Year of Award

2015

Document Type

Dissertation

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Cellular, Molecular and Microbial Biology

Department or School/College

Division of Biological Sciences

Committee Chair

Scott Miller

Commitee Members

J. Stephen Lodmell, Frank Rosenzweig, Jeffrey Good

Abstract

Symbiosis occurs between organisms in all domains of life. The evolution of obligate symbionts from free-living bacteria typically results in the loss of genes involved in metabolic independence and an overall reduction in genome size. Outside the organelles, the most extreme examples of genome reduction come from the intracellular symbionts of sap-feeding insects. The genomes of these bacteria encode very few genes other than those involved in translation, replication, and amino acid synthesis. Candidatus Hodgkinia cicadicola (Hodgkinia) and Candidatus Sulcia muelleri (Sulcia) live in specialized insect cells (bacteriocytes) of the cicada Diceroprocta semicincta, and have undergone severe gene loss. Hodgkinia in particular retains one of the smallest gene sets of all bacteria, and even less than many organelles. As a result, the Hodgkinia genome is left with a seemingly incomplete set of genes that are required for cellular life, including core genes in the translational machinery. I analyzed a set of Hodgkinia genomes and performed several experiments to uncover the constraints guiding the evolution of Hodgkinia. What mutational and selective pressures are acting on the Hodgkinia genome? How do essential cellular enzymatic reactions occur in Hodgkinia cells? Does the cicada host complement Hodgkinia's limited genetic repertoire? How does the evolution of insect endosymbionts compare to the evolution of organelles? My work provides answers to many of these questions, and deepens our understanding of intracellular symbioses.

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© Copyright 2015 James Theodore Van Leuven