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Multiple infections within the same host are now understood to be common and important determinants of the outcomes of disease processes. Multiple infections are particularly important in insects, which are often infected by vertically transmitted symbionts that are passed from the mother to her offspring. In many cases, these symbionts have evolved to confer high levels of protection against co-infecting parasites, pathogens, or other natural enemies. Despite widespread examples of symbiont-mediated defense, there are key outstanding questions in the ecology and evolution of defensive symbiosis. These include the mechanisms through which protection is conferred, the specificity of defensive effects against different parasites and pathogens, and the overall roles of defensive and other symbioses in host communities and ecosystems. To address these questions, I used a model of defensive symbiosis in which the bacterium Spiroplasma protects the woodland fly Drosophila neotestacea from the nematode parasite Howardula aoronymphium. First, I conducted a series of experiments that included transcriptome sequencing of \textit{D. neotestacea} infected by Howardula and Spiroplasma to uncover the mechanistic basis of defense in this symbiosis. Through these experiments, I found evidence of a putative protein toxin encoded by Spiroplasma that might contribute to defense. Following this, we characterized the protein as a novel member of a class of toxins known as ribosome-inactivating proteins (RIPs). RIPs are important virulence factors in bacteria such as enterohemorrhagic E. coli; I exploited recent approaches for quantifying RIP activity to design sensitive assays that demonstrate that Howardula suffers a high degree of ribosome cleavage specific to RIP attack during Spiroplasma-mediated defense. This is among the first demonstrations of a mechanism of defense against a specific enemy in an insect defensive symbiosis. I next worked with collaborators to culture and characterize a novel trypanosomatid parasite of Drosophila that I uncovered during the above transcriptome sequencing. Trypanosomatids are protist parasites that are common in insects, and the causes of important human diseases that include Chagas disease and African sleeping sickness. Despite Drosophila's history as an important model of infection and immunity, little is known of its trypanosomatid parasites, and we describe this parasite as a new genus and species: Jaenimonas drosophilae, the first trypanosomatid formally described from a Drosophila host. We conduct a series of experiments to understand infection dynamics, immune responses and interactions with other parasites and symbionts within the host, beginning to establish Drosophila-Jaenimonas infections as a tractable model of trypanosomatid infection in insects. Finally, though examples of ecologically important defensive symbioses accumulate, an understanding of their overall roles in ecosystems is lacking. I close with a synthesis of the ways in which symbioses - defensive or otherwise - can affect ecosystem structure and function through their effects on food webs. This work will help to develop a conceptual framework to link reductionist findings on specific symbioses to larger scale processes. / Graduate

Coral reefs are declining globally, calling for better ways to quantify coral health and predict resilience to future stress. The relationship between bleaching and fitness is key, as is reserve capacity to deal with physical trauma. This dissertation is an integrative study of the coral-algal symbiosis, holobiont performance under varied environmental conditions, and interactions between holobiont and environment on coral colony health and ability to recover from routine partial damage. I utilize the facultatively symbiotic, temperate coral Astrangia poculata as a natural model to explore the dynamics of colony health, performance, and the influence of environmental and nutritional stress under stable aposymbiotic and symbiotic states. Unlike most tropical hermatypic corals that rely heavily upon photosynthetic symbionts for energy, A. poculata can (1) flexibly use both heterotrophic and autotrophic nutritional pathways and (2) exist in naturally occurring, stable, and measurable aposymbiotic and symbiotic states. I begin by describing the impacts of environmentally relevant (winter, summer, and above range) temperatures on small-scale wound healing and recovery. Next, I explore the effects of nutritional and symbiotic states by comparing wound recovery, total colony health, host behavior, and symbiont performance in fed and starved colonies. Finally, I generate a novel reference transcriptome for A. poculata, and use computational approaches to characterize variation in gene expression between the symbiotic and aposymbiotic states. This analysis reveals that regardless of temperature, and with or without the potential for heterotrophic nutritional sources, a relationship with Symbiodinium enhances wound recovery and resilience to stress. Compromised healing ability and tissue cover at low temperatures suggest that in temperate stony corals, recovery and survival are more impacted by winter conditions than by exposure to high summer temperatures. Differential expression analysis revealed predictable enhancements to photosynthesis-related gene expression in symbiotic colonies. Together these results illuminate the complex interactions among symbiotic state, stress, recovery, and performance. We propose that studies like ours that examine the effects of combined stressors, as opposed to a monotonic focus on coral bleaching per se, are essential to clinical diagnosis and stewardship for coral reefs subjected to intense, cumulative human impacts.

acase@tulane.edu / Microbial organisms are powerful symbionts of macroorganisms due to their high abundance and diversity in every ecosystem as well as their wide range of metabolic pathways. Microbes have the potential to affect host fitness, and the microbial community function can range from mutualist to pathogenic. The composition of the community may be determined by the local microbial diversity, host-level selection, and effects of environmental chemistry and ambient conditions. In this dissertation, I use two systems, leaf-cutting ants and Jatropha curcas, to determine the factors driving symbiotic microbial community assembly as well as assess the effects of the microbial community on host characteristics. With leaf-cutting ants, I collected microfungi acting as symbionts within the leaf-cutting ant system through leaf-tissue symbionts (endophytes) and fungal garden/ant colony contaminants. Through micro-colony assays, my data suggest that the worker ants moderate their behavior in a species-specific rather than generalized fashion when responding to different types of microfungi. This demonstrates the power of a biotic selective agent in driving the microbial community assembly. With J. curcas, I use field and greenhouse studies to compare assembly of bacterial symbionts as well as greenhouse studies to assess the effects of different soil microbial guilds. I found the bacterial community within J. curcas is plastic and is influenced by major plant nutrients, the soil microbes, and the ambient environment. In the field, where the ambient environment was not constant and where environmental stresses were the strongest, we saw divergence in community based on location. In a greenhouse experiment, we grew J. curcas in soil with high microbial (fungal and bacterial) diversity, low microbial diversity, AMF diversity, and bacterial diversity (fungi suppressed). We found that soil with the highest diversity of microbes hampers plant growth, but AMF alone can increase plant growth. However, we saw no evidence for the ability of soil microbiota to increase resistance to generalist foliar pathogens. These studies highlight the importance of examining multiple factors influencing microbial community assembly and microbial effects on host traits. / 1 / Kimberly Mighell

Thesis (Ph.D.)--Murdoch University, 2005. / Thesis submitted to the Division of Science and Engineering. Includes bibliographical references (leaves 185-209)

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