Dissertation Defense Schedule
Academic Excellence
Sharing original dissertation research is a principle to which the University of Delaware is deeply committed. It is the single most important assignment our graduate students undertake and upon completion is met with great pride.
We invite you to celebrate this milestone by attending their dissertation defense. Please review the upcoming dissertation defense schedule below and join us!
PROGRAM | Biological Sciences
Genotypic to phenotypic attributes of spontaneously-produced lysogenic phages of soybean bradyrhizobia
By: Vanessa Richards Chair: Jeffry Fuhrmann
ABSTRACT
Soybean (Glycine max) is a popular widely-grown legume rich in protein and oil used for human consumption, animal feed, and industrial purposes. As a cost effective and environmentally sustainable alternative to synthetic nitrogen fertilizers, soybean bradyrhizobia (Bradyrhizobium spp.) can meet the nitrogen requirements for high crop yields. Soybean bradyrhizobia in soil form a symbiotic relationship with soybean, fixing nitrogen (N2) into bioavailable ammonia (NH3) in root nodules. However, bradyrhizobia differ in their symbiotic effectiveness and saprophytic ecology, both of which may be altered by phage-mediated horizontal gene transfer (HGT).
The University of Delaware Bradyrhizobium Culture Collection (UDBCC) contains root nodule soybean bradyrhizobia isolates collected from 31 Delaware farms plus reference USDA strains. The collection (n=352) consists of species common to North America: Bradyrhizobium japonicum, B. diazoefficiens, B. elkanii, B. ottawaense. Many UDBCC strains spontaneously produce high numbers of phage in culture (without chemical induction). Characterization of these lysogenic phages and their potential for horizontal gene transfer is critical to understanding possible viral impact on the bradyrhizobia-soybean symbiosis.
In this dissertation research, a representative subset of 98 phenotypically and phylogenetically diverse strains from UDBCC was chosen for phage morphology screening. The majority of strains (69%) spontaneously produced virus-like particles when examined by epifluorescence microscopy. Transmission electron microscopy revealed variety among tail types, capsid shapes and collar or baseplate presence. Twenty-one UDBCC bradyrhizobia accessions, selected based on genotypic and phenotypic data, were sequenced and assembled to understand the genomic context between the four species. This analysis also highlighted differences of nodulation, nitrogen fixation and rhizobitoxine islands among genomes. Whole genome sequencing helped identify 39 putative prophages, representing the diversity of spontaneously produced phages in soybean bradyrhizobia which have the potential to mobilize bacterial DNA through HGT. Finally, a large-scale greenhouse experiment showed soybean plant growth phenotypes when inoculated with each of the genomically-characterized bacterial strains. The research in this dissertation connects bacteriophage genotypes, bacterial genotypes, and bacterial phenotypic characteristics to better understand how viral-host interactions impact the bradyrhizobial-soybean symbiosis.
The Process
Step-by-Step
Visit our “Step-by-Step Graduation Guide” to take you through the graduation process.From formatting your Dissertation to Doctoral Hooding procedures.
Dissertation Manual
Wondering how to set up the format for your paper. Refer to the “UD Thesis/Dissertation Manual” for formatting requirements and more.
Defense Submission Form
This form must be completed two weeks in advance of a dissertation defense to meet the University of Delaware Graduate and Professional Education’s requirements.
