Jardine, Michael; (2022) The maintenance of genetic variation by balancing selection. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Adaptive evolution occurs when selection acts on genetic variation for phenotypic traits. In doing so, selection is expected to remove fitness variation in the population. Contrary to this expectation, DNA sequencing has shown that populations harbour high levels of standing genetic variation for fitness. This paradox results in a long-standing question: what maintains genetic variation? One possible mechanism is ‘balancing selection’, where selection actively maintains polymorphism. Once considered unlikely, studies using genomic and phenotypic approaches have recently given new support for balancing selection and have provided evidence of balancing selection in several species. However, it is often difficult to connect genetic and phenotypic evidence for balancing selection with evidence of the action of selection in real time. This limits our understanding of how balancing selection occurs and its contribution to maintaining genetic variation. To address these knowledge gaps, I first assayed the fitness effects of a polymorphism in the Drosophila melanogaster gene fruitless, which shows a signature of balancing selection in wild populations. I show that this polymorphism displays antagonistic pleiotropy, a possible mechanism for balancing selection at this locus (Chapter 2). I next used experimental evolution and pool-sequencing to track the frequency of the fruitless polymorphism over time in laboratory populations (Chapter 3). I was able to demonstrate that the fruitless polymorphism is probably evolving under balancing selection in these populations, although this result is complicated by 44% of putatively neutral SNPs also being diagnosed as under balancing selection. I next expanded this approach to diagnose selection at 397 candidate sexually antagonistic SNPs. 60% appeared to be under balancing selection (Chapter 4). The equilibrium allele frequency of these SNPs was positively related to that in two wild populations, illustrating that the short-term evolution in the cages is correlated to long-term evolution in wild populations. That shows that selection is consistent and supports the inference of balancing selection. Overall, this thesis describes the action of balancing selection in maintaining fitness influencing polymorphisms in D. melanogaster and develops methods to diagnose active balancing selection at the population level.

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