PhD Candidate The Pennsylvania State University State College, Pennsylvania
Body of Abstract: Legumes such as Medicago truncatula form mutualistic relationships with nitrogen-fixing rhizobial bacteria housed in nodules that exhibit variation in size, shape, and number. Host plants may influence strain diversity in nodules by controlling nodule morphology through developing lobed nodules or increasing nodule number. This potential has yet to be explored. Here we use a ‘Select and Resequence’ assay with 202 M. truncatula accessions and 88 strains of Sinorhizobium meliloti to measure the contribution of host genetic variation to nodule morphology and strain diversity. Nodules were harvested, imaged to measure morphological traits, and processed to identify the relative abundance of each strain within. Redundancy analysis and regression models revealed host genotype to explain 20% of the variation in strain diversity and 34% in nodule morphology. Further analysis of morphologies highlighted a weak but significant relationship between nodule number, soil sand content, and precipitation at the site of plant origin. Based on previous work, we expected increased strain diversity in samples with many lobed nodules. Using machine learning methods (e.g., lasso regression), we found that nodule number was the main driver of strain diversity. To remove this confounding signal, we simulated community assembly with random draws determined by nodule number, calculated the strain diversity, and compared this to the observed strain diversity. We reran our analysis on this novel metric of host selectivity and found that shorter nodules exhibited greater selectivity of rhizobia strains. We also conducted a GWAS to identify host genes contributing to variation in nodule morphology. Our results provide the first look at how nodule morphology influences the evolutionary ecology of symbiotic nitrogen fixation by uncovering components of host genetic control, signals of local adaptation, and relationships with rhizobia strain diversity and host selectivity.