Date of Thesis
Hybridization is an important evolutionary pathway that has contributed to the world’s vast biodiversity. In plants, especially angiosperms, hybridization is known to be an important mechanism for speciation, phenotypic divergence, and changes in reproductive systems. Solanum species present an ideal system to investigate how hybridization between two different sexual systems impacts the reproductive and phenotypic biology of the hybrid progeny. Hybrid seeds were acquired from crosses between Australian Solanum species Solanum dioicum (dioecious) and S. ultraspinosum (andromonoecious) in order to track what happens when you cross two plants with different sexual systems. Vegetative and floral morphological measurements were conducted, and the data was analyzed using an ANOVA and PCA to evaluate phenotypic differences across generations. Pollen tube growth was evaluated under a microscope using fluorescent microscopy technique to observe whether pollen tube growth occurred and whether it reached the ovary, providing insight into crossing success or failure. The only successful hybrids from the original crosses were those derived from S. dioicum as the pollen donor and S. ultraspinosum as the pollen recipient. Due to strong maternal effects, all F1 hybrids resembled S. ultraspinosum, thus all F1 plants were andromonoecious. The F2 and F3 hybrids demonstrate variability in inflorescence architecture, specifically the persistence of cosexual flowers in the staminate position of an andromonoecious inflorescence and the abortion of staminate buds, which may be suggestive of a change in sexual system. A principal component analysis supported that the F1 and F2 hybrids were distinct from both parents, but were most similar to S. ultraspinosum, the pollen recipient, while the F3 hybrids clustered independently. In attempts to create an F3 and F4 hybrid generation, nearly all of our crosses have failed—suggesting that a hybrid breakdown is occurring. The observation of pollen germinating but failing to reach the ovary by fluorescent microscopy technique suggests that pollen tube abortion in the style is contributing to hybrid breakdown. This study should promote a better understanding of hybridization—a driving force in plant diversification—among Australian Solanum, a group in which hybridization is known to be widely possible but rarely confirmed in nature. Likewise, hybridization between taxa with two distinct sexual forms may shed light on the evolution of reproductive strategies in this clade.
dioecy, hybridization, andromonoecy, Solanum, sexual systems, fluorescence microscopy
Bachelor of Arts
Minor, Emphasis, or Concentration
Melody P. Sain
Zizis, Diamanda A., "Heading for a Breakdown: Assessing Evolution Through the Hybridization of Two Sexual Systems" (2023). Honors Theses. 650.