The Effects of Asexuality and Selfing on Genetic Diversity, the Efficacy of Selection and Species Persistenc

Understanding the origin, maintenance and distribution of nature's astonishing diversity of reproductive systems is a major endeavour in evolutionary biology. One of the oldest evolutionary puzzles is the rarity of species that undergo high rates of asexual reproduction or self-fertilization despite their significant benefits over sex and outcrossing. My thesis investigates the evolutionary consequences of asexuality and selfing using experimental evolution, genomic analyses and theoretical modeling. In chapter 2, I performed a mutation accumulation experiment using the facultatively sexual rotifer, Brachionus calyciflorus, under different population sizes with UV-C radiation. Our finding suggests that large effect mutations are relatively common in B. calyciflorus, at least under UV radiation. In chapter 3 and 4, I examined genomic diversity in natural isolates of the facultatively sexual duckweed, Lemna minor, and B. calyciflorus . We found genomic signatures suggesting L. minor reproduces mostly through asexual propagation but also evidence that it reproduces sexually more often than a related duckweed species, Spirodela polyrrhiza . We found that B. calyciflorus possessed low neutral diversity and relatively efficient purifying selection, but anomalies in the data limited the reliability of our results. These chapters are among the few studies that analyze genome wide diversity in facultative sexual species. In chapter 5, I performed simulations of highly selfing populations experiencing selection that fluctuates in magnitude (not in direction) over time. We found that highly selfing populations accumulate deleterious mutations more quickly when selection fluctuates and that mutations fix disproportionately at loci in the genome that responds to fluctuating selection compared to loci that are constantly selected. This highlights the importance of environmental heterogeneity when considering fitness decline and extinction in selfing populations. In chapter 6, I extend a classic species selection model to track the abundance of asexual and sexual species when diversification rates of asexuals decline with age. Our results broaden the parameters space where sexual and asexual species can coexist. In summary, my thesis contributes data related to selection efficacy and genomic diversity of facultatively sexual species, as well as providing novel theoretical insights into the evolutionary fate of asexual and selfing species.