Date of Award
Doctor of Philosophy (PhD)
Plant and Environmental Science
Sarah A. White, Committee Chair
The genus Magnolia comprises over 250 species naturally occurring in Eastern North America, Central America, South America, islands of the Gulf of Mexico, and Eastern Asia. A long history of cultivation and breeding going back to the early nineteenth century reveals a common trend of reproductive compatibility throughout this group of basal angiosperms. Interspecific hybridization has led to plethora of ornamental cultivars that have proven adaptable to natural and cultivated habitats beyond the range of each species. Understanding of taxonomic relationships between species has evolved as observations of morphological characteristics and genetic analyses have been more thoroughly investigated. The objectives of this research were to 1) Perform crosses that lead to novel hybridization, 2) improve understanding of inheritance of morphological traits through the phenotypic variation observed in hybrid progeny, and 3) improve understanding of inheritance of morphological traits through the analysis of the molecular variation observed in hybrid progeny.
Over 30 novel crosses were performed with species and previously developed hybrids from each of the 3 recognized subgenera and 9 of the 12 recognized sections. Progeny were raised, and hybridity verified by intermediate morphological characteristics. In selected crosses, hybridity was also supported by intermediate relative genome size determined by flow cytometry or by DNA separation by gel electrophoresis. Successful interspecific crosses were achieved within and between sections, and intersubgeneric crosses were achieved with Magnolia lotungensis of Section Gynopodium. Intersectional crosses typically provided fewer offspring for evaluation than intrasectional crosses. A comparative study of sibling variation was performed with an intrasectional cross, Magnolia foveolata × Magnolia laevifolia (Section Michelia), and an intersectional cross, Magnolia sieboldii (Section Rytidospermum, Subsection Oyama) × Magnolia insignis (Section Manglietia). The morphometric comparisons of hybrid populations displayed a gradient of intermediacy typically seen in inheritance influenced by quantitative trait loci in the intrasectional hybrid for the majority of the measured traits. However, in the intersectional hybrid, the presence of extreme phenotypes evidenced by multiple traits exhibiting negative heterosis indicated inheritance influenced by transgressive segregation. Inter Simple Sequence Repeat (ISSR) markers revealed 35.7% polymorphism among siblings in the intrasectional cross and 45.2% polymorphism in the intersectional cross. These polymorphism percentages are comparable to other woody plant species indicating adequate genetic variation is present for interspecific magnolias to undergo speciation in the absence of reproductive barriers. In a combined analysis of each set of hybrids and parental taxa, a pairwise matrix of relative genetic distances revealed no significant difference in the distance between parental species in the intersectional (0.362) and intrasectional (0.392) cross. An unrooted Neighbor Joining dendrogram clustered the taxa into two groups with hybrid progenies distinct from their parents. The greatest genetic distance was between each set of new hybrids (0.763) illustrating ongoing divergence via hybridization. These genetic variations also present many opportunities for selection of superior ornamental plants.
Parris, James Kevin, "Magnolia: Impact of Interspecific Hybridization on Genetic Variation and Ongoing Breeding Initiatives" (2018). All Dissertations. 2538.