Evolution of cooperation in the legume-rhizobium symbiosis

Mutualistic symbioses include a wide range of interactions among a diverse set of organisms. The symbiosis between legumes and rhizobia (Rhizobia , Bradyrhizobium, Mesorhizobium, Sinorhizobium species) is a classic mutualistic relationship. In return for carbohydrates provided by the host legume, the rhizobia supply nitrogen to the legume. Although reciprocally beneficial, the stability of this cooperative relationship poses a dilemma to evolutionary theory. Why do symbioses persist over millennia, if additional resources could be gained by either partner through the exploitation of the mutualism? N2 fixation is clearly beneficial to the host plant because it supplies nitrogen needed for growth and photosynthesis. But N2 fixation is energetically costly to the bacteria, and hence reduces the resources that could be allocated to their growth and reproduction. Why then do rhizobia fix N2 to benefit the host plant when expending those resources on reproduction could increase the rhizobia's own fitness?; We have suggested that legumes penalize rhizobia that defect from their N2-fixing duties. In a series of laboratory experiments, we prevented a normally mutualistic rhizobium strain from cooperating and demonstrated a fitness cost to those rhizobia failing to fix N2. Additionally, by altering the gas treatments, we simulated intermediate variation in N 2-fixing investment and found that host sanctions can operate at intermediate levels in response to rhizobium performance. Both natural and human-mediated selection pressures have shaped sanctions in the legume-rhizobia mutualism. Agronomic practices (plant breeding, fertilizer use, crop rotation) create selection pressures that influence symbiotic functioning of plant mutualists in cropping systems. We review the evolutionary implications of these selection pressures for agricultural productivity and present two years of field work investigating differences in the ability of older and newer soybean cultivars to limit losses to ineffective rhizobia.; The most durable and long-term solutions to increasing symbiotic gains will involve an integration of agronomic and evolutionary perspectives. Our research suggests that sanctions selectively favor the most beneficial rhizobia, and hence can stabilize the mutualistic interaction.