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

Abstract

Measuring selection acting on microbial populations in natural or even seminatural 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 22% and 12% of the variance in the strain composition of nodule communities in the second cohort, respectively. Although 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.

Whose trait is it anyways? Coevolution of joint phenotypes and genetic architecture in mutualisms

“…Genomic complexity could influence genetic variance and evolutionary trajectories, as in multi-nucleate arbuscular mycorrhizal fungi [60]. Furthermore, our model comprises a panmictic population in a constant environment; yet environments fluctuate in space and time, especially for horizontally transmitted microbes [61], and such variation in selection could alter evolution [62] or generate spatial correlations between host and symbiont breeding values. In our model, partnerships form at random, so genetic correlations do not arise pre-selection through spatial structure or other mechanisms (e.g.…”

Diversity and plant growth-promoting functions of diazotrophic/N-scavenging bacteria isolated from the soils and rhizospheres of two species of Solanum

“…Rhizobia are best known to develop symbiotic interactions with legumes, where mutualistic relationships progress to modifications of the root architecture of the host plant by the formation of nodules where nitrogen fixation occurs [60]. Nevertheless, nonsymbiotic and even parasitic rhizobial strains can be found in abundance in rhizosphere and nonrhizospheric soils, and the resulting plant-rhizobium interaction can be determined by the genetic content of both partners and environmental pressure [61][62][63][64].…”

Evolving together, evolving apart: measuring the fitness of rhizobial bacteria in and out of symbiosis with leguminous plants

“…Genomic basis of fitness alignment: One host genotype (A17) associated with more beneficial bacterial strains than the other (Burghardt et al, 2018). By looking for overlap between top GWAS candidates for plant size in single-strain inoculations and strain fitness, Epstein et al (2019) identified bacterial genes, including NifA and QueC, that may influence both traits.…”