Fine-Scale Patterns of Genetic Structure in the Host Plant Chamaecrista fasciculata (Fabaceae) and Its Nodulating Rhizobia Symbionts

In natural plant populations, a fine-scale spatial genetic structure (SGS) can result from limited gene flow, selection pressures or spatial autocorrelation. However, limited gene flow is considered the predominant determinant in the establishment of SGS. With limited dispersal ability of bacterial cells in soil and host influence on their variety and abundance, spatial autocorrelation of bacterial communities associated with plants is expected. For this study, we collected genetic data from legume host plants, Chamaecrista fasciculata, their Bradyrhizobium symbionts and rhizosphere free-living bacteria at a small spatial scale to evaluate the extent to which symbiotic partners will have similar SGS and to understand how plant hosts choose among nodulating symbionts. We found SGS across all sampled plants for both the host plants and nodulating rhizobia, suggesting that both organisms are influenced by similar mechanisms structuring genetic diversity or shared habitat preferences by both plants and microbes. We also found that plant genetic identity and geographic distance might serve as predictors of nodulating rhizobia genetic identity. Bradyrhizobium elkanii was the only type of rhizobia found in nodules, which suggests some level of selection by the host plant.

Additive genetic variance for lifetime fitness and the capacity for adaptation in an annual plant

The immediate capacity for adaptation under current environmental conditions is directly proportional to the additive genetic variance for fitness, VA(W). Mean absolute fitness, , is predicted to change at the rate , according to Fisher’s Fundamental Theorem of Natural Selection. Despite ample research evaluating degree of local adaptation, direct assessment of VA(W) and the capacity for ongoing adaptation is exceedingly rare. We estimated VA(W) and in three pedigreed populations of annual Chamaecrista fasciculata, over three years in the wild. Contrasting with common expectations, we found significant VA(W) in all populations and years, predicting increased mean fitness in subsequent generations (0.83 to 6.12 seeds per individual). Further, we detected two cases predicting “evolutionary rescue”, where selection on standing VA(W) was expected to increase fitness of declining populations ( < 1.0) to levels consistent with population sustainability and growth. Within populations, interannual differences in genetic expression of fitness were striking. Significant genotype-by-year interactions reflected modest correlations between breeding values across years (all r < 0.490), indicating temporally variable selection at the genotypic level; that could contribute to maintaining VA(W). By directly estimating VA(W) and total lifetime , our study presents an experimental approach for studies of adaptive capacity in the wild.

Draft Genome Sequence of Bradyrhizobium sp. Strain LVM 105, a Nitrogen-Fixing Symbiont of Chamaecrista fasciculata (Michx.) Greene

Here, we present the draft genome sequence of Bradyrhizobium sp. strain LVM 105, a soil bacterium that forms nitrogen-fixing nodules on the roots of partridge pea.