University who was not involved with the study. Though amoebas would seem unlikely to coordinate interactions with one another over much more than microscopic distances, the discovery of such a massive clonal colony, he said, “raises the possibility that cells might evolve to organize on much larger spatial scales.”

Thoughts of a giant organized amoeba colony can conjure up visions of the 1958 horror classic “The Blob,” but these social amoebas, a species known as Dictyostelium discoideum, a kind of slime mold, are infinitely more subtle. Microscopic and tucked away in the dirt, the billions of amoebas would have gone unnoticed by anyone driving past the pasture. Joan Strassmann, an author on the paper along with another evolutionary biologist at Rice University, David Queller, said she and a team of undergraduates searched for the species by sticking drinking straws into dirt and cow dung to retrieve materials where the amoebas might be living. In the laboratory, they spread the samples on Petri dishes and waited to see what would grow. DNA analyses later showed that the huge numbers of amoebas collected from the pasture were genetically identical.

Bernard Crespi, an evolutionary biologist at Simon Fraser University in Canada, said the study was the first to clearly demonstrate “the extreme of relatedness” in social microbes, a population of genetically identical individuals. Such a colony provides the ideal conditions to foster the evolution of behaviors like cooperation, because the more genetically similar two organisms are, the more natural selection will favor their assisting each other.

Dictyostelium, for example, can carry out stunning feats of cooperation, engaging in what’s known as suicidal altruism, a behavior in which individual amoebas come together to form a single body, with some amoebas sacrificing themselves to allow for more effective reproduction of amoebas in other parts of the body.