Geographic adaptation in plant-soil mutualisms: tests using Acacia spp. and rhizobial bacteria

1. Variation in the effectiveness of mutualistic associations is well characterized in plantsoil symbiont interactions, yet there are little empirical data providing insight into how such variation evolves and persists in natural settings. 2. Heterogeneity in the strength and direction of co-evolutionary selection among spatially discrete demes is predicted to be important for the maintenance of genetic variation in species interactions. Here, we experimentally test the potential for local adaptation to generate phenotypic divergence among wild host-symbiont populations using two leguminous host species that differ in their specificity for rhizobial partners. 3. Molecular characterization of host populations and associated rhizobial communities revealed significant among-population genetic differentiation. Reciprocal cross-inoculation experiments testing for variation in the fitness of nine populations of Acacia salicina and A. stenophylla in response to inoculation with rhizobia revealed variation in host response to the mutualism (both host species), and the benefit conferred by different rhizobial populations (A. salicina only). However, there was no indication that host population-by-rhizobial population interactions influence the outcomes of mutualism for the host. 4. We further examined potential correlations between (i) plant response to inoculation and (ii) rhizobial effectiveness, with variation in soil fertility at the sites from which plant and rhizobial samples originated. Data from the cross-inoculation experiments revealed no correlations between soil chemistry, water availability and either host or rhizobial performance. However, analysis of results from an extensive whole-soil inoculation trial including nearly 60 A. salicina and A. stenophylla sites showed a significant negative correlation between levels of soil nitrogen and plant response to inoculation. 5. Overall, these findings suggest that selection for local adaptation may play little role in maintaining phenotypic variation in these interactions. We hypothesize that mutualistic interactions occurring among communities of hosts and symbionts do not favour co-evolutionary divergence among populations.