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UBC Theses and Dissertations

The effects of a FLOWERING LOCUS T polymorphism on phenology, reproductive isolation and hurricane survival in the silverleaf sunflower Pascual Robles, Mariana Alicia


Speciation is the process by which barriers to gene flow evolve between previous interbreeding populations. In this dissertation, I explore the earliest stages of speciation process, using the silverleaf sunflower (Helianthus argophyllus) as an experimental system. The species occurs on the coastal plain of southern Texas and is comprised of two ecotypes that differ in flowering time due to a presence/absence polymorphism for a major flowering time gene, FLOWERING LOCUS T (HaFT1). Asynchrony in flowering time can facilitate divergence and speciation. Therefore, I mapped the distribution of the HaFT1 polymorphism and explored its the effects on phenology and other traits. I found that HaFT1 was rare on the mainland, but both haplotypes were abundant in the barrier islands. The polymorphism at HaFT1 was closely correlated with variation in flowering time, plant height, seed size, and habitat. The rarity of HaFT1 is most likely due to high rates of immigration of the late flowering ecotype to the islands, combined with anthropogenic disturbance, which may ease establishment. I also asked if differences in flowering phenology cause assortative mating, thereby reducing gene flow and increasing reproductive isolation (RI). Parentage analyses revealed that RI was strong early and late in the flowering season when only one of the ecotypes was flowering, but weak during the middle of the season, when both ecotypes were flowering. A genomic scan based on re-sequencing data further revealed that even in the face of high gene flow in the present, assortative mating has had a genomic-wide impact, particularly in a region of low recombination where HaFT1 is found. Lastly, I took advantage of Hurricane Harvey, which hit my H. argophyllus sites, to ask what factors might aid survival of an extreme flooding event. I found that individuals that were shorter were more likely to survive. In the year following Hurricane Harvey, I observed massive establishment of H. argophyllus at sites where it was not present the previous year, suggesting the importance of seed banks in adaptation of the species to extreme climate events. Overall, H. argophyllus appears to be both resistant and resilient to extreme climatic events.

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