Data from: 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 Bradyrhizobium strains displayed conserved growth effects on hosts, and host control did not decline under experimental fertilization. Host sanctions were robust in all hosts, but host lines from different populations varied significantly in measures of host investment in both single- and co-inoculation experiments. Variation in host investment could promote variation in symbiotic effectiveness and prevent the extinction of ineffective Bradyrhizobium from natural populations.高效的宿主调控理论可预测低效共生菌会被清除，但这类共生菌在自然界中却依然广泛存在。本研究针对与本土豆科植物共生的根瘤菌（rhizobia）的共生变异维持机制，检验了三项假说：一是宿主与共生菌的匹配错配，即共生菌的共生效率随宿主基因型的不同而变化；二是资源饱和效应，即外源营养供应会削弱宿主的调控能力；三是不同宿主基因型间的调控能力差异。我们选取6个种群的糙荚山黧豆（Acmispon strigosus），用3株在同域宿主上共生效率存在差异的慢生根瘤菌属（Bradyrhizobium）菌株进行接种处理。在不施氮肥和接近生长饱和浓度的氮肥两种施肥处理下，我们分别测定单接种和共接种体系中宿主与共生菌适合度的替代指标。我们同时检测了宿主调控的两个组成部分：单接种与共接种条件下宿主对根瘤大小的“宿主投资”，以及共接种条件下宿主对低效菌株的“宿主制裁”。慢生根瘤菌菌株对宿主的生长效应具有保守性，且实验施加氮肥并未削弱宿主的调控能力。所有宿主均表现出稳定的宿主制裁效应，但不同种群来源的宿主株系在单接种与共接种实验中的宿主投资指标均存在显著差异。宿主投资的差异可促进共生效率的变异，并阻止低效慢生根瘤菌在自然种群中灭绝。