Fall 2009


As they slice perfect circles from leaves to carry back to their nests, leaf-cutter ants practice one of the oldest forms of agriculture on the planet.

IT’S AN AGE-OLD STORY. A young woman sets out on her own. She leaves behind the only place she’s ever known, a city crawling with millions of citizens engaged in the activity of making a metropolis run. Workers build new homes and roads. Farmers plant, weed and harvest their gardens. Others are busy cleaning the streets or guarding important landmarks or deploying to fight disease.

Our heroine seeks a life less crowded. A place to call her own. So she heads somewhere new. Takes a flight. Her only carry-on a little memento from home.

When she settles down, though, her story veers from the familiar script.

Reaching her final destination, she digs a tunnel into the earth, removes the piece of home from a pouch in her cheek and tears the wings from her body. The memento is actually a fungus. Those wings are its first meal. And that piece of home grows and grows under her care—the key to sustaining the new colony that will soon spring from the eggs she lays.

Every rainy season in Central and South America, this story unfolds a million times over. It is the saga of the leaf-cutter ant, and it may one day influence everything from how we produce fuel to how we fight disease to how we think about evolution.

But first, it just might make us reconsider what it means to be human.

Ants in Currie’s lab navigate their plastic environment just as they would in the wild, carrying leaves back to a vast spongy garden of fungus, which supplies the colony with its primary source of food.

Thirty years ago, noted physician and essayist Lewis Thomas observed that it is a generally accepted metaphor that humans, when viewed from a great height, resemble ants at work. However, Thomas continued, it’s frowned upon to peer through the magnifying glass and make the opposite analogy. Which is odd, he wrote in his book The Lives of a Cell: Notes from a Biology Watcher, considering that “ants are so much like human beings as to be an embarrassment.”

We regard our ancestors’ transition from hunting and gathering to agriculture as the accomplishment of human industry, one that allowed us to thrive upon this earth. But ants have been growing their own food for 50 million years, which is about 50 million years longer than we have.

Think the discovery of antibiotics was only made possible because of our big brains? For millennia, ants have used some of the same bacteria we “discovered” to create disease-fighting antibiotics. What’s more, they’ve managed to avoid the problem of antibiotic resistance that plagues our medical field.

Of course, the human-ant comparison falls apart at the individual level. A lone ant has little in the way of self-awareness or cognitive thought. The ant harbors no brain, just a few thousand neurons strung together. But once that ant joins the swarm teeming about a nest, it becomes a model for societal organization and technological accomplishment. Then, Thomas writes, “you begin to see the whole beast. And you now observe it thinking, planning, calculating. It is an intelligence, a kind of live computer, with crawling bits for its wits.”

From the fourth floor of UW-Madison’s Microbial Sciences Building, associate professor of bacteriology Cameron Currie is trying to crack the code of this “live computer.” An entomologist by training, he has since ventured into realms as diverse as microbial ecology, evolutionary biology and genomics to help him sort out the myriad interactions that help fungus-growing ants thrive. His work has led to grants from places like the U.S. Department of Energy and the pharmaceutical giant Roche. He’s also garnered several awards, the most recent (and prestigious) being the Presidential Early Career Award for Scientists and Engineers, which he’ll have handed to him by President Barack Obama this November.

It’s an enviable career arc, but it didn’t begin with the desire to study ants. Currie was first a student of ecology, finding inspiration in Darwin’s description of the “tangled bank”—an attempt, Currie says, to explain how “birds and worms and insects all interact and (how) these complex communities shape each others’ evolution and are shaped by evolution.”

Like most young students, Currie was first interested in applying that theory to more charismatic fare. “I started (out) being more interested in bigger animal interactions,” he says, “like in the Serengeti—lions, wildebeest, stuff like that. But it’s hard to work on lions and wildebeest, right? You can’t collect them, you can’t contain them in the lab, you can’t sample them.” So Currie looked to smaller organisms that could be raised and studied in the lab. Soon he realized that many insects lived in close association with microbes around them, and that these relationships drove the evolution and ecology of the whole system. He’d found his bank—still tangled, just smaller.

Currie’s particular interest in tropical ants was lit during graduate studies at the University of Toronto. His advisor, mycologist David Malloch, was in Costa Rica for a workshop on fungal diversity when Currie called to get ideas for a dissertation topic. Malloch, intrigued by what he was learning at the conference about leaf-cutting ants and their fungal gardens, suggested that he study ants.

MORE THAN 200 known species of ants grow fungus as their primary food source. The leaf-cutter ant is the most highly derived from an evolutionary perspective. In layman’s terms, that means it is by far the coolest. Leaf-cutters have the biggest nests, largest populations and the most complex societal organization.

