Investigating physiological collaborations between a lower termite and its symbionts
This project was completed in an effort to better understand the contributions of symbiotic microbes to the biology of Reticulitermes flavipes, the eastern subterranean termite. Lower-termites, like R. flavipes, house symbionts from all three domains of life within their hindgut paunch. This intimate association is reflected in nearly every aspect of termite biology. Here, I investigate these physiological collaborations as they relate to digestion and immunity. My efforts focused on 1) quantifying the role of bacteria in wood digestion within the termite gut, 2) evaluating the role of symbionts in protection against pathogens, and 3) identifying gene products that bacterial symbionts contribute to naïve and pathogen-challenged termites. Bioassays coupled with in vitro enzyme assays, gene expression analysis, and symbiont population estimations show that termites with reduced gut fauna have less lignocellulolytic potential. Using a suite of antimicrobial compounds, bacterial contributions (direct or indirect) to wood digestion were calculated on average at ∼23-50%. Apart from digestive potential, termite gut symbionts were also implicated in pathogen-resistance. Defaunated workers were 2-3X more susceptible to fungal infections and had significantly altered the expression of endogenous, immune-associated genes in response to challenge with the bacterial entomopathogen, Serratia marcescens. Using recombinant enzymes, in vitro assays, and bioassays, two protist-derived glycosyl hydrolase family 7 (GHF7) enzymes showed promise as a potential mechanism for symbiont-derived, anti-fungal defense in R. flavipes. These signatures of symbiont- mediated immunity/protection were further explored using RNAseq to capture a snapshot of the termite holobiont transcriptional response following pathogen challenge. This strategy served as a means of pinpointing critical taxa and physiological roles of microbes in this system by taking a global, metatranscriptomic approach. Differential expression analysis identified a bacterially-encoded amidohydrolase that may be important for anti-fungal defense. Overall, this project has significantly expanded our perspective on the importance of microbes in termite physiology as a whole. With examples from digestion and immunity, this research lays the groundwork for future explorations of termite-symbiota collaborations including, but not limited to, the collaborations described herein. Together, these results highlight the importance of a holistic “systems biology” approach to understanding termite biology from the perspective of the termite’s intimate associations with microbes. Assessing termite and microbe responses in isolation (as independent systems) may not provide an accurate account of the collaborative nature of this relationship. Additionally, my findings emphasize the importance of considering both organismal (termite and microbe) and sub-organismal (cellular and molecular) level processes when investigating physiology, symbiosis, and the link between them.
Scharf, Purdue University.
Entomology|Microbiology|Evolution and Development
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