ENHS Nature Trails May 99 Article 2

Natural History and You - The President's Forum
by Nathan Tublitz

Biological true love: A satisfying menage à trois

No, this is not a review about a post-modernism French novella nor is it about a new NC-17 rated movie at the local Cinemark 324-screen theatre. It's about some very unusual yet wonderous goings-on in the biological world. A natural Ripley's-believe-it-or-not tale. So hold on to your hats and fasten your seatbelts, ladies and gentlemen, as you are in for a wild ride into the "Outer Limits" of biology.

This story starts simply with the ever-so-resourceful and humble little ant. Ants are one of the major biological success stories on this planet (for more information, read the collected works of E.O. Wilson or if you don't have 300+ spare hours, see the films A Bug's Life and Antz). With the exception of Antarctica, ants have colonized every continent and sub-arctic ecosystem in the world. Ants are colonial, highly social, have different castes for different tasks (queen, guard, etc), use a frighteningly wide variety of ingenious chemical defenses to protect themselves from predators, build elaborate nests, and are possessed with an amazing ability to navigate long distances, sometimes in nearly clueless environments (like the flat sandy stretches of the Sahara Desert). But this is all secondary to today's factlet: that some ants have developed a strange and fascinating symbiotic relationship with other species.

The stars of today's column are the taxa of ants known as the Attini, 210 species found mostly in the neotropics of South America. Perhaps the best known representatives of the Attini are the leaf cutter ants, known for their elaborate range of behaviors involving the cutting of living plant tissues. The primary herbivores of the Amazon basin, leaf-cutter ants have an unceasing need to eat. As adults they chow down on tons of leaves daily - estimations suggest that a single colony of leaf cutters eat as much in one day as an adult dairy cow.

The larvae of these ants also have a voracious appetite, however in their case their primary food source is a fungus, Attamyces bromatificus, which is cultivated inside the nest by their adult nest mates. Leaves are brought to the nest by forager ants and once inside, the leaves are mashed up and sprayed with faecal material by a special caste of leaf cutters. This leaf "mash" is used as a "soil" to grow the Attamyces fungus, which is only found in the "fungal gardens" of leaf cutter ant nests. A different subset of worker ants dedicate themselves to tending these fungal gardens; they control humidity, pH, and temperature in order to harvest specific parts of the fungus called gongylidia, which are fed to the ever-hungry ant larvae. Undesirable invading fungi are weeded out by yet another group of workers in order to maintain a monoculture of Attamyces.

As any farmer knows, a major drawback of growing a monoculture is the worry that the crop could be destroyed by a single predator. The leaf cutters have solved this problem in a unique way by recruiting a third species to "defend" their fungal gardens. The final partner in this unusual menage à trois has no legs and no mouth but is quite effective; it is a bacterium belonging to the genus Streptomyces. This bacteria hitch-hikes into the colony on the bodies of the ants and once inside lives on the fungus where it secretes an antibiotic that prevents a parasitic fungus, Escovopsis, from taking over and killing the Attomyces in the fungal garden. The ants, the Attomyces fungus, and the Streptomyces bacteria live together in a thriving, mutualistic community where each depend on the combined efforts of the others for survival. Evolutionarily speaking, this system appears to be the first example of the use of antibiotics for biological control, predating the use of antibiotics by humans by about 50 million years. And we think we're so smart.

This fascinating and fantastic system of co-evolution between three species from different kingdoms is probably one of many found in nature. Complex biological interactions such as these potentially holds the key to understanding the nature and control of many diseases including those affecting humans. Most of these complex biological communities are found in the tropics which has the greatest biodiversity on our planet. Unfortunately understanding these communities and utilizing the knowledge from these discoveries will not occur if we continue to destroy the rainforests at the current rate. C'est dommage.

Nathan Tublitz
Professor of Biology
Institute of Neuroscience
University of Oregon
Eugene OR 97403
Phone: 1-541-346-4510 FAX: 1-541-346-4548

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