Abstract
dc:description.abstractThe genetic material of all animals, plants and fungi is organised into chromosomes. While each species tends to have a fixed number of chromosomes in their genome, chromosome numbers vary widely across the tree of life. Changes in chromosome number occur due to rearrangements, such as the fusion and fission of chromosomes, and have been hypothesised to contribute to speciation and adaptation. However, we have only recently been able to reconstruct the exact evolutionary histories of chromosome rearrangements thanks to the availability of chromosome-level genomes. Consequently, the evolutionary forces and mechanisms that shape the varying rates of chromosome rearrangement across eukaryotes remain unclear. In this thesis, I use comparative genomic approaches to explore the evolution of chromosome structure across the insect order Lepidoptera, the butterflies and moths. I explore patterns of chromosome evolution across the whole order, and over a short timescale in Polyommatinae, a subfamily of blue butterflies. First, using over 200 chromosome-level genomes, I traced the dynamics of chromosome rearrangement across Lepidoptera. I found that the last common ancestor of Lepidoptera had 32 chromosomes and that the chromosomes of butterflies and moths have remained remarkably stable despite 250 million years of evolution and diversification. However, change does happen, I found that fusions, particularly between small chromosomes and sex chromosomes, have been the dominant form of karyotypic change. In contrast, I found that fissions were extremely rare. Against this stable backdrop, I identified several groups of species that have evaded the typical constraints and instead have undergone many fusion and/or fission events. Second, to understand the evolution of chromosome fission, I characterised the history of fission in Polyommatus atlantica, a species with 229 chromosomes — the highest known number of chromosomes of any non-polyploid eukaryote. I demonstrated that aspects of genome architecture, such as chromatin structure, influenced where fissions occurred. Further, I found that fission tended to occur in longer chromosomes. I also found that the genomes of various Polyommatus species that have undergone many fission events have hundreds of internal arrays of telomeric repeat sequences, suggesting a potential link to the evolution of fission. Third, I examined the parallel evolution of fission in Polyommatinae, a rapidly-radiating group of blue butterflies. Using 30 chromosome-level genomes, I inferred the phylogenetic history of this young group and found that three independent transitions from a low rate of chromosome rearrangement to a high rate of chromosome rearrangement have happened in genus Lysandra, and in Polyommatus subgenera Agrodiaetus and Plebicula. I identified several commonalities in the process of fission by characterising the complex histories of chromosome rearrangements in each group. While fission occurred at distinct precise sites in each group, similar regions of loosely-packed chromatin experienced fission each time. Overall, this work highlights the evolutionary constraints that shape the chromosome complements of animals and the repeatability of large-scale changes to chromosome structure. This thesis provides a framework for using comparative genomic approaches to study the mechanisms and evolutionary consequences of chromosome rearrangement across the diversity of eukaryotes.
Degree
thesis:*- Name dc:type.qualificationname
- Doctor of Philosophy (PhD)
- Level dc:type.qualificationlevel
- Doctoral
- Grantor dc:publisher.institution
- University of Cambridge
- Year dc:date.issued
- 2024
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Wright, Charlotte
- Advisors dc:contributor.advisor
-
- Blaxter, Mark
- Lawniczak, Mara
Subjects
dc:subject × 7Rights
dc:rightsIdentifiers
dc:identifier.*- DOI dc:identifier.doi
- https://doi.org/10.17863/CAM.119720
- OAI identifier oai:identifier
- oai:www.repository.cam.ac.uk:1810/386561