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Insights into the role of diptericin A on Drosophila immunity, the microbiome, and life history traits.
Mullinax, Sarah Rebecca Wenske
Mullinax, Sarah Rebecca Wenske
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Abstract
All organisms must mount effective immune responses against pathogens. However, an immune response can come at a cost for the organism. These costs include expending energy and resources, autoimmune diseases, and imbalances of the gut microbiota. Understanding how an effective immune response balances suppressing systemic pathogens while not harming the host is an important and active area of research.In most eukaryotes, antimicrobial peptides (AMPs) play a major role in the innate immune response. AMPs combat Gram-positive and Gram-negative bacteria, and fungi; thus, in the face of increasing antimicrobial resistance, development of new antimicrobial agents focuses on AMPs. Understanding the evolution of AMPs as an effective innate immune response both systemically and within the gut, and whether naturally occurring variants are functionally equivalent, may aid the development of robust antimicrobial medicines. Diptericin is a Drosophila AMP that is an effector gene of the NF-kB pathway, IMD, which is the more Gram-negative specific pathway in Drosophila. It is a secreted peptide with no apparent structure. Diptericin is interesting to this research because it has an amino acid polymorphism (S/R) which relates to the immune response to the Drosophila bacterial pathogen, Providencia rettgeri, and is believed to be under balancing selection. Homozygous serine flies are better able to survive systemic infection better than homozygous arginine flies, yet the homozygous arginine allele is maintained at the population level. Why the arginine allele, which does not contribute to the fitness of a fly after infection with P. rettgeri, is maintained in populations is unknown. Here, I used CRISPR/Cas9 genome editing to edit the diptericin gene in D. melanogaster, which resulted in fly lines with the serine, arginine and null alleles on an otherwise controlled genetic background. In chapter 1 I show that CRISPR/Cas9 genome edited flies exhibit very similar survival after P. rettgeri infection as observed in inbred lines. I then expand my analysis to assess how the polymorphism effects survival after systemic and oral infection with other types of bacteria. Although homozygous serine flies consistently have a fitness advantage after systemic infection in our lab, when studying the microbiome, homozygous arginine flies possess a fitness advantage. Specifically, homozygous arginine flies survive an association with the common Drosophila gut bacteria, Lactobacillus plantarum and Acetobacter tropicalis, better than homozygous serine flies. We then studied potential fitness tradeoffs between diptericin genotype and other life history traits. Chapter 2 addresses the effects of diptericin genotype on survival after non-infection stresses. I show a difference in resistance to stresses based on diptericin genotype, specifically in starvation resistance. Homozygous arginine flies survive starvation stress better than other genotypes, contrary to infection stress where homozygous serine flies show a fitness advantage. This dissertation characterizes some of the possible mechanisms of balancing selection for the amino acid polymorphism in Diptericin that leading to population level maintenance of allelic variation.
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Date
2022-08-31
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University of Kansas
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Keywords
Molecular biology, antimicrobial peptide, diptericin, Drosophila, evolution, immunity