Mackintosh, Carl J; (2023) Ecological and evolutionary impacts of chromosomal inversions. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Inversions are genomic structural variants in which a segment of a chromosome is reversed orientation. One consequence is the near-total suppression of recombination within the inverted region. This region of tight linkage can have profound implications for the evolution of the genome, by enabling sets of alleles to be inherited as a single unit without being in close proximity. This thesis investigates how inversions evolve and influence evolution, from the genome up to the level of populations. One example is the architecture of meiotic drivers --- genetic elements that have a transmission advantage over their wild-type counterparts. These systems often comprise multiple loci within an inversion. The first chapter models the spread and demographic consequences of X-linked meiotic drivers in the face of the associated fitness costs. When the costs are such that X-drive can remain polymorphic, the resulting female-biased sex ratio increases the equilibrium population size and persistence time relative to a wild-type population. Inversions may also promote the evolution of local adaptation under gene flow by linking coadapted alleles and preventing the production of hybrid, maladaptive genotypes. Work on this phenomenon often considers the limited case of a ‘’continent-island’’ model. We extend this and include the probability of the formation of locally adaptive inversions. Our results contrast with a simple interpretation of existing literature, suggesting strongly selected loci are likely to underpin locally adaptive inversions. Higher mutation load is expected in regions of suppressed recombination, due to the reduced efficacy of purifying selection on deleterious alleles. Furthermore, in the inversion case, the mutation load captured by an inversion is likely to persist throughout future lineages. The relative importance of each of these phenomena depends on how long the inversion spends at a low frequency. We use simulations to determine the conditions in which the accumulation of deleterious mutations during the early stages of inversion spread is important in deciding the fate of an otherwise adaptive inversion. This is placed in the context of the existing literature, which underestimates the effect of mutation accumulation during the early stages of inversion lineages.

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