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Although there are approximately 250,000 extant angiosperm species, we still have much to learn about the speciation process, including the ways in which species boundaries are maintained among closely related taxa. Species are formed when populations become reproductively isolated from each other via genetic and morphological barriers that act before hybrid formation (prezygotic) or after hybrids are formed (postzygotic).
Erythronium albidum Nutt. and Erythronium mesochoreum Knerr (Liliaceae) differ in ploidy, are likely sister species, and have been reported to hybridize, making them well-suited for assessing the strength of multiple reproductive barriers. First, I assessed the frequency of hybrid occurrence at four contact zones throughout the U.S. Midwest. Hybrids were identified based on genome size using flow cytometry. I found that hybrids occurred infrequently, indicating that reproductive isolation between the study species is strong.
Next, I assessed the contributions of numerous pre- and postzygotic barriers to species boundary maintenance between E. albidum and E. mesochoreum. Using herbarium records from Midwestern states and study plots in eastern Nebraska, I found that flowering phenology for each species differs significantly on a broad geographic scale but can overlap substantially on local scales. This indicates that flowering asynchrony is not a consistently strong reproductive barrier. Further, by capturing insects visiting E. albidum and E. mesochoreum flowers, I found that the plants’ pollinator assemblages are significantly non-overlapping, which may serve as a strong reproductive barrier by severely limiting interspecific pollen transfer. Finally, in a hand-pollination experiment, I found that hybrid seed set was significantly lower than conspecific seed set.
Overall, these studies show that multiple reproductive barriers contribute to the maintenance of species boundaries between E. albidum and E. mesochoreum. However, these barriers varied in strength. Though many previous studies have emphasized the role of individual reproductive barriers for species formation and perpetuation, my results highlight the importance of considering the role of multiple barriers in species boundary maintenance among plants.
Advisors: Sabrina E. Russo and Diana Pilson