Environmental and Genetic Contributions to Symbiosis Traits in a Wild Legume

Plants can gain substantial growth benefits from microbial symbionts, but these benefits are threatened by ineffective symbionts that infect plants without providing a service. To minimize this threat, plants can preferentially associate with effective symbionts and avoid or punish ineffective symbionts. Although these ‘host control traits’ are central to our understanding of mutualism evolution, we know little about how much they genetically vary within plant species or how they perform in different environments. Here, I investigated variation in host control traits using the California native legume Acmispon strigosus and wild strains of its nitrogen-fixing symbiont, Bradyrhizobium. In the first chapter, I tested for genetic variation in host control traits among six population sources of A. strigosus by inoculating plants with pure cultures of effective and ineffective Bradyrhizobium. In all hosts, the strain content of root nodules was biased toward the most effective Bradyrhizobium strain, but hosts varied genetically in mean nodule size as well as growth benefits. These patterns did not change under experimental nitrogen fertilization. In the second chapter, I examined host genetic variation in nodule size in a new experimental setting, inoculating plants with soil slurries rather than pure Bradyrhizobium cultures. I found similar variation in nodule size and host benefits as I observed in chapter 1, indicating that host control trait variation was robust to the biotic complexity of inocula. Furthermore, plant growth benefits from soils were more strongly driven by plant genotype than soil source, further highlighting the importance of plant genotype. In the third chapter, I investigated host and symbiont contributions to strain content of nodules by inoculating four plant genotypes with nine combinations of effective and ineffective rhizobia strains. I found significant variation among ineffective strains in the relative fitness they achieved in nodules. However, the dominant force shaping strain relative abundance was the nitrogen fixation phenotype, consistent with hosts being in control of this trait. Overall, my dissertation research provides evidence of limited genetic variation in host control traits of A. strigosus in a variety of experimental settings