Mutualism in metapopulations of legumes and rhizobia

Coevolution in mutualisms may result in a stable mosaic pattern of spatial differentiation, with regions occupied by different coadapted pairs of mutualists each being resistant to invasion by organisms with alternative phenotypes. The evolution of geographic mosaic patterns was analyzed for symbioses of legume plants and root-nodule bacteria (rhizobia), where plants are commonly polymorphic for genes affecting mutualism specificity. Typically, some alleles confer broad compatibility, and others restrict the set of bacterial genotypes accepted by plants as mutualist partners. Metapopulation simulation models with genetic assumptions matching the observed form of symbiotic specificity show that selection can generate a stable geographic mosaic if certain conditions are satisfied regarding competitive abilities, fitness benefits from mutualism, and migration rates. The structure of geographic variation in natural populations of the annual legume Amphicarpaea bracteata, together with patterns of variation in symbiotic fitness, is consistent with the interpretation that a mosaic pattern of differentiation may exist. Experimental and observational studies necessary to test more rigorously for mosaic distributions are outlined.