Host control by Acmispon strigosus constrains fitness gains of ineffective Bradyrhizobium symbionts in mixed infections

Plant hosts can gain significant growth benefits from symbiosis with microbes, but these benefits could be threatened by divergent fitness interests among partners. Here, we measured fitness outcomes in symbiosis, by varying the genotypes of both microbes and hosts, to examine scenarios that might favor uncooperative symbionts. We studied associations between Acmispon strigosus, an annual legume native to California, and its nitrogen fixing symbionts in the genus Bradyrhizobium. Bradyrhizobium symbionts form root nodules on compatible hosts, with strains varying from effective, fixing substantial nitrogen for the host, to ineffective strains that do not fix nitrogen and provide no benefit to host growth. We co-inoculated four A. strigosus plant lines with nine combinations of effective and ineffective Bradyrhizobium strains and measured the relative fitness of ineffective strains within individual nodules, as hosts must select against uncooperative symbionts to maintain benefits. In mix..., Data analysis – Generalized linear mixed models (GLMMs) were used to test hypotheses using JMP Pro 13.0.0 (SAS Institute Inc., Cary, NC, USA). Dependent variables were log10-transformed as needed to improve normality. Proportional data was logit-transformed after applying a linear transformation to account for zeros and ones in the dataset (i.e., 1% was added to all datapoints except ones, from which 1% was subtracted). All models included a random effect of harvest week; models using plant biomass data (and not nodule culturing data) also included a random effect of block nested within harvest week. For each GLMM, all possible interactions among main effects of interest were initially tested. Nonsignificant interactions were removed from the model if this reduced the corrected AIC (AICc) by at least 2 units, and results from trimmed models were reported. Significant differences among levels of main effects were assessed with pairwise t-tests (Tukey’s HSD) of least squares means. The Te..., , # Host control by Acmispon strigosus constrains fitness gains of ineffective Bradyrhizobium symbionts in mixed infections

[https://doi.org/10.5061/dryad.2ngf1vj04](https://doi.org/10.5061/dryad.2ngf1vj04)

## Description of the data and file structure

*Data analysis –* Generalized linear mixed models (GLMMs) were used to test hypotheses using JMP Pro 13.0.0 (SAS Institute Inc., Cary, NC, USA). Dependent variables were log10-transformed as needed to improve normality. Proportional data was logit-transformed after applying a linear transformation to account for zeros and ones in the dataset (i.e., 1% was added to all datapoints except ones, from which 1% was subtracted). All models included a random effect of harvest week; models using plant biomass data (and not nodule culturing data) also included a random effect of block nested within harvest week. For each GLMM, all possible interactions among main effects of interest were initially tested. Nonsignificant interactions were removed f...