The maintenance of genetic variation in mutualism-related traits is key for understanding mutualism evolution, yet the mechanisms maintaining variation remain unclear. We asked whether genotype-by-environment (G×E) interaction is a potential mechanism maintaining variation in the model legume–rhizobia system, Medicago truncatula–Ensifer meliloti. We planted 50 legume genotypes in a greenhouse under ambient light and shade to reflect reduced carbon availability for plants. 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. We also found G×E 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. Our results suggest that G×E can maintain genetic variation in a mutualism-related trait that is under consistent positive directional selection across light environments.

维持共生相关性状的遗传变异是理解共生进化的关键，但维持变异的机制尚不清楚。我们询问基因型与环境 (G×E) 的相互作用是否是维持模型豆科植物 - 根瘤菌系统，Medicago truncatula - Ensifer meliloti变异的潜在机制. 我们在环境光照和阴影下的温室中种植了 50 种豆科植物基因型，以反映植物的碳供应量减少。我们发现在阴影条件下植物性能性状（例如叶数、地上和地下生物量）以及与共生相关的性状、根瘤数等预期减少。我们还发现了结节数的 G×E，由于基因型适应度等级顺序在光照环境中的变化，加上与光照环境无关的结节数的强正方向选择，这种相互作用的 83% 左右。我们的研究结果表明，G×E 可以保持共生相关性状的遗传变异，该性状在光照环境中处于一致的正向选择下。