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Bountiful harvest. A queen ant presides over her workers as they tend their fungal garden.

Three's Company Down on the Ant Farm

One of nature's oddest partnerships exists between certain ants and the fungi they cultivate. Now, researchers have found a third player in this collaboration, a pathogen that infects the fungi. Moreover, the pathogen has been associated with the two for as long as the ants have tended their moldy crop. This is one of the first demonstrations of a long-term, three-way relationship.

Attine ants, which include leaf-cutters that can defoliate a tree in one night, can't digest plant matter themselves and instead retrieve it as fodder for the fungi. In return, the fungi supply the ants with nutrient-filled threads. But infection by a microfungus called Escovopsis can reduce the size of the “farm” and of the ant workforce. Sometimes it can even destroy an entire colony.

The ants and fungi have been together for 50 million years, maintaining their tight association even as new ant species arose. Cameron Currie of the University of Kansas, Lawrence, has now shown that over that same time, the pathogen has been as tightly entwined with the ant and the fungi as they have been with each other.

To trace the pathogen's history, Currie and his colleagues analyzed DNA from 17 strains. The resulting evolutionary tree pointed to a common ancestor that dated back to the days of the first cultivation of fungi by ants, he reports in the 17 January issue of Science. Today, there are four lineages of the microfungus, and each is associated with a particular ant-fungi system.

“It looks to me as if the pathogen was locked into the relationship” early on, notes Daniel Janzen, an evolutionary biologist at the University of Pennsylvania in Philadelphia. All in all, Janzen adds, “it's a nice, clean example” of coevolution.

Related sites
Cameron Currie's Science paper
General info on the fungus-growing ants
An evolution Web site that features Currie