Shafer, Aaron AB

Hybridisation is an important evolutionary mechanism with diverse outcomes, including the formation of new lineages, the exchange of alleles between species, or their extinction through genetic swamping. In some cases, hybrids exhibit higher fitness than their parental species, which can lead to hybrid invasions that threaten ecosystems. In North America, hybridisation between the native cattail Typha latifolia and the non-native T. angustifolia produces the hybrid T. × glauca, which is a highly impactful invader in wetlands across large areas of southern Canada and northern USA; contrastingly, in Europe and Asia, where its parental species also co-occur, T. × glauca is rare to non-existent. This thesis examines the evolutionary history of T. latifolia and T. angustifolia, which, in addition to being the parental species of T. × glauca, are two of the most globally widespread Typha species, a genus of plants foundational to wetlands. First, we developed genomic resources, including ~12M nuclear SNPs and plastome assemblies to facilitate genetic research on Typha; we also described a cost-effective library preparation and genotyping protocol that makes population genetic studies of freshwater plants accessible. Then, we applied those genomic resources to investigate the roles of drift, selection, and hybridisation in the divergence of T. angustifolia, T. latifolia, and their sister species, T. domingensis and T. shuttleworthii. We found that speciation in these taxa was driven by drift under allopatry, resulting from historical bottlenecks, and that natural selection has played a minimal role in the divergence of these species; additionally, we observed introgressive hybridisation from T. latifolia into T. angustifolia. Finally, we reconstructed the demographic histories of T. angustifolia and T. latifolia from North America and Europe. We observed that reproductive isolation is strong in Europe, where the two species have potentially been sympatric for ~800,000 years, and weak in North America, where they have been sympatric for only a few centuries. Our results exemplify how the divergence and demographic histories of species can correlate with their strength of reproductive isolation. We emphasise that preventing invasions by hybrids will require limiting the movement of Typha and other historically allopatric species, which likely lack reproductive barriers.

Author Keywords: biological invasions, cattails, demographic histories, evolutionary histories, population genomics, reproductive isolation