Host investment into symbiosis varies among genotypes of the legume Acmispon strigosus, but host sanctions are uniform

Efficient host control predicts the extirpation of ineffective symbionts, but they are nonetheless widespread in nature. We tested three hypotheses for the maintenance of symbiotic variation in rhizobia that associate with a native legume: 1) partner mismatch between host and symbiont, such that symbiont effectiveness varies with host genotype, 2) resource satiation, whereby extrinsic sources of nutrients relax host control, and 3) variation in host control among host genotypes. We inoculated Acmispon strigosus from six populations with three Bradyrhizobium strains that vary in symbiotic effectiveness on sympatric hosts. We measured proxies of host and symbiont fitness in single- and co-inoculations under fertilization treatments of zero added nitrogen and near-growth-saturating nitrogen. We examined two components of host control: ‘host investment’ into nodule size during single- and co-inoculations, and ‘host sanctions’ against less effective strains during co-inoculations. The Bradyr...