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UBC Theses and Dissertations

A clone together : exploring the causes and consequences of range divergence between sexual and asexual Easter daisies Hersh, Evan Whitney


Sexual and asexual organisms exhibit a wide variety of biological differences that can impact their ecological and evolutionary trajectories. One result of these differences is that closely related sexual and asexual taxa often exhibit range divergence, with asexuals typically having larger geographic ranges and being found at higher latitudes and elevations. This pattern, termed “geographical parthenogenesis”, has been documented in numerous plant and animal systems and a variety of potential mechanisms have been proposed. Hypotheses relate to differences in reproductive assurance (asexuals can reproduce without mates, while most sexuals require mates), genetic consequences of sexuality vs asexuality, selection on clonal lineages, ecological impacts of sexuality and asexuality, and demographic differences between reproductive modes. In this thesis, I explore several of these potential drivers of geographical parthenogenesis in Townsendia hookeri, a subalpine perennial flowering plant in the Asteraceae that has diploid sexual and polyploid apomictic (reproducing asexually through seed) forms with divergent but overlapping ranges. Population genomic analyses of apomicts in T. hookeri revealed largely monoclonal populations and geographically widespread clones, suggesting that apomictic range expansion may have been aided by “general-purpose genotypes” that can withstand an array of environmental conditions. Results from a large-scale, multi-year reciprocal transplant garden experiment showed that sexual populations had comparable performance when planted into the apomictic range as into their own, but that fitness of apomictic individuals generally declined in sexual regions. This provides evidence that while sexuals are likely limited by dispersal (they cannot reach suitable habitat outside of their current range), apomicts are not well adapted to the ecological conditions (biotic and/or abiotic) in the sexual range. When comparing early life history traits between sexuals and apomicts, apomicts were found to have increased germination success and improved seed dispersal traits in comparison to sexuals. These traits are expected to have given apomicts a colonization advantage, which (along with reproductive assurance) has likely contributed to their increased range size. Overall, the work presented in this thesis highlights the intricate nature of geographical parthenogenesis in Townsendia hookeri, and underscores the need to investigate complex biological phenomena using a diverse suite of approaches.

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