Such mutualism is puzzling, however, Simms said.
"Mutualism is a paradox because natural selection acts on individuals, so why would an individual do something costly that benefits another individual?" she said. "Such a system is also vulnerable to exploitation - one species taking advantage of another. What mechanisms could prevent this?"
She and colleagues proposed several years ago that in cooperative relationships, one member must have the upper hand in enforcing good behavior. This could involve shutting down resources to nodules inhabited by poorly performing bacteria, or preventing the aging of nodules more effective at fixing nitrogen.
In studies at the University of California's Bodega Marine Reserve, a 362-acre preserve located about 60 miles north of the UC Berkeley campus, Simms and her colleagues set out to test some of these hypotheses about mutualism. She chose as her subject the lupine, one of the plants she has researched during the nearly 15 years she has studied the ecology of nitrogen fixation in legumes.
Lupines are widespread and beautiful wildflowers, but they also have been used for human and animal food for more than 2,000 years in parts of Europe and South America. Before new "sweet" breeds were developed 80 years ago, the often toxic alkaloids found in wild lupines had to be leached out. In addition, native Californians used strong thin lupine roots to make snares and fishing nets.
In a preliminary study, she, Taylor and Povich used molecular markers to identify the different strains of the bacteria Bradyrhizobia that interact with roots of the six species of lupine found on the reserve site.