Year of Award

2026

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

Brandon Cooper

Commitee Members

Jeffrey Good, Scott Miller, Beverly Piggott, Christopher Vassallo

Keywords

Drosophila, endosymbiosis, evolution, life history, titer, Wolbachia

Abstract

Symbiotic interactions are widespread across the tree of life, and many insects associate with vertically transmitted intracellular bacteria whose fitness is tied to host reproduction. Because these endosymbionts transmit through the maternal line, selection often favors traits that increase the production and success of transmitting females. Over time, endosymbionts can evolve functions such as nutrient provisioning, defense against predators and pathogens, and contributions to environmental stress tolerance, thereby shaping host ecology and evolution. Yet these effects depend critically on the abundance and localization of endosymbionts within host tissues. This dissertation examines these processes in Wolbachia, the most common known animal endosymbiont and a powerful system for linking endosymbiont density and localization to variation in host fitness that shapes population-level outcomes. Chapter 1 reviews the origins, consequences, and biology of endosymbiosis, with emphasis on Wolbachia spread, titer regulation, and the evolutionary importance of host-endosymbiont interactions. Chapter 2 shows that Wolbachia density is dynamically partitioned across tissues in divergent Drosophila hosts: conserved ovarian enrichment arises through distinct developmental routes, whereas somatic titer across systems ranges from nearly absent across all tissues to strong enrichment in specific tissues, and brain-associated density better predicts Wolbachia-associated behavioral effects than does whole-body density. Chapter 3 demonstrates that Wolbachia fitness effects can vary markedly, from strongly positive to negative across host systems and genomic backgrounds. Chapter 4 shows that this variation is explained in part by tissue-specific titer and genomic context: negative fitness effects can stem from elevated Wolbachia density in somatic or reproductive tissues, while system-specific contributions of host nuclear, cytoplasmic, and cytonuclear interactions underlie variation in components of host fitness. Together, these results demonstrate that Wolbachia-associated phenotypes emerge from interactions among tissue-specific bacterial regulation and host and endosymbiont genomes. By linking within-host mechanisms like titer to host fitness and evolutionary dynamics, this dissertation advances our understanding of endosymbiosis in natural systems.

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© Copyright 2026 John Paul Statz