Genetic variation in host selectivity and adaptive strain enrichment in legume-rhizobia symbiosis

Mutualism breakdown can be prevented if partner species preferentially select and reward partners that provide greater benefit. We examined these two components using the legume Medicago truncatula and its nitrogen-fixing symbiont Sinorhizobium meliloti. The first dataset focuses on reanalyzing strain composition data from the nodules of 202 accessions to show significant genetic variation in the capacity of Medicago to restrict strain diversity when controlling for nodule number variation using a simulation. By performing an augmented image analysis of nodules pool images to produce scores of morphological parameters, we found that hosts with a suite of nodules traits, including shorter nodules, tended to be more selective. Using previously published SNP data for these ~200 accessions, we performed a genome-wide association study on host selectivity to identify candidate genes. The second dataset uses two well-studied Medicago genotypes (A17 and R108) with contrasting nodule morphologies to assess the effectiveness of adaptive enrichment mechanisms by sampling the relative frequencies of rhizobial strains in pools of small nodules that we show have smaller rhizobia population sizescompared to large nodules. We pair these results with previous single-strain assessments of strain benefits to hosts to show that hosts enriched beneficial strains in large nodules, the host that formed larger and more variably sized nodules and thus had greater 'potential' to increase rhizobial populations was less effective. Together this package includes the data, code, and figures to reproduce our analysis.