In mutualism, hosts select symbionts via partner choice and preferentially direct more resources to symbionts that provide greater benefits via sanctions. At the initiation of symbiosis, prior to resource exchange, it is not known how the presence of multiple symbiont options (i.e., the symbiont social environment) impacts partner choice outcomes. Furthermore, little research addresses whether hosts primarily discriminate among symbionts via sanctions, partner choice or a combination. We inoculated the legume, Acmispon wrangelianus, with 28 pairs of fluorescently labeled Mesorhizobium strains that vary continuously in quality as nitrogen-fixing symbionts. We find that hosts exert robust partner choice, which enhances their fitness. This partner choice is conditional such that a strain's success in initiating nodules is impacted by other strains in the social environment. This social genetic effect is as important as a strain's own genotype in determining nodulation and has both transitive (consistent) and intransitive (idiosyncratic) effects on the probability that a symbiont will form a nodule. Furthermore, both absolute and conditional partner choice act in concert with sanctions, among and within nodules. Thus, multiple forms of host discrimination act as a series of sieves that optimize host benefits and select for costly symbiont cooperation in mixed symbiont populations.

We transformed eight Mesorhizobium strains to express fluorescent markers. We grew Mesorhizobium in symbiosis with a single genotype of the native Acmispon wrangelianus host plant as both 1-strain inoculations (8 treatments) and all possible 2-strain combinations (28 treatments), replicated over 8 blocks. Rhizobia and host plant fitness data were measured in the greenhouse experiment (symbiosis_phenotype_data.csv).  We also measured reproductive fitness of rhizobia in nodules (cfu_data.csv) and measured each strains intrinsic axenic growth rate (growth_data.csv).

We provide three datasets as csv files and one R code file that contains code for analyzing the data in these files. In addition, two more csv files are provided to save time but these files can also be regenerated using code in the R file. See the README file for metadata for each data set. The contents of each file are as follows:

* symbiosis_phenotype_data.csv contains Mesorhizobium and host plant fitness data from the greenhouse experiment. This file contains a metadata description in the first column.

* cfu_data.csv contains nodule culturing data (counts of colony forming units). This file contains a metadata description in the first column.

* growth_data.csv contains each of the eight Mesorhizobium strains growth rate. This file contains a metadata description in the first column.

* noduleproportion_models_randomized.csv is an extra file that is provided to save time when reproducing part of figure S2 (nodule proportion), or the code that generated this output file is included in the R code file (Montoya_et_al_2022_code.R)

* shootmass_models_randomized.csv is an extra file that is provided to save time when reproducing part of figure S2 (shoot mass), or the code that generated this output file is included in the R code file (Montoya_et_al_2022_code.R)

* Montoya_et_al_2022_code.R contains R code for importing, processing, and analyzing data from the above datasets. It also contains code for producing figures from these data.