Invasion of legume-rhizobium mutualists across a heterogeneous environment: Microevolution during colonization of a serpentine grassland

Background/Question/Methods

A growing body of ecological evidence suggests that soil microbes play a critical role in plant invasions, yet the role of microevolution in plant-microbe range expansion is largely unexplored. Numerous legume species are prominent invasive plants, and most rely upon their ubiquitous association with nitrogen fixing rhizobia to persist. Burclover (Medicago polymorpha), a widespread exotic, was introduced to a serpentine grassland ~150 years ago, along with its symbiotic partner, Sinorhizobium medicae. Serpentine soil is a physiologically harsh environment due in part to low macronutrients and toxic soil chemistry, and often occurs as patchy outcroppings adjacent to more benign non-serpentine soil. While burclover’s population is centered on non-serpentine soil, it appears to be expanding its range onto serpentine habitats. To examine whether microevolution in legumes or rhizobia facilitate this expansion, we tested whether M. polymorpha and S. medicae exhibit local adaptation to the abiotic environment (serpentine/non-serpentine soil) and to the biotic environment (local vs. foreign partners). We manipulated three factors in a factorial design: plants and rhizobia genotypes collected from replicated serpentine or non-serpentine sites (host origin soil and symbiont origin soil), grown in pasteurized serpentine or non-serpentine soil in the greenhouse (destination soil). We measured standard fitness proxies for the legume and nodule number and weight for rhizobium fitness.

Results/Conclusions Results suggest that a trend for local adaptation of M. polymorpha to the abiotic environment facilitates its persistence on serpentine, while S. medicae did not show abiotic adaptation in the symbiotic state. Serpentine M. polymorpha genotypes increased their access to nitrogen-fixing bacteria (nodule weight) when challenged with serpentine destination soil, whereas non-serpentine M. polymorpha genotypes showed a decrease in nodule weight. This suggests the rhizobia may benefit from legume abiotic local adaptation, not vice versa, and co-adaptation was not detected. Invasion of serpentine habitats represents a threat to California’s remnant floristic diversity, and these invaders are of particular concern, as they may increase available nitrogen in thin soils, facilitating other non-natives. Future research will further explore this phenomenon of legume local adaptation to heterogeneous environments mediated by an increased ability to associate with mutualistic microbes in the home environment.