Data from: Legacy of prior host and soil selection on rhizobial fitness in planta

Measuring selection acting on microbial populations in natural or even
semi-natural environments is challenging because many microbial
populations experience variable selection. The majority of rhizobial
bacteria are found in the soil. However, they also live symbiotically
inside nodules of legume hosts and each nodule can release thousands of
daughter cells back into the soil. We tested how past selection (i.e.,
legacies) by two plant genotypes and by the soil alone, affected selection
and genetic diversity within a population of 101 strains of Ensifer
meliloti. We also identified allelic variants most strongly associated
with soil- and host-dependent fitness. In addition to imposing direct
selection on rhizobia populations, soil and host environments had lasting
effects across host generations. Host presence and genotype during the
legacy period explained respectively 22% and 12% of the variance in the
strain composition of nodule communities in the second cohort. While
strains with high host fitness in the legacy cohort tended to be enriched
in the second cohort, the diversity of the strain community was greater
when the second cohort was preceded by host rather than soil legacies. Our
results indicate the potential importance of soil selection driving the
evolution of these plant-associated microbes.

在自然甚至半自然环境中测量作用于微生物群体的选择压力极具挑战，因为许多微生物群体面临可变选择。大多数根瘤菌存在于土壤中，但它们也能与豆科宿主的根瘤形成共生关系，且每个根瘤可向土壤释放数千个子细胞。我们测试了两种植物基因型及仅土壤环境的既往选择（即遗留效应，legacy）如何影响101株苜蓿中华根瘤菌（Ensifer meliloti）群体的选择压力与遗传多样性。我们还鉴定了与土壤依赖型及宿主依赖型适应性关联最紧密的等位变异（allelic variants）。除了对根瘤菌群体施加直接选择压力外，土壤与宿主环境还能跨宿主世代产生持久影响。遗留期内宿主的存在与否及基因型，分别解释了第二队列根瘤群落菌株组成变异的22%和12%。尽管遗留队列中宿主适应性较高的菌株在第二队列中富集，但当第二队列的前置条件为宿主遗留效应而非土壤遗留效应时，菌株群落的多样性更高。研究结果表明，土壤选择压力在这些植物关联微生物的进化过程中具有潜在重要性。