| Microbes have fundamentally shaped the evolution of metazoans on Earth by influencing the chemistry of the planet and altering the ecology of multicellular life-forms through symbioses. Herbivores present an example in which both of these influences are prominent; often, not only do symbiotic communities of microbes convert plant biomass into usable nutrients for their host, they also play a critical role in carbon cycling that influences global nutrient conditions and the climate. In this dissertation I explore symbioses between herbivores and their symbiotic microbial communities using fungus-growing insects, especially leaf-cutter ants, as a model. In Chapter 1, I review recent research into the composition and function of the fungus gardens of leaf-cutter ants to establish the current breadth of knowledge into these symbiotic ecosystems. In Chapter 2, I present a metagenomic and metaproteomic characterization of leaf-cutter ant fungus gardens that explores the identity and potential symbiotic role of bacteria that reside in these environments. I show that similar bacterial communities are present in the fungus gardens of different leaf-cutter ant species, and that these microbes likely play a role in nutrient biosynthesis and the provisioning of nitrogenous compounds. In Chapter 3, I present a draft genome of Leucoagaricus gongylophorus, the fungus cultivated by leaf-cutter ants, and use metaproteomic techniques to verify that this microbe produces a variety of enzymes for the degradation of plant biomass in leaf-cutter ant fungus gardens. Lastly, in Chapter 4, I use metagenomic techniques to compare the bacterial communities associated with leaf-cutter ant fungus gardens to those of other fungus-growing insects, including fungus-growing termites, Ambrosia beetles, mountain pine beetles, and southern pine beetles. Here I present evidence consistent with evolutionary convergence of these fungus-growing insects towards highly similar bacterial communities. The work presented in these chapters suggests that the fungus-bacteria communities cultivated by fungus-growing insects are simple, highly specialized, and have drastically affected the evolution of their host insect lineages. Moreover, due to the role of some fungus-growing insects as dominant herbivores and invasive pests of trees, this work underscores the importance of symbiotic microbial communities in shaping ecosystems around the planet. |
