Graduation Semester and Year
Spring 2026
Language
English
Document Type
Dissertation
Degree Name
Doctor of Philosophy in Quantitative Biology
Department
Biology
First Advisor
Jeffery P. Demuth
Second Advisor
Todd Castoe
Third Advisor
Esther Betran
Fourth Advisor
Daniela Palmer Droguett
Abstract
Sex chromosome evolution generates imbalances in gene dosage that can disrupt gene expression and organismal function. These imbalances are often resolved through dosage compensation mechanisms, yet the factors that drive the emergence and diversity of these systems remain poorly understood. This dissertation addresses a central question in evolutionary genomics: what governs the evolution of dosage compensation?
Focusing on dosage sensitivity as a potential driver, I evaluate the Insensitive Sex Chromosome Hypothesis (ISCH), which predicts that chromosome-wide compensation is more likely to evolve in genomic contexts that are not depleted of dosage-sensitive genes. Using flour beetles (Tribolium spp.) as a model system, this work integrates comparative and functional approaches to test this framework. In Chapter 1, I synthesize the current understanding of dosage compensation across taxa, highlighting the diversity of regulatory mechanisms and challenging the long-standing view that heterogamety predicts compensation outcomes. Comparative analyses indicate that neither male (XY) nor female (ZW) heterogametic systems are more likely to evolve complete dosage compensation or dosage balance. I further emphasize key limitations in the field, particularly the difficulty of defining and measuring dosage sensitivity, and outline the ISCH as a testable framework. In Chapter 2, I directly test the relationship between dosage sensitivity and expression evolution using genome-wide RNAi knockdown data. Dosage-sensitive, or more lethal genes are not depleted from the X or neoX chromosomes, consistent with ISCH predictions. However, gene-by-gene analyses among chromosomes reveal limited evidence that lethality strongly constrains expression evolution, suggesting that individual gene sensitivity does not predict expression divergence. In chapter 3, I evaluate expression breadth as an alternative proxy for dosage sensitivity. Broadly expressed genes (measured as low Tau, or expression in all tissues) exhibit greater constraint on expression evolution, supporting expression breadth as an indicator of pleiotropic constraint. However, differences in expression breadth among chromosomes are minimal and do not strongly explain patterns of dosage compensation. Together, my findings suggest that dosage sensitivity is not a primary determinant of dosage compensation evolution. Although the autosomal ancestor of the neoX chromosome retained a typical complement of dosage-sensitive genes, enabling hemizygosity to evolve within a permissive genomic context, sensitivity itself does not predict patterns of expression evolution or compensation. Instead, the evolution of dosage compensation is more likely shaped by higher-level regulatory mechanisms and evolutionary history.
Keywords
Evolution, gene expression regulation, Tribolium castaneum, Tribolium confusum, dosage balance
Disciplines
Biology | Computational Biology | Evolution | Genetics | Genomics
License
This work is licensed under a Creative Commons Attribution 4.0 International License.
Recommended Citation
Pau, Shana, "Dosage Sensitivity and the Evolution of Dosage Compensation: Tests of the Insensitive Sex Chromosome Hypothesis in Flour Beetles" (2026). Biology Dissertations. 1.
https://mavmatrix.uta.edu/biology_dissertations2/1
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Biology Commons, Computational Biology Commons, Evolution Commons, Genetics Commons, Genomics Commons