The ecological and evolutionary outcomes of host-microbe symbioses have the potential to shift dynamically between mutualism and antagonism depending on environmental context. Environmental conditions can alter the fitness benefits of host-symbiont cooperation, drive patterns of environmental adaptation, and impact patterns of coevolution. Therefore, an integrated perspective on how environmental variation shapes the evolutionary ecology of symbioses requires an understanding of: 1) How do the costs and benefits of cooperation differ across environmental conditions? 2) Do tradeoffs between environmental adaptations and cooperation constrain partner evolution? And 3) Are host and symbiont environmental and coevoltutionary history important to the expression of local environmental adaptation? To address these questions, I examine naturally coevolving populations of the legume, Acmispon wrangelianus, and symbiotic nitrogen-fixing Mesorhizobium rhizobium bacteria. These mutualists inhabit both physiologically stressful serpentine soils enriched in the toxic levels of nickel, as well as more benign non-serpentine soils. In this talk I integrate the results of cross-inoculation experiments in the greenhouse with rhizobium draft genome sequencing and laboratory assays of environmental tolerance to connect environmental and co-evolutionary adaptation.
Results/Conclusions
Cross-inoculation experiments in which host legumes were grown with and without their rhizobium partners demonstrate that rhizobia confer greater relative fitness benefits to their hosts in more stressful nickel-enriched soils than on benign soils. This stronger relative fitness benefit from cooperation could intensify co-evolutionary dynamics in a more stressful environment. Laboratory and cross-inoculation growth assays indicate that adaptation of rhizobia to nickel does not tradeoff with the amount of cooperative benefit a strain provides to a legume. Thus symbiont environmental adaptation does not appear to constrain the potential for highly mutualistic symbionts. This finding is supported by population genomic data indicating that the genetic bases for nickel tolerance and symbiotic benefit reside in distinct mobile genomic compartments. Finally, both host and symbiont genotype origins are shown to contribute to the expression of local adaptation in symbioses grown in stressful nickel-enriched soil or benign soil. Thus the expression of local adaptation to nickel is a phenotype modulated by symbiosis. Together, these findings demonstrate paths by which environmental adaptation and host-symbiont coevolution interact to determine the fitness outcomes of symbiosis.