Fungus-farming ants cultivate multiple species of fungi for food, but the history of fungus-ant co-evolution is poorly known. In a new study published this week in the journal Science, researchers found that fungus-ant agriculture originated approximately 66 million years ago when the end-of-Cretaceous asteroid impact temporarily interrupted photosynthesis, causing global mass extinctions but favoring the proliferation of fungi.
Nearly 250 different species of ants in the Americas and Caribbean farm fungi.
Entomologists organize these ants into four agricultural systems based on their cultivation strategies.
Leafcutter ants are among those that practice the most advanced strategy, known as higher agriculture.
These ants harvest bits of fresh vegetation to provide sustenance for their fungi, which in turn grow food for the ants called gongylidia.
This food helps fuel complex colonies of leaf cutter ants that can number in the millions.
“Ants have been practicing agriculture and fungus farming for much longer than humans have existed,” said Smithsonian Institution entomologist Ted Schultz.
“We could probably learn something from the agricultural success of these ants over the past 66 million years.”
“We believe decaying leaf litter likely became the food of many of the fungi that grew during this period, which brought them in close contact with ants.”
“The ants in turn began to use the fungi for food and have continued to rely on and domesticate this food source since the extinction event.”
“To really detect patterns and reconstruct how this association has evolved through time, you need lots of samples of ants and their fungal cultivars.”
In the research, Dr. Schultz and colleagues analyzed genetic data from 475 species of fungi and 276 species of ants to craft detailed evolutionary trees.
This allowed the researchers to pinpoint when ants began cultivating fungi millions of years ago, a behavior that some ant species still exhibit today.
“Historical ideas about fungal farming by ants generally assumed there was a single origin of fungal cultivation, but what was hampering deeper insight into questions regarding how ants began farming was trying to capture sufficient DNA sequence data from the fungi that ants consumed,”
“During the past 15 years, the cost of genome sequencing has plummeted, and the techniques for collecting many types of genomic data have significantly improved — enabling this and many other studies.”
“If you have a mushroom, it’s relatively straightforward to sequence its genome.”
“But when you have teeny-tiny fragments of a fungus that an ant is carrying inside of it, it’s hard to get enough fungal material to generate sufficient genome sequence data to analyze. That’s where the fungal bait sets we created come in.”
“They allowed us to pull the DNA from miniscule bits of fungi, amplify it, sequence it and analyze it.”
The data revealed that ants and fungi have been intertwined for approximately 66 million years. This is around the time when an asteroid struck Earth at the end of the Cretaceous period.
This cataclysmic collision filled the atmosphere with dust and debris, which blocked out the Sun and prevented photosynthesis for years.
The resulting mass extinction wiped out roughly half of all plant species on Earth at the time.
However, this catastrophe was a boon for fungi. These organisms proliferated as they consumed the plentiful dead plant material littering the ground.
“Extinction events can be huge disasters for most organisms, but it can actually be positive for others,” Dr. Schultz said.
“At the end of Cretaceous, dinosaurs did not do very well, but fungi experienced a heyday.”
The study also revealed that it took nearly another 40 million years for ants to then develop higher agriculture.
The authors were able to trace the origin of this advanced practice back to around 27 million years ago.
At this time, a rapidly cooling climate transformed environments around the globe.
In South America, drier habitats like woody savannas and grasslands fractured large swaths of wet, tropical forests.
When ants took fungi out of the wet forests and into drier areas, they isolated the fungi from their wild ancestral populations.
The isolated fungi became completely reliant on ants to survive in the arid conditions, setting the course for the higher agriculture system practiced by leafcutter ants today.
“The ants domesticated these fungi in the same way that humans domesticated crops,” Dr. Schultz said.
“What’s extraordinary is now we can date when the higher ants originally cultivated the higher fungi.”