Polyploid speciation in plants: Influences on hybridization and ecological differentiation in the native, perennial herb, Houstonia Open Access
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Speciation of plant lineages can be shaped by a variety of factors. For instance, polyploidy, hybridization and ecogeographic isolation can independently or collectively contribute to diversification. Polyploidy is prominent within the plant kingdom (e.g., approximately 35% of plant species contain a polyploid lineage) and it is estimated to be responsible for 15% of all speciation events within angiosperms. To this end, polyploidy can effectively reproductively isolate individuals due to genetic incompatibilities caused by differences in ploidy-level between parents and offspring or closely related taxa, and thus prevent backcrossing or hybridization. Notably, it is estimated that approximately 40-70% of angiosperms have hybridization events in their evolutionary histories. Despite this, some extant plant species do not hybridize. It is hypothesized that polyploidy may break down species barriers and lead to hybridization between species that may not hybridize as diploids. Further, ecogeographic isolation or the distribution of populations due to ecological factors can affect reproductive isolation between populations. Models of polyploid establishment suggest that environmental divergence is important in the establishment of polyploid individuals in a population. Consequently, ecological factors can affect the establishment and distribution of hybrid and polyploid offspring. My dissertation focused on the roles of whole genome duplication and environmental habitat differences in hybridization and speciation within the native plant genus, Houstonia. The main hypothesis tested the role of polyploidy on hybridization patterns. My results demonstrated that polyploids hybridize more readily than their diploid counterparts. Further, I examined hybridization patterns between a rare species and its common congener using a combination of morphological and molecular data. This study found hybridization had occurred in natural populations and outlined the role of hybridization in the delicate relationship between rare and common species. Lastly, I compared environmental habitats among species, between ploidy-levels of a species as well as between hybrid populations and their parent species. Results showed that habitat differences contributed to the distribution and potential ecogeographic isolation of Houstonia species. Taken together, these results demonstrate the impact of polyploidy, hybridization and habitat on the evolution of Houstonia species and contribute to our understanding of how these factors jointly influence plant speciation.