After a queen makes her nuptial flight, digs a tunnel and lays eggs, she focuses solely on tending to that tiny piece of fungus she snipped from the old nest. Soon, new ants hatch into the colony all with pre-assigned roles. Some grow into minims. The smallest members of the colony, minims tend to the fungus and weed out any dead or diseased material. Other eggs produce foragers, which venture into the forest and bring back morsels of leaves, arranging them like scaffolding in football-sized chambers. The fungus then grows onto the fragments and consumes them for food. Guarding the whole enterprise are the soldiers. Aside from the queen, soldiers are the largest members of the colony, big enough to literally straddle the columns of foragers as they return to the nest with leaves. The soldiers stand alert, ready to turn back any threat with fierce mandibles that can easily cut through human skin.

All of them—minims, foragers and soldiers—are females. Males appear only once or twice a year, when the queen lays eggs that grow into reproductive males and females. When mature, these winged ants fly from the nest in a great swarm. The males, as is the common fate of many in the insect world, mate during the flight and die soon after. The females collect sperm from several different males, which fertilize the eggs that will start their new colonies.

Ants in Currie’s lab navigate their plastic environment just as they would in the wild, carrying leaves back to a vast spongy garden of fungus, which supplies the colony with its primary source of food.

The whole operation takes on a scale vastly outsized for the ants’ tiny bodies. Their nests thrive for as long as the queen lives (up to 15 years) and can be home to more than 5 million ants. If you were to dig the entire thing out of the ground, the resulting hole would be bigger than your living room. In the tropics, leaf-cutter ants consume up to 20 percent of the fresh vegetation. A single nest is the equivalent of a cow settling in and eating non-stop.

Scientists have spent decades studying fungus-growing ants, mostly in an attempt to understand their behavior and the structure of their societies. They have discovered that each species tends to its own distinct species of fungus, which cannot survive outside of the ants’ nests. Scientists also know that the ants play an integral role in cycling nutrients through the rainforest. But there is almost nothing known about how any of this came to be.

“The mystery I’d like most to get to the bottom of is how … this complex suite of behaviors was assembled over evolutionary time,” says Ted Schultz, an entomologist with the Smithsonian Museum of Natural History. “How did it begin?”

But scientists have only recently stumbled upon the complex inner workings that surround the ants’ society. When Currie began studying leaf cutters in the late 1990s, the system was considered a fairly straightforward model of symbiosis, where the ants nourish the fungi that in turn nourishes them. But one facet of the system bothered Currie. No one had ever spotted disease in the fungus gardens. Currie knew that any human crop grown as a monoculture for generations would battle wave after wave of opportunistic parasites and diseases. Surely these fungus gardens weren’t exempt.

Currie approached the system from a new perspective: He got out the microscope. Looking closer, he found that disease was indeed present, but it wasn’t an assortment of attackers. The fungus was battling a single species of parasitic mold. It turns out that this mold is not some new invader, but an organism that has been part of the system for millions of years.

“What I showed is that, when the ants are there, they’re suppressing the disease,” says Currie, “but the disease is having an impact and it’s performing a persistent infection. So it’s not like ‘Ant there, clean. Ants gone, overgrown.’ It’s a continuous kind of interplay between the players.”

But that’s not all the microscope revealed. Currie noticed the ants had a waxy, white residue on their carapaces, which he discovered were bacteria that produced antibiotics that helped suppress the parasitic mold. The four organisms—ant, fungus, parasite and bacterium—were like players in a long-running drama, where each had something to gain by cooperating and everything to lose by taking advantage of the system.

Further studies have added more members to the cast. Currie and others have discovered yeast that live on the ants and feed the antibiotic-producing bacteria, as well as a bacterium in the fungus garden that fixes nitrogen and fertilizes the fungus. As Rebeccah Steffensen BS’08 one of Currie’s lab managers puts it, “When most people think about symbiosis, they only think about two things. We think that if you keep looking you’ll find more and more interactions. So it’s more like this network of (different) symbioses.”

In a long, windowless room in the back of the Currie lab, Steffensen pops the lid off one of dozens of Tupperware bins that house colonies of fungus-growing ants. There is a sweet, surprisingly earthy smell. Inside are several clear plastic cubes with holes cut in their corners, a leaf-cutter colony laid bare, doing everything their wild counterparts would. Foragers travel in and out, harvesting food—typically oak and maple leaves with a steady supply of ground corn—from an adjacent bin. Minims weed the fungi that grow on a latticework of decaying plants. A separate cube, set off from the others, serves as the ants’ compost pile, where they take dead chunks of fungus and other refuse. Peering into the cubes, it becomes apparent that the fungus is more than a food source—it is the world the ants inhabit. The white, spongy mat is crisscrossed with well-traveled tunnels. Small chambers deep inside house the ants’ brood, larvae tended and fed by minims until they mature and assume adult roles. The ants build and the fungus grows until it completely fills each chamber.

A wince crosses Cameron Currie’s face as a soldier ant bites into his thumbnail.

