The Effect of Abiotic and Biotic Factors, Symbiont Exchange Between Host Species, and Host Migration on Fungal Symbiont Community Composition and Diversity

Abstract

Microbiomes, the ubiquitous communities of microbial symbionts residing within hosts, play important roles in host health, development, and fitness. While recent research has characterized the microbiomes of many different host species, our understanding of how environmental factors affect microbial community dynamics is still in nascent stages. In this dissertation, I investigate how biotic and abiotic environmental factors affect the diversity and composition of microbial symbiont communities within different host species. In chapter 1, I use a full factorial experimental plot design and culture-based methods to examine the effects of nutrient addition, large animal herbivore exclusion, and host tissue specificity on the fungal symbiont communities, known as endophytes, residing within the grass species Andropogon gerardii. While neither nutrient addition nor herbivore exclusion alone significantly affect the diversity or composition of culturable endophytes, in combination, nutrient addition and herbivore exclusion were associated with greater fungal symbiont diversity than found in other treatments. Further, while different host tissues harbored distinct fungal communities, diversity was greater in all plant host tissues sampled from plots with both nutrient addition and herbivore exclusion treatments. In chapter 2, using field collections and both culture- and sequence-based methods I compare fungal diversity and community composition in a migratory agricultural pest insect, Spodoptera frugiperda, its plant host, Sorghum bicolor, and soil collected beneath infested host plants. Finding fungal communities in insects were much more variable compared to fungal communities in plants and soil, I estimated contributions of these differing sources of fungal symbionts to the insect microbiome. Surprisingly, I find that insect fungal communities were more commonly attributed to other insect sources than to the plants on which they were feeding or to soil sources. In chapter 3, I examine the fungal symbiont communities in an overwintering population of S. frugiperda to the fungal symbiont of a migratory population to ask whether fungal symbiont communities differ over the course of migration. Specifically, I ask if the prevalence and abundance of entomopathogenic taxa is different in overwintering and migratory populations. In an analysis of environmental sources of insect fungal symbionts, I examined the fungal symbiont communities of plant hosts on which they feed at the overwintering and migratory sites. Fungal communities were surprisingly similar over the course of migration in insect hosts. While most fungal entomopathogens occurred in similar prevalence and abundance, there were two OTUs that were significantly different in abundance in the two states with one more abundant in overwintering populations and one more abundant in migratory populations. There was little evidence of fungal symbiont exchange between insects and plants. Together, these chapters characterize how both abiotic and biotic environmental factors affect fungal symbiont communities in plants and insects, the extent to which fungal symbionts may transmit among different hosts and ecological compartments, and how migration impacts microbial symbiont communities. This work has important implications for our understanding of the factors affecting microbial symbiont community dispersal and our ability to predict the effect of the environment on microbial symbiont communities in agriculturally significant species.