Graduate Theses & Dissertations

Clonal structure and mating patterns in a natural population of Sagittaria latifolia
Increased plant size is expected to have negative consequences for mating by increasing pollen transfer among the same plant. However, recent theoretical studies have demonstrated that this may not be true for clonal plants. Instead, clonal expansion could enhance outcrossing opportunities without increasing selfing by reducing distances to potential mates. I investigated how the spatial structure of clones influences patterns of pollen dispersal, selfing rates and siring success in a natural population of Sagittaria latifolia. I found that pollen dispersal distances typically exceeded the spatial extent of clones and there was a positive association between clone size and the likelihood that clones were intermingled. Together, this resulted in a weak positive association between clone size and selfing rates, and a strong positive association between clone size and outcross siring success. This is the first empirical support for the theoretical expectation that any negative effects of selfing in large clones might be offset by increased siring success. Author Keywords: clonal growth, fitness gain curve, geitonogamy, plant mating, plant reproductive ecology, sex allocation theory
Evidence for hybrid breakdown in the cattail (Typha) hybrid swarm in southern Ontario
Heterosis, expressed as phenotypic superiority over parental species, typically peaks in first generation hybrids (F1s), while later generations (F2 +) exhibit lower fitness. The decrease in hybrid fitness is called hybrid breakdown. The overall incidence of hybrid breakdown in invasive hybrid zones remains poorly understood. The Laurentian Great Lakes (LGL) region contains a hybrid zone comprised of: native Typha latifolia, Typha angustifolia, and hybrid Typha × glauca. F1 T. × glauca display heterosis and are invasive, while later generation hybrids are relatively rare. To investigate possible hybrid breakdown, I compared seed germination and plant growth of backcrossed and advanced-generation (F2) hybrids to F1s and T. latifolia. I found evidence for hybrid breakdown in F2s and backcrossed hybrids, expressed as reduced growth and germination rates. Expression of hybrid breakdown in F2s and backcrosses may explain their relative rarity in the LGL hybrid zone. Author Keywords: Advanced-generation hybrids, Backcrossed hybrids, Hybridization, introgression, Invasive species, plant competition
Investigating the regional variation in frequencies of the invasive hybrid cattail, Typha × glauca
Interspecific hybridization rates can vary depending on genomic compatibilities between progenitors, while subsequent hybrid spread can vary depending on hybrid performance and habitat availability for hybrid establishment and persistence. As a result, hybridization rates and hybrid frequencies can vary across regions of parental sympatry. In areas around the Laurentian Great Lakes, Typha × glauca is an invasive plant hybrid of native Typha latifolia and introduced Typha angustifolia. In areas of parental sympatry in Atlantic Canada and outside of North America, T. × glauca has been reported as either rare or non-existent. I investigated whether the low frequencies of hybrids documented in Nova Scotia, Atlantic Canada, are influenced by reproductive barriers that prevent hybrid formation or environmental factors (salinity) that reduce hybrid performance. I identified an abundance of hybrids in the Annapolis Valley (inland) and a scarcity of hybrids in coastal wetlands through preliminary site surveys throughout Nova Scotia. In Annapolis Valley populations, flowering times of progenitor species overlapped, indicating that asynchronous flowering times do not limit hybrid formation in this region. Viable progeny were created from interspecific crosses of T. latifolia and T. angustifolia from Nova Scotia, indicating that there are no genomic barriers to fertilization and germination of hybrid seeds. Typha × glauca germination in high salinity was significantly lower than that of T. latifolia, but there was no difference at lower salinities. Therefore, while germination of hybrid seeds may be impeded in the coastal wetlands where salinity is high, inland sites have lower salinity and thus an environment conducive to hybrid germination. However, I found that once established as seedlings, hybrids appear to have greater performance over T. latifolia across all salinities through higher ramet production. Moreover, I found that T. latifolia sourced from Ontario had reduced germination and lower survivorship in high salinities compared to T. latifolia sourced from Nova Scotia, which could indicate local adaptation by T. latifolia to increased salinity. These findings underline that interactions between environment and local progenitor lineages can influence the viability and the consequent distribution and abundance of hybrids. This, in turn, can help explain why hybrids demonstrate invasiveness in some areas of parental sympatry but remain largely absent from other areas. Author Keywords: flowering phenology, Hybridization, invasive species, physiology, pollen compatibility, salinity tolerance

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Graduate student theses produced at Trent University. You may also browse theses and dissertations by year, program, or author.

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