Effects of genetic variation in microbial mutualists on host-parasite interactions in the legume-rhizobia mutualism

Microbial symbiotic mutualists play important roles for their hosts. Many host traits are in fact joint traits determined by contributions from both host and microbial mutualists. Mutualist effects on host traits can even extend to traits involved in host-parasite interactions. Microbial mutualists also have extensive genetic variation, which has not been well accounted for when considering its effects on host-parasite interactions, especially in systems outside of defensive mutualisms. This is despite the fact that microbial mutualists harbor genetic variation for host-parasite interactions could have profound effects on the ecology and evolution of mutualist and host-parasite systems. Here I present work exploring whether strain level variation in mutualistic rhizobia (Ensifer meliloti) may alter traits related to parasitism of legumes (Medicago truncatula) by root knot nematodes (Meloidogyne hapla).In chapter 1, I present an overview of what is known about strain effects of microbial mutualists on host traits and its relevance for the ecology and evolution of hosts, parasites, and mutualists and present the gaps in our knowledge that motivate this work. In chapter 2, I present work that establishes that different strains of rhizobia can alter host-parasite interactions. This is done by comparing infection-related and mutualism-related traits and gene expression at the site of host-parasite and host-mutualist interactions across plants inoculated with two different rhizobia strains. I found that the two strains of rhizobia showed gene expression changes that may alter biological processes involved in host-parasite interactions, and parasite infection rates differed across plants inoculated with the two strains. Having established that these two rhizobia strains altered host-parasite interactions, in chapter 3, I investigated whether rhizobia populations harbor genetic variation for infection-related traits. If mutualist strains can harbor genetic variation that influences traits involved in host-parasite interactions, genetic variation in mutualists may be an under appreciated variable influencing host and parasite evolution. Determining whether rhizobial effects on host-parasite interactions have the ability to influence evolution requires investigating variation across a broad sampling of the genetic variation present in populations. I compared infection-related and mutualism-related phenotypes across a large sampling of host genotype and rhizobia strain combinations and find that rhizobia strains have effects on host susceptibility, parasite virulence, and host tolerance and that at times these effects may be dependent on interactions between host genotype and rhizobia strain identity. And in chapter 4, I briefly discuss the relevance of this worth for the broader field and lay out future research directions that build off of this work. This work highlights the potential importance that mutualist genetic variation may play in determining host-parasite interactions even in systems outside of defensive mutualisms. This underscores the importance of considering not only the ecology of microbial mutualists (i.e., presence absence of microbial mutualists) but also the evolution of microbial mutualists (i.e., genetic variation in microbial mutualists and its change over time) when considering hosts involved in multiple species interactions. Future work is needed to examine whether these microbial mutualists influence host-parasite interactions in other systems, the biological mechanisms of strain effects, and whether these effects are present in ecologically realistic situations.