提出假设：We asked whether genotype-by-environment (GxE) interaction is a potential mechanism maintaining variation in the model legume-rhizobia system, Medicago truncatula-Ensifer meliloti.

结果1-试验设计：We planted 50 legume genotypes in a greenhouse under ambient light, and shade to reflect reduced carbon availability for plants.

结果2-表型：We found an expected reduction under shaded conditions for plant performance traits such as leaf number, aboveground, and belowground biomass, and a mutualism-related trait, nodule number.

结果3-GxE与根瘤：We also found GxE for nodule number, with ～83% of this interaction due to shifts in genotype fitness rank order across light environments, coupled with strong positive directional selection on nodule number regardless of light environment.

结论：Our results suggest that GxE can maintain genetic variation in a mutualism-related trait that is under consistent positive directional selection across light environments.