The potential for genotype-by-environment interactions to maintain genetic variation in a model legume–rhizobia mutualism

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.