PARENT SESSION
Organized Oral Session 3: Mutualists as parasites: How mutualistic are mutualists, and why?
Organizer(s): JR Powell, BE Wolfe, and JN Klironomos
Monday, August 8, 8:00 AM - 11:30 AM, Meeting Room 511a, Level 5, Palais des congrès de Montréal
The evolutionary persistence of diversity in rhizobium lifestyles.
Denison, R. Ford*,1, 2, Kiers, E. Toby2, 1 Ecology, Evolution, and Behavior, St. Paul, MN, USA2 Agronomy and Range Science, Davis, CA, USA
ABSTRACT- Rhizobium bacteria vary greatly in the net benefits they provide their legume host plants. Some rhizobium strains that lack symbiotic genes are nonetheless abundant in soil. Can we explain this diversity in rhizobium lifestyles? The several different rhizobium strains infecting each individual plant are each others most likely future competitors. This creates a potential tragedy of the commons, such that collective benefits to rhizobia from a healthier host can not explain the evolutionary persistence of individual investment in symbiotic N2 fixation. If plants treated all rhizobia the same, once inside their root nodules, then rhizobia that invested more in their own reproduction would outcompete those that invest in the N2 fixation. Recent experiments show that at least some legumes monitor the actual symbiotic performance of individual nodules -- rhizobium behavior, not identity -- and reduce O2 supply to the interior of nodules that fix less N2. Rhizobia prevented from fixing N2 (using an N2-free atmosphere) reproduced less inside nodules and had lower subsequent survival in soil, the opposite of what would be expected in the absence of these plant sanctions. But, given the severe fitness consequences for rhizobia of plant sanctions, why are "cheating" rhizobia, which infect legumes but then fix little or no N2, still common in some soils? We are exploring a range of hypotheses, from mixed nodules to more sophisticated rhizobium strategies for evading plant sanctions. Although a single rhizobium that successfully founds a nodule may have millions of descendants in the soil a few months later, competition for nodulation may reduce the chances of success enough that nonsymbiotic strains have similar fitness to symbiotic strains. Despite the experimental advantages of the legume-rhizobium symbiosis as a model for mutualism, many aspects of rhizobium natural history apart from their role in symbiotic N2 fixation have been neglected.
Key words: rhizobium, legume, symbiosis, mutualism