Ants that tend and harvest gardens of fungus have been closely studied for dozens of years, and are often cited as a model system for symbiosis. However, two other key players–an antibiotic-producing bacteria and a garden parasite, which have likely been co-evolving with the ants and the fungus for tens of millions of years–have been identified in recent years by Cameron Currie, a newly hired professor of bacteriology in the College of Agricultural and Life Sciences.

“The larger implications of this work are an applied understanding of how hosts and pathogens evolve,” explains Currie, who came to the University of Wisconsin-Madison from the University of Kansas. “Among other things, we might be able to use this particular model to learn more about antibiotic resistance, and possibly develop new natural products.”

About 210 species of ants-found from the southern United States to South America-are known to tend gardens of fungus. This well-documented phenomenon is a classic example of mutualism, or a mutually beneficial symbiosis, says Currie: the ants get nutrients from the fungus, and the fungus is cultivated, spread and protected by the ants. The two organisms have likely been co-evolving for 50 to 65 million years, fine-tuning their delicate relationship.

Currie says he became interested in the ant-fungus partnership because of a curious footnote in the story: although fungus reproduces asexually-which usually makes an organism a target for parasites-the ants” gardens seemed to be free of disease. His investigation revealed that the gardens are actually frequently infected by a specialized fungal parasite, but that the ants fight to protect their crop, both by physically weeding and removing the parasite and by culturing bacteria, on their own bodies, that produce antibiotics.

Currie describes parasitism as a potential arms race-as hosts and their parasites co-evolve, each must continually evolve to resist or attack the other. In this case the ants, fungus and antibiotic-producing bacteria are all aligned in opposition to the parasite, which is trying to overcome the defense mechanisms set up to protect the fungus.

“This is a 65-million year old system that spans biogeographical realms,” Currie explains. “It can be a useful model in examining some human disease issues, including the evolution of antibiotic resistance, and it shows us how mutualism and symbiosis shape species diversity.”

Currie conducts ongoing field research on several of the species including a group called the leaf-cutters, which live in Panama and other countries. He notes that a typical colony might have five million workers, and consist of hundreds of chambers. “It”s a massive underground city of ant and fungus,” he says, “and the soldier ants can bite through shoe leather!”

Currie adds that he hopes to explore natural products based on his work. “The UW is very supportive of this, and it”s one of the reasons I came here,” he says.