Some of the world’s greatest gardeners are mere millimeters long. The leafcutter ants of the Americas, for example, slice off chunks of leaves, haul them back to their nests, and feed the fresh greens to fungi, which the ants carefully tend in special climate-controlled chambers within their colonies. Just as humans can’t eat the hay we feed livestock, the ants can’t eat the leaves—only the fungi that flourish on them.
Hundreds of ant species farm fungi today, and studies of ant evolution suggest the adaptation goes back tens of millions of years. Now, scientists have sharpened the picture by bringing in the fungal family tree as well. Today in Science, they pinpoint a date for the origins of the partnership and suggest a surprising catalyst: the asteroid that killed the dinosaurs 66 million years ago.
This “really impressive” project builds on decades of fieldwork and genomic data, says Peter Biedermann, an entomologist and evolutionary biologist at the University of Freiburg who was not involved in the study.
Since ants’ fungal gardens were first described 150 years ago, entomologists have uncovered 247 species of ants that tend them and rely on this fungal crop to survive. Researchers surmise that the ants descend from a common ancestor that later evolved into separate species nurturing different types of fungi. But fungal family trees, or phylogenies, are less precise and complete than those for ants, which Ted Schultz, a research entomologist at the Smithsonian Institution, calls “one of the most glaring problems in the ant-fungus agriculture world.” The result has been an incomplete picture of how and when ants and fungi became bedfellows.
One challenge is that researchers have lacked fungi-specific DNA probes to examine fungal genomes and decipher their relationships. The research has also moved slowly because scientists lacked enough samples of the free-living relatives of ant-cultivated fungi, Schultz says.
Now, he and his colleagues have turned to recent advances in fungal genetic analysis to mine the genomes of 475 species of fungus—most, but not all, cultivated by ants—and trace the roots of their tangled family tree. The team also created a complementary phylogeny of 276 ant species. In total, these species represent more than 30 years of field collections, Schultz says. The team calibrated the phylogenies with ant and fungus fossil records to establish the age of each branch.
With both dated trees in hand, the team drew connections between known ant-fungus pairs and made a timeline of key events in their evolution. To Schultz’s surprise, matching dates popped up in both family trees.
The ant and fungal taxa involved in farming both arose about 66 million years ago, which coincides with the massive asteroid strike that drove nonavian dinosaurs and many other species extinct. That cataclysmic impact produced lingering clouds of debris that shut down photosynthesis across the planet for several months, possibly even years. It was a catastrophe for most organisms, including plants and the animals that eat them—but not all. “Fungi that decompose plant material had a heyday,” Schultz says.
The researchers suggest ants that had already developed a loose relationship with fungi were ready to take advantage of this newly abundant source of food. “It makes sense” that farming evolved when plants were scarce but fungus was abundant,
Biedermann says. “Full-blown agriculture arose presumably fairly rapidly,” Schultz says.
Indeed, the fungal phylogeny suggests the innovation arose not once, but twice in the wake of the catastrophe. With two origins in fungi, the mutualistic relationship “is more complex than we thought,” says Gabriela Camacho, a taxonomist and systematist at the Museum of Zoology of the University of São Paulo who was not involved in the study.
For the first few million years, the ants tended fungal species also found in the wild. Then, about 27 million years ago, a subset of ants completely domesticated their fungal cultivars, just as humans have done with most of our staples, which are now remote from their wild roots. The fully domesticated fungi include Leucoagaricus gongylophorus, the species most leafcutter ants favor for its specialized, highly nutritious fruiting bodies.
The start of domestication corresponds with a period of global cooling, when droughts in South America created large swaths of arid grasslands across a previously wet, forested landscape. As fungus-farming ants adapted to the drier conditions, they likely carried their fungi with them into new habitats. “This resulted in the fungi losing contact with their gene pools and triggered domestication, basically total dependence on their ant farmers,” Schultz says.
The team can’t prove the asteroid impact originally sparked this mutualistic relationship. But the fact that the same “beautiful pattern” emerges from two distinct sets of data adds confidence to this origin scenario, Camacho says. The narrative links the evolution of both groups to the same periods of ecological upheaval, whose impact we can still see today, she says. “It’s a great story.”