20 January 2003
Ants The Oldest Farmers
by Kate Melville
Fungus-growing ants practice agriculture and have been doing so for the past 50 million years according to research published in the Jan. 17 issue of Science. These ants not only grow fungus gardens underground for food but also have adapted to handling parasitic "weeds" that infect their crops.
The team of scientists who collaborated on this analysis includes Ted Schultz of the Smithsonian's National Museum of Natural History, Bess Wong of the Smithsonian Tropical Research Institute, Cameron Currie and Alison Stuart of the University of Kansas, Stephen Rehner of the U.S. Department of Agriculture, Ulrich Mueller of the University of Texas at Austin, Gi-Ho Sung and Joseph Spatafora of Oregon State University, and Neil Strauss of the University of Toronto.
"The ants, garden fungi, and weeds have all been co-evolving since ant agriculture first got started - that's around 50 million years of symbiosis," said Dr. Ted Schultz, research entomologist in the Entomology Section of the Smithsonian's National Museum of Natural History.
By studying DNA sequences from ants, garden fungi and fungal weeds, the research team was able to peer millions of years into the past to see how this co-evolutionary system evolved. The researchers learned that the ants, their garden fungi and the parasitic fungal weeds have been living in a co-evolved, complex system for a very long time, probably 50 million years or longer. During that time, they have been locked in a never-ending evolutionary "arms race," in which the ants and garden fungi are perpetually evolving new ways to control the parasitic fungal weeds, and the weeds are perpetually developing new ways to continue to infect fungus gardens.
There is a fourth factor in the ant colonies, a kind of bacteria that the ants cultivate on the outsides of their bodies. These bacteria produce an antibiotic that specifically suppresses the growth of the weed fungi, and the ants use this antibiotic to keep their gardens healthy.
"We suspect that it's going to turn out that this antibiotic use also goes back to the beginning of ant agriculture," said Schultz.
Past work by researchers established phylogenies (evolutionary histories) for the ants and their cultivated fungi, and it also established that the ant gardens almost always contain weed molds in the genus "Escovopsis," which are found nowhere else in nature - only in ant gardens. One of the new findings in this research paper is that the scientists now have a phylogeny for the weed fungi, an association that appears to be very ancient.
The collaboration that produced this work is supported by a five-year National Science Foundation special program (Integrated Research Challenges in Environmental Biology) grant, and the Smithsonian Institution is the designated permanent repository for all of the project's ant, fungal, and bacterial specimens, preserved both for morphological and molecular study. The museum has set up an archival liquid N2 DNA repository for the molecular collections generated by the study.
"The Smithsonian's repository of ant, fungal and bacterial specimens is an extremely important resource because most of these organisms have never been collected before," said Schultz. "Ant agriculture has become a model system that will be studied for decades or even centuries into the future - and the Smithsonian's morphological and DNA specimen collections will be the source of the data for many of those studies."