By Mari N. Jensen

In the middle of southern Arizona rangeland, researchers gather at a brightly lit table intently staring at moths, slides and a notebook. One woman regularly says things like,

“Number 5, Hyles lineata, male.”

Lightning flashes in the distance, lighting up the night sky. A lone figure does an erratic dance behind a white screen draped with two purple lights. It’s just another Tuesday evening for the Davidowitz field crew.

Goggy Davidowitz and his team of researchers are part of a larger effort to investigate the relationship between sacred datura plants and the hawk moths that pollinate them. The datura-hawk moth association is one of the many cooperative interspecies interactions studied by ecology and evolutionary biology professor Judith Bronstein.

Such interactions, known as mutualisms, pose a puzzle for evolutionary theory, because one partner in a mutualism puts him or herself out a bit to help the other. At first glance, helping another species doesn’t look like the most efficient way to pass on one’s own genes.

However, such between-species mutual aid societies abound. A garden-variety one is that formed by flowers and their pollinators. Bees gather nectar and pollen, and the plant gets fertilized. Each party benefits.

Such relationships in nature have always intrigued people, Bronstein says. “Throughout human history, people have studied mutualisms. The Greeks used them as examples of cooperation,” she said, adding that many writers pointed to such interspecies interactions as natural models for how to run human society.

Although Darwin recognized the more selfish aspects of mutualism, it wasn’t until the 1970s that biologists began to consider the shady underside of such associations, she said. “Someone characterized mutualisms as reciprocal parasitism, and that’s what they are.”

But recasting mutualisms from partnerships based on cooperation and friendship to relationships between adversaries doesn’t explain why mutualisms are so common or so successful, Bronstein said. Mutualisms persist even in the face of cheating. She wants to tease apart what makes mutualisms tick. Ultimately, she wants to develop an overarching theory of mutualism that can predict the circumstances under which mutualisms will be stable and the circumstances under which they fall apart.

To do that, Bronstein looks at the costs the partners incur compared with the benefits they receive. Initially she studied several unusual plant-insect relationships, ones like the fig-wasp system in which the pollinators fertilize the plant but then eat some of the seeds, the plant’s offspring.

However, she realized that those unusual pollination systems, although classic ones for figuring out the costs of mutualisms, might not yield answers that could be applied to less-specialized interspecies associations.

In her search for better partnerships to study, she turned to one found in Southern Arizona, the association between sacred datura, Datura wrightii, and its major pollinator, the hawk moth Manduca sexta. Here again, the plant seemed to be getting a bad deal. The moth pollinates the plant while slurping up sweet nectar and then lays eggs on the plant. So in return for the moth’s pollination services, the plant is eaten down to a nub by the moth’s larvae.

Although there’s a clear advantage to the plant to be pollinated, Bronstein wants to know, “Why does the plant rely on a pollinator that inflicts such a huge cost on the plant?”

To answer that and other questions about the sacred datura-Manduca sexta mutualism, Bronstein is teaming up with two other members of the department of ecology and evolutionary biology, assistant research scientist Goggy Davidowitz and assistant professor Travis Huxman.

It’s an unusual approach to studying mutualisms, Davidowitz says. “Typically people have looked at mutualisms from the plant’s perspective or the insect’s perspective. What’s unique about this project is we have a plant physiologist looking at the plant perspective, an insect physiologist looking at the insect perspective and a mutualism expert who’s tying the plants and insects together to understand their interaction.”

To compare how each partner in a mutualism fares overall requires measuring the costs and benefits in a common currency. For interactions in human society, that’s easy: the currency is money. But evaluating the pollination services the plant receives against the leaves it loses to caterpillars is more complex. Bronstein says, “The reason the three of us are working together is we think we have a way to measure the costs in terms of insect and plant physiology by using energy budgets.”

With the help of a team that includes eight enthusiastic undergraduates, the multi-year research project, launched in 2004, is already unearthing the secret life of night-blooming daturas and their not-so-benign moth partners.

The results are surprising the researchers.

Plants capture energy in the form of sunlight through photosynthesis, but a defoliated datura plant can’t do that until it grows new leaves. Because growing new leaves takes energy and resources, the researchers wanted to know whether the datura plants have any reserves stored in their roots. The answer is yes. Some of the tuberous roots are so large that digging them up took four people two hours, said Kelly Mackay, one of the undergraduate members of the team. Having such a storage organ means the plant is socking away water, energy and nutrients in the plant version of a savings account.

“The plant has an enormous storage organ that we had never paid attention to,” said Bronstein. “But because it’s there, the plant can quickly replace those lost leaves.” This is one of the first pieces of the puzzle of this mutualism, she said. “Our work is starting to show that the cost to the plant is not as dramatic as we thought it would be.”

For another component of the research, Davidowitz’s crew goes to UA’s Santa Rita Experimental Range on Tuesday evenings to “black light” for moths at the same location where the other team members are studying a select group of datura plants. The black-lighting crew members erect a flat, white screen about seven feet tall and five feet wide, drape it with UV lights, illuminate it with mercury vapor lamps, and wait for the moths to fly in.

One team member, frequently Rebecca Ruppel, catches the moths, many of which are the size of an adult’s palm, as they flutter and dance across the screen. She then passes the moths to a crew member, often Davidowitz, who identifies the moth and measures it. He also uncoils the moth’s tongue and drags it through a red bead of gel on a slide, thereby capturing the grains of pollen stuck to the moth’s tongue. A third person labels the slide with the name of the moth species, melts the gel with a cigarette lighter, and plops a coverslip on it. Some moths visit other plants besides datura. By censusing the moths and also identifying the pollen on the slide, the researchers will learn what species of moths are available to visit datura plants and what species of plants various moths actually visit.

The researchers will combine the information from the plant study with the information from the moth study to figure out the benefits to the plants of being pollinated.

Figuring out the costs to the plant, is the purview of plant physiologist Travis Huxman. He says, “One of the neatest things we’re doing in this project is taking the physiological view of measuring the costs and benefits.” He’s studying how a datura plant copes once the moth’s caterpillars have stripped it of its leaves. The key turns out to be the plant’s underground storage organ, the tuber.

“The plant has to choose between growing roots, growing shoots and growing new flowers,” he says. “We want to know how the plant balances those competing choices.”

One way Huxman and his crew are answering those questions is by using a “root cam.”

The researchers grow pairs of sacred datura plants in the greenhouse in 25-gallon Rubbermaid tubs. Buried in each container are two 2.5-inch-diameter plexiglass tubes that run the length of the tub, parallel to the ground. A plant is planted over each tube. One plant gets loaded up with moth caterpillars; the other plant remains uneaten. About every other day, the researchers run a camera into the tubes and take pictures of the plants’ roots as they grow toward and eventually surround the tube.

“There’s all this exciting stuff underground that we don’t even know about,” Huxman says. Some of the excitement goes on in a plant’s roots. “You watch their birth, and you watch their extensions, and you watch their death.”

By using the root cam, the researchers can watch how having its leaves eaten affects root growth. Not surprisingly, the plant that’s getting munched has to draw on its savings account, the tuber. Huxman says, “Root growth goes down when plants have been eaten ­ just as we suspected.” The team is finding other changes too: when defoliated plants manage to put out new growth, the new leaves aren’t as good. Huxman says, “The plants might grow back just as big, but might not be as tolerant of the many environmental stresses they face.”

By combining their expertise, the researchers will get a well-rounded picture of the mutualism’s costs and benefits for the hawk moth and for the sacred datura plant. It’s a unique approach to studying mutualisms, Bronstein says. “To me, it’s the triumph of interdisciplinary science.”