Showing papers by "James D. Bever published in 1998"

Maintenance of diversity within plant communities: soil pathogens as agents of negative feedback

Abstract: The effect of soil pathogens on plant communities was investigated using four old-field perennial plant species and five isolates of a pathogenic oomycete in the genus Pythium These Pythium strains were isolated from the roots of two of the plant species, Danthonia spicata and Panicum sphaerocarpon, used in a previous experiment on the consequences of changes in the soil community on plant growth In this previous experiment, Danthonia and Panicum changed the soil community in a manner that reduced their growth relative to that of a third plant species, Anthoxanthum odoratum In the current experiments, we found that inoculation with Pythium reduced overall plant mass and root: shoot ratios, but Danthonia and Panicum were more susceptible to the presence of Pythium than the other two plant species, Anthoxanthum and Plantago lanceolata In addition, Pythium ac- cumulates at different rates on different plant species, with a greater than tenfold higher population observed in association with Panicum compared to Anthoxanthum The results of these experiments suggest that the accumulation of species-specific soil pathogens could account for the previous observation of negative feedback on plant growth through changes in the soil community As negative feedback may act to maintain plant species diversity within a community, these results suggest that soil pathogens may themselves contribute to the maintenance of plant species diversity

Evolutionary dynamics of rhizopine within spatially structured rhizobium populations

Abstract: Symbiosis between legumes and nitrogen-fixing bacteria is thought to bring mutual benefit to each participant. However, it is not known how rhizobia benefit from nodulating legume hosts because they fix nitrogen only after becoming bacteroids, which are terminally differentiated cells that cannot reproduce. Because undifferentiated rhizobia in and around the nodule can reproduce, evolution of symbiotic nitrogen fixation may depend upon kin selection. In some hosts, these kin may persist in the nodule as viable, undifferentiated bacteria. In other hosts, no viable rhizobia survive to reproduce after nodule senescence. Bacteroids in these hosts may benefit their free-living kin by enhancing production of plant root exudates. However, unrelated non-mutualists may also benefit from increased plant exudates. Rhizopines, compounds produced by bacteroids in nodules and catabolized only by related free-living rhizobia, may provide a mechanism by which bacteroids can preferentially benefit kin. Despite this apparent advantage, rhizopine genotypes are relatively rare. We constructed a mathematical model to examine how mixing within rhizobium populations influences the evolution of rhizopine genotypes. Our model predicts that the success of rhizopine genotypes is strongly dependent upon the spatial genetic structure of the bacterial population; rhizopine is more likely to dominate well-mixed populations. Further, for a given level of mixing, we find that rhizopine evolves under a positive frequency-dependent process in which stochastic accumulation of rhizopine alleles is necessary for rhizopine establishment. This process leads to increased spatial structure in rhizobium populations, and suggests that rhizopine may expand the conditions under which nitrogen fixation can evolve via kin selection.