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.