Rapid adaptation can occur across a wide range of taxa and in many biological contexts, and has the potential to alter ecological interactions and ecosystem-level processes. Although many studies have documented rapid adaptation, it can be difficult to identify the environmental factors that drive adaptation, known as agents of selection. In a previous study, I found evidence of rapid evolutionary changes in two recently restored populations of the annual legume Chamaecrista fasciculata. Chamaecrista form facultative mutualistic interactions with nitrogen-fixing rhizobia, and one of these restored populations has evolved to produce significantly more rhizobia-housing root nodules than the other population. To explore whether rhizobia are potential agents of selection in this system, I conducted a greenhouse experiment growing Chamaecrista from both populations with multiple rhizobia strains from each population’s home site. I examined whether rhizobia differ between these two sites, whether plants differ in their dependence on rhizobia, and whether populations are locally adapted to rhizobia.
Results/Conclusions
Rhizobia differed between the two sites, with rhizobia from one site providing growth benefits to plants, and rhizobia from the other site providing little to no benefit. Plants from the site with beneficial rhizobia appear to be adapted to these rhizobia, producing more biomass than when inoculated with other or no rhizobia. Plants from the non-beneficial rhizobia site don’t benefit from rhizobia at all, even when inoculated with beneficial rhizobia, suggesting these plants may have abandoned the mutualism. This suggests that these Chamaecrista populations have evolved different strategies to interact with rhizobia, and that rhizobia potentially play a role in driving plant evolution in this system. The presence and identity of rhizobia could be important for the ecological and evolutionary outcomes of restoration.