Exploring the evolutionary significance of the mitochondrial genome

Abstract

Energy production in eukaryotic cells is mediated by oxidative phosphorylation (OXPHOS), which requires tight co-ordination between genes that span mitochondrial and nuclear genomes. Given the importance of OXPHOS to organismal function, it was traditionally assumed that strong selection would prevent non-neutral genetic variation from accumulating in the mitochondrial genome. Over the past decade, numerous studies have challenged this assumption by showing an abundance of mitochondrial genetic variation underpinning life-history trait expression. Currently, however, it is unclear how this variation is primarily shaped - by adaptive (natural selection) or non-adaptive (drift) processes. Furthermore, because of the maternal inheritance of the mitochondria, natural selection can only directly shape the mitochondrial DNA (mtDNA) sequence through females, thus facilitating the accumulation of mutations with male-biased effects (the Mother’s Curse hypothesis). The aim of my thesis is to explore the modes of coevolution between mitochondrial and nuclear (mito-nuclear) genomes. My first aim was to test the effect of different mtDNA haplotypes on male reproductive success, and investigate the hypothesis that flies with coevolved mito-nuclear genotypes will have higher fertility than flies with evolutionary novel mito-nuclear genotypes. Mitochondrial haplotypes affected male fertility. Furthermore, coevolved combinations of mito-nuclear genotype were associated with higher fertility than evolutionary novel counterparts. This suggests coevolved nuclear backgrounds harbour compensatory adaptations that offset the costs to males associated with the accumulation of male-harming mtDNA mutations. I then hypothesized that the Y chromosome would be enriched for adaptations involved in compensatory mito-nuclear coevolution. This is because the Y chromosome is only present in males, and should thus evolve to maximise male fitness. Furthermore, placement of nuclear counter-adaptations on the Y chromosome offers a potential resolution to the inherent inter-sexual conflict brought about by the Mother’s Curse hypothesis, since these compensatory adaptations would only be expressed in males and not interfere with female fitness. We found that interactions between the mtDNA haplotypes and Y chromosomes affected male mating outcomes, but these interactions were complex and contingent on the day of the mating assay. Overall, our results did not support the hypothesis that the Y chromosome harboured compensatory adaptations involved in male-mediated modes of mito-nuclear coevolution. Finally, I harnessed an experimental evolution approach to test the emerging ‘mitochondrial climatic adaptation’ hypothesis, which posits that the pool of standing genetic variation in the mitochondrial genome has been shaped under thermal adaptation. Following long-term selection of replicate populations to temperatures of 18 or 25°C, we report divergence of a solitary SNP in the mtDNA sequence, located in the mitochondrial large ribosomal RNA gene, across the selection treatment. While this indicates this SNP was a target of thermal selection, we were however unable to map this SNP to the capacity of flies to tolerate thermal stress. My PhD research indicates both non-adaptive and adaptive processes are likely to shape the mitochondrial genome, but further work is required to determine the relative contributions of each. I conclude the thesis by outlining worthy avenues for further research.