Simm's Lab:

Cheaters never prosper, or so the saying goes. But in a recent painstaking study of soil bacteria, IB professor Ellen Simms and her team of researchers dug up evidence to the contrary.

Early in her career in evolutionary biology, Simms studied interactions between plants and their pathogens and herbivores--antagonistic relationships, in which only one party benefits. But that field felt filled up with people asking similar questions, so she started casting about for a different line of inquiry--and found herself attracted to mutualisms, or relationships in which both parties receive a fitness benefit. Besides bees and flowers, another familiar--or at least close to home--mutualism is the relationship between humans and the bacteria living in our intestines. The single-celled organisms help us digest dinner, and in return, we offer them free food in the form of simple sugars.

"At the time, the field of mutualistic interactions was very descriptive and was considered to be very narrow but it was just becoming apparent that these cooperative interactions are not rare at all," Simms says. "But the theory underlying them was not very well developed."

She set to work developing a theoretical framework to help develop answers for basic questions about why such mutualistic relationships exist and persist--particularly given the fact that providing a fitness benefit to another species is often energetically expensive. Why do organisms of different species help each other out? And wouldn't natural selection lead to the evolution of cheaters--individuals who figure out how to get something in return for little or nothing?

"A cheater could just take advantage of the situation and get a free ride," says Simms. "Because it's not paying the cost of providing a fitness benefit to its partner, the cheater would have more offspring, so cheating would spread and the whole mutualism would fall apart. So why doesn't that happen?"

That question has been driving Simms' research for the last ten years, and to answer it, she studies the partnership between legumes and bacteria.

Plants need nitrogen to grow and thrive, but they can't absorb atmospheric nitrogen, which comprises some three-quarters of the air we breathe. Instead, that gas has to be transformed into reactive nitrogen. Enter a class of soil bacteria called rhizobia, which "fix" nitrogen--turn it into a form that plants can use.

Legumes, a group of plants that includes beans, peas and alfalfa, have formed a special relationship with rhizobia whereby bacteria colonize a plant, forming a nodule in the root. There, they provide usable nitrogen in return for food--the sugars that are one byproduct of the plants' photosynthesis. It's a relationship that's long been crucial for farmers, who often rotate their fields between nitrogen-hungry plants (like corn) and legumes (like soybeans), whose bacterial partners replenish the soil with nitrogen.

Simms says that this theory about the persistence of mutualisms is called "partner choice": cheaters don't arise in the legume-bacteria system, she thinks, because in a marketplace with lots of potential partners, plants can choose to deal only with good bacteria that provide a real benefit. Bad partners--those that don't provide a benefit--are either ignored or abandoned, so cheaters never get a chance to get an evolutionary foothold.

To test that theory, she and her team collected lupine seeds from their field site at Bodega Bay, and then returned to the lab to inoculate plants with different strains of soil bacteria. After allowing the plants to grow for several months, they measured the size of the root nodules, a proxy for the amount of space and food provided by the plant to the bacteria, and the plant's size--a proxy for the bacteria's degree of helpfulness. What they found supported Simms' partner choice model.

"The bacteria that provide higher fitness tend to end up in larger nodules," Simms says. "So it seems that plants are preferentially rewarding bacteria that are more beneficial."

Simms and Joel Sachs, a postdoctoral researcher in her lab who's now an assistant professor at UC-Riverside, wanted to know more about which bacteria were actually providing a benefit. They developed a technique for gathering bacteria not just from nodules but also from the plants' roots' surfaces. Then they cultured the bacteria, sequenced a region of their DNA to begin constructing a phylogenetic tree of the various strains, and inoculated plants with each different strains.

That's when they found the cheater. One strain of bacteria provided no fitness benefit to the plant, but received a fitness benefit from the plant that was two orders of magnitude higher than most of the "honest" bacteria. It was just one out of some 300 isolates they cultured. But still--there it was, "the first evidence that we've been able to find in the literature," Simms says, "that cheaters do, in fact, prosper."

A closer look at the molecular data led Simms and Sachs to hypothesize that the cheater arose through lateral gene transfer, a chance event. But if the genotype is so wildly successful, then why isn't it proliferating?

That's Simms' next question, and she thinks the answer has something to do with the plant's ability to identify and punish cheaters. In her previous experiment, the plant was inoculated only with the cheater--it had no better alternative. Now, by inoculating one plant with two bacterial strains--one that plays by the rules and one that doesn't--she'll examine how plants sanction cheaters when they have the choice.

"This is really the intellectual center of my work right now," says Simms. But like most professors in IB, a series of other fascinations and side projects orbit around that center. She recently provided her expertise in evolutionary genetics to help a colleague assess selection pressures on invasive trees growing in Florida. And now, she's coming up with a way to measure the traits of the beautiful, polychromatic lupine seeds she's been working with for the past decade and a half.

Appreciating nature's aesthetics, Simms says, was a gift from her father, an artist. "Naturalists and artists see in similar ways," she says. "When you get two people like that together to go for a hike, it's really interesting. Because you both see the same things and you both respond, while people around you don't even see those things."

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