Graduation Semester and Year




Document Type


Degree Name

Doctor of Philosophy in Quantitative Biology



First Advisor

Jeffery Demuth


Cytogenetic data in the form of karyotypes are complex, highly-variable traits that offer opportunities to detect changes in genome organization, uncover phylogenetic history, and distinguish cryptic species. However, the synthesis of cytogenetic data across large taxonomic scales has been rare. Here I report the insights gained through a synthesis of all available karyotype data from Arthropoda. The first chapter focuses on two databases that were built to make these data openly available, and broad insights into the evolution of sex determination and chromosome number in Arthropoda that the collected data have made possible. This is followed by four chapters that use these data to address fundamental questions in evolutionary biology. In chapter two I address the question, why do some clades frequently lose Y chromosomes while they are rarely lost in others? I propose the fragile Y hypothesis that suggests meiotic mechanisms are of central importance in explaining the phylogenetic distribution of Y chromosome loss. In chapter three I address the question, why do some clades exhibit near stasis in chromosome number while closely related clades show great variation? Using data from beetles in a comparative framework I show that when species evolve traits that reduce effective population size rates of chromosome number evolution increase dramatically suggesting that observed differences in chromosome number are underdominant while segregating and are fixed through random drift in small populations. In chapter four I address the question, does low chromosome number increase the probability of evolving haplodiploidy? I develop a novel comparative method to test this long-standing hypothesis first proposed by Bull (1983). The results indicate that low chromosome number increases the probability of transitions to haplodiploidy. In chapter five I address the question, do eusocial Hymenoptera have higher chromosome number than solitary Hymenoptera? Using a larger dataset than previous studies, I show that there is no support for an absolute difference in chromosome number of solitary and eusocial Hymenoptera. Instead, I find that eusocial Hymenoptera have much higher rates of chromosome evolution when compared to their solitary relatives suggesting variable selection pressure or reduced effective population size in eusocial hymenoptera. As a body of work these analyses illustrate that chromosome number and meiotic mechanisms can impact the evolution of sex determination systems and that the evolution of chromosome number is often strongly influenced by the traits an organism exhibits.


Biology | Life Sciences


Degree granted by The University of Texas at Arlington

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