BORIS Theses

BORIS Theses
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On the balance of drift and selection: the evolution of the Orkney vole

Wang, Xuejing (2023). On the balance of drift and selection: the evolution of the Orkney vole. (Thesis). Universität Bern, Bern

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Demography and selection have been under the spotlight for a long time in evolutionary biology. As human activities lead to increased risk of population fragmentation and biological invasion, both of which involve drastic demographic changes, the importance of advanced knowledge about the impacts of genetic drift in the short and long term is increasing. Populations on islands are ideal models to study adaptive and non-adaptive evolutionary processes simultaneously. In isolated island populations, the efficacy of purifying selection is reduced by genetic drift, leading to accumulation of deleterious variants in homozygous state, hence reduced fitness (mutation load). On the other hand, island populations often show phenotypic differences when compared to the continental populations (island syndrome), which are considered to be related to divergent selection posed by the contrast in environmental factors. In this thesis, I used the Orkney vole (Microtus arvalis orcadensis) as my model to investigate the genomic consequences of bottlenecks and long-term isolation. The Orkney vole was introduced by Neolithic farmers from the European continent, and isolated since introduction for over 5,000 years, providing a unique opportunity to study the long-term effects of isolation in nature. In Chapter 1, I reconstructed the detailed demographic history of Orkney populations and found that Orkney voles have been through a strong bottleneck related to the introduction. I further investigated the mutation load in Orkney populations and found high fixation of potential deleterious alleles. In Chapter 2, I looked at the genomic landscape of Orkney voles and found genome-wide relaxation of purifying selection. I performed genomic tests for divergent selective sweeps, and detected signatures indicating the reduction of positive selection in Orkney voles related to their increase of body size. The research of mutation load and selection in most species has been mainly constrained to autosomes so far. Inherited along with the autosomes, the sex chromosomes undergo disparate evolutionary paths not only because of their functions but also differences in ploidy. In Chapter 3, I first assembled the sex chromosomes of the common vole. With population genomic data, I found that the autosomes, X and Y chromosomes had different levels of genetic diversity, accumulation of deleterious alleles, and genetic responses to severe bottlenecks. Such differences are likely correlated to the ploidy of the chromosomes and sex-biased mutation rates.

Item Type: Thesis
Dissertation Type: Cumulative
Date of Defense: 6 June 2023
Subjects: 500 Science > 570 Life sciences; biology
Institute / Center: 08 Faculty of Science > Department of Biology > Institute of Ecology and Evolution (IEE) > Population Genetics
Depositing User: Sarah Stalder
Date Deposited: 12 Jul 2023 06:35
Last Modified: 06 Jun 2024 22:25

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