Evolution of nitrogen fixation in spatially structured populations of Rhizobium

Symbiosis between legumes and nitrogen-fixing bacteria is thought to bring mutual benefit to each participant. However, it is not known how rhizobia b enefit from nodulation of legume hosts because they fix nitrogen only after differentiating into bacteroids, terminally differentiated cells that cann ot reproduce. Because free-living rhizobia can reproduce, and may benefit f rom the increase of plant root exudates stimulated by nodulation, evolution of symbiotic nitrogen fixation may depend upon kin selection. However, unr elated nonmutualists may also benefit from increased plant exudates and nit rogen-fixing populations are therefore vulnerable to invasion by nonfixing, saprophytic Rhizobium. The access of nonfixing Rhizobium to the plant exud ates associated with nodules depends upon the spatial structure of the Rhiz obium populations within the soil. We investigate the influence of spatial structure on the evolution of N-fixation within a Rhizobium population usin g a mathematical model. Our model demonstrates that spatial structure is ne cessary for the evolution of N-fixation and that N-fixation is more likely to evolve with increasing degrees of spatial structure. In fact, we identif y three dynamic outcomes that depend upon the relative strength of the cost s of N-fixation relative to the degree of spatial structure and benefits re sulting from nodulations. If the costs are relatively high, N-fixation will not evolve; if the costs are relatively low, N-fixing genes will fix in th e population, but at intermediate conditions, a stable mixture of N-fixing bacteria and nonfixing bacteria will be maintained. The conditions for coex istence of N-fixing bacteria and nonfixing bacteria expand under a saturati ng relationship between nodule numbers and N-fixing genotype frequency.