A UW-Madison researcher has discovered a startling relationship between a breed of ants and the fungus from which the ants derive nutrients. The ants produce chemicals that protect the fungus from bacterial parasites. As the bacteria evolve to survive the pesticides, the ants evolve new pesticides against the bacteria, in a 50-million-year-old chemical arms race that could one day help humans design more precise antibiotics.
UW-Madison microbiologist Cameron Currie studies leaf-cutter ants and their fungus parasites as leading examples of mutual evolutionary adaptation.
The leaf-cutter ant, native to Central America, is one of over 200 species of fungus-farming ants and exists only in North and South America. This species carries leaf fragments to an underground nest, a five-meter-deep network that looks from the surface like a pile of tilled earth, almost like a baseball pitching mound. In the nest, worker ants \farm"" or digest the leaves into a fungus garden. The fungus yields nutrients for the colony and relies on the ants to protect it and help it grow.
But fungus is prone to disease since it reproduces asexually and thus introduces no new genetic information from generation to generation. If the fungus has any genetic weaknesses, those weaknesses would be reproduced continuously, leaving the fungus vulnerable to parasites that could take advantage of those weaknesses. So the farmer ants must continually produce new pesticides to protect their food sources from these opportunistic parasites.
""This is about the evolutionary arms race,"" Currie said. The host develops a defense and the invader evolves an improved ability to overcome that defense; the two continue to hone their developments to the point of consistent adaptation and counter-adaptation.
While examples of symbiotic relationships abound, the degree to which the ants and fungus have united against the bacteria is especially remarkable.
""Typically, we don't [find] quite the same relationship,"" said Richard Lindroth, a UW-Madison professor of entomology. ""It's generally not as tightly coupled. In [the] case [of Currie's research], they're closely related.""
Over the past few decades, researchers tracked molecular changes in ant species and have recently run DNA analysis on fungus and parasites. The results show a tight line of co-evolution-imagine an evolutionary tree that starts with one ant species, then branches into over 210 species, each associated with its own genetically-unique fungus farm, over a span of 50 million or more years.
The fungus can still survive without the ants, but not as well. Currie's research shows fungal growth and colony production slow down without the ants' specialized pesticide, but the fungus doesn't die off completely.
""If the parasite becomes too virulent and kills the host, that's bad for the host as well as the virulent strain,"" said Robert Jeanne, UW professor of entomology, noting the parasite would have destroyed its own food source.
When the ants are present, though, the parasites seem to have little success.
""Ants are so good at agriculture that they maintain their garden free from disease,"" Currie said.
Scientists hope these studies will lead to the design of more intelligent pesticides and antibiotics.