Despite being removed from their habitat, fed a foreign diet and confined in unnatural environs, the ants carry on without hiccup. Put a handful of leaf cutters in an empty bin with a small piece of fungus and a few leaves and you’ll soon see their whole natural structure duplicated. It’s just what they do. “Imagine if you took 17 carpenters, 14 masons and 13 electricians, and you put them all together without a foreman or a blueprint. Do you think they could build a house?” says Garrett Suen, a postdoctoral researcher in the lab. “Ants can do this.”

The mechanism that enables ants to organize so productively is called swarm intelligence, and Suen got interested in it while working on his master’s degree in computer science. He was dumbfounded, he says, by the fact that, even with millions of individuals moving through narrow tunnels and all heading to the same place, ants don’t get into traffic jams.

“But look at us,” Suen says. “Think about even in Madison, when construction cuts down one lane on University Avenue. What happens? It takes 10 times longer to get anywhere. Ants have clearly figured out a way (around) this. And that is clearly an intelligence we call ‘lower’ intelligence, but really from a computer science perspective, we have not unlocked that secret.”

Swarm intelligence can be striking in its effectiveness. For example, when researchers lace maple leaves with a fungicidal agent, the ants soon discover those leaves are killing their fungus, and an announcement somehow goes out to the entire colony. After that, not a single ant brings a maple leaf back to the nest. No one knows how an individual ant identifies threats to the fungus. Or it could be the fungus alerting the ant. It’s still unclear how the rest of the colony somehow receives the news. Even though it is operating in plain sight, caught in the Tupperware bin, it is a mystery how such a complex system keeps humming along.

The answer may be found, Suen says, via genomics. The Currie lab was recently awarded a grant from Roche pharmaceuticals for 10 gigabases of genome sequencing. That’s the equivalent to the complete genetic sequences of three humans. The award is not monetary—it’s a service. Currie’s lab isolates the DNA and sends it to Roche, where scientists sequence the genome. The result will be a genetic picture of the entire ecosystem.

Once they have the genome decoded for each organism in the system, Currie’s lab will be able to use something called microarray technology to see what genes get turned off or on under certain conditions. They can then start tweaking the conditions to see how the ant, fungus, parasite and bacteria all respond to one another. For example, in the experiments where ants identify and avoid tainted leaves, scientists could look at what genes were activated or shut down in the fungus when it encounters the leaf. Then they could do the same for the ant and the parasite to look for similar responses, which will eventually lead to better understanding of what chemicals and stimuli are enabling them to all interact.

“What’s really different about this project is that so far genome sequencing has been about understanding the biology of a particular organism,” says Nicole Gerardo, an assistant professor of biology at Emory University and one of the co-investigators on the Roche grant. “And what we’re asking is can you use genome sequencing to understand the associations between an entire group of organisms.” She says taking a wider view will allow scientists to get at the inner workings of systems “we used to see as just cool natural history stories.”

The hidden mechanism that most intrigues Gerardo is the ants’ use of antibiotics to battle mold in their gardens. She already knows that microbes in the system can recognize each other, which means that there’s some kind of chemical communication going on. Unlocking that communication on a genetic level could teach scientists more about how pathogens sense and respond to antibiotics, information that could help improve antibiotic drugs for human use.

The ants’ strong mandibles aren’t their only line of defense. Currie was among the first scientists to notice that the white spots that cover many leave cutter ants were antibiotic-producing bacteria that the ants harbored to help keep their gardens disease-free.

But the fascinating aspect of Currie’s lab is that no one of these potential applications holds center stage. His team is a melting pot of academic disciplines, where behavioral ecologists work alongside bacteriologists and bioinformaticists. This interdisciplinarity has led to collaborations with units such as the Great Lakes Bioenergy Research Center, which is funding part of Currie’s work in hopes of learning how the ants’ gardens break down and process the cellulose in plant leaves.

The emergence of these avenues of research are a function, Currie says, of the changes taking place in the way we study life. Science has moved from an era of field work, carried out by naturalists such as Darwin traipsing around in tangled banks, to lab work, where organisms are grown and observed in controlled conditions. Both methods have their drawbacks: Field work was hard to control, but lab work imposed a kind of false reality that didn’t capture what was really happening in the environment. Genomics is opening the door to a new kind of science, where you can just bring the whole natural system indoors, preserving the diverse interactions of nature while providing the sophisticated observational tools of the lab.

And that brings an interesting twist in the saga of the leaf-cutter ant. Consider our heroic queen. It is a half year later. Her colony is up and running, boasting a few hundred workers and a fungus garden the size of a baseball. Suddenly, the wall of the chamber gives way. A pair of gloved hands reaches in and gingerly transfers the queen and her garden into a container. Then the queen takes another flight, this time tucked into the luggage of an overhead bin.

When she lands, she finds herself in a Wisconsin lab, unknowingly helping humans uncover the secrets of how to become one of the most successful animals in the world.

This article was posted in Fall 2009, Features, Food Systems and tagged , , , .

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