Year of Award

2026

Document Type

Dissertation

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Ecology and Evolution

Department or School/College

Division of Biological Sciences

Committee Chair

Scott R. Miller

Commitee Members

Matthew Church, Anna Sala, Frank Rosenzweig, J. Patrick Kociolek

Keywords

comparative genomics, cyanobacteria, diatoms, endosymbiont evolution, endosymbiosis, nitrogen-fixing

Abstract

Endosymbiosis, where one cell lives inside of a different cell, is ubiquitous in nature and has enormous impacts on the ecology and evolution of both the host and the endosymbiont [1, 2]. These relationships exist along a continuum ranging from facultative—where both partners are able to live independently—to completely obligate—where partners can only survive in the presence of the other. The most famous examples being the chloroplast and mitochondrion [3]. After hundreds of millions or even billions of years of coevolution, highly-integrated endosymbionts are coordinated with the host on the genetic, metabolic, and cellular levels, but we do not understand the mechanisms that drive this integration, or if there is a particular order in which these changes must occur. Addressing the mechanisms of early integration between partners is crucial for understanding how long-lasting endosymbiotic relationships successfully establish.

In my dissertation, I explore the early stages of host-endosymbiont evolution using diatoms in the family Rhopalodiaceae that host nitrogen-fixing endosymbionts called spheroid bodies (SBs) [4]. In chapter 1, I introduce the Rhopalodiaceae-SB system and the culture collection I developed for the work in subsequent chapters. In chapter 2, I examine changes in SB gene expression since host-restriction and report drastically reduced variation over diel cycles [5]. In chapter 3, I expand on SB evolutionary history using phylogenetics and comparative genomics, providing a better understanding of the closest free-living relatives to SBs and the general progression of SB genome reduction, and bring insights on how SBs are integrated with their hosts [6]. In chapter 4, I shift to exploring the relationship from the host perspective by examining the costs and benefits of maintaining SBs. This chapter provides an unprecedented look at the cellular integration between hosts and SBs, and addresses the context-dependent metabolic burden imposed on the hosts by SBs. Broadly, my dissertation furthers our understanding of the functional and evolutionary dynamics of the Rhopalodiaceae-SB endosymbiosis and what enables these long-term relationships to succeed. Further, by leveraging a well-established, yet evolutionarily young, endosymbiotic relationship, my work identifies key processes during endosymbiont integration and evolution.

In addition to the work presented here, the culturing and genomic resources I have established are a part of a broader research effort to expand and revise our knowledge of the diversity and evolution of the diatom order Rhopalodiales, which has led to multiple other related projects and publications [7–9]. Collectively, my dissertation and related projects contribute to fundamental knowledge about the evolution of endosymbiosis.

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© Copyright 2026 Heidi Elizabeth Abresch