UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
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Item The role of host-plant hybridization in host-associated population divergence in Phytomyza glabricola (Diptera: Agromyzidae)(2012) Hebert, Julie Byrd; Hawthorne, David J.; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Phytomyza glabricola (Diptera: Agromyzidae) is a leaf-mining fly native to the eastern United States that mines two sympatric native holly species, Ilex coriacea and I. glabra. Recent work revealed significant genetic divergence between host-associated populations of flies in North and South Carolina, suggesting the populations are host forms and recent work in Ilex phylogenetics hint the two holly hosts may hybridize. In this work, I investigated potential ecological speciation in P. glabricola, hybridization in its host plants, and how the hybridization among host plants may affect gene flow between host forms of the flies. No-choice mating trials in a greenhouse revealed reproductive isolation between host forms of P. glabricola and suggested female flies are capable of making oviposition mistakes resulting in adult offspring on the non-natal host. Based on these results, I used sequences of the nuclear gene EF-1α and AFLPs to genetically confirm host form status of the flies, and identify I. glabra as the ancestral host. In addition, genome scans revealed several loci under divergent selection among the hosts, suggesting the flies may be undergoing ecological speciation. To investigate the role host plants may play in the genetic divergence among flies, I first used AFLPs to confirm hybridization between I. coriacea and I. glabra. Hybridization rates differed across the geographic range of the species, which was also reflected in the morphology of the leaves. There were no general patterns, however, in the phenotypes of hybrid plants, and no single morphological trait that could reliably identify the hybrids. Finally, I combined genetic data of the flies and the plants to determine whether hybrid plants serve as bridges or barriers for the flies. Population comparisons revealed a significant positive relationship between hybridization in the plants and gene flow in the flies, and individual comparisons indicated flies are using the hybrid plants, albeit at low levels. The results suggest hybrids could serve as bridges between parental species, helping explain how a species from a typically monophagous lineage could expand its host range.Item Geography and Genetics of Ecological Speciation in Pea Aphids(2008-01-23) West, Joan Alette; Hawthorne, David J; Behavior, Ecology, Evolution and Systematics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)During ecological speciation, divergent natural selection drives evolution of ecological specialization and genetic differentiation of populations on alternate environments. Populations diverging onto the same alternate environments may be geographically widespread, so that divergence may occur at an array of locations simultaneously. Spatial variation in the process of divergence may produce a pattern of differences in divergence among locations called the Geographic Mosaic of Divergence. Diverging populations may vary in their degree of genetic differentiation and ecological specialization among locations. My dissertation examines the pattern and evolutionary processes of divergence in pea aphids (Acyrthosiphon pisum) on alfalfa (Medicago sativa) and clover (Trifolium pretense). In Chapter One, I examined differences among North American aphid populations in genetic differentiation at nuclear, sequence-based markers and in ecological specialization, measured as aphid fecundity on each host plant. In the East, aphids showed high host-plant associated ecological specialization and high genetic differentiation. In the West, aphids from clover were genetically indistinguishable from aphids on alfalfa, and aphids from clover were less specialized. Thus, the pattern of divergence differed among locations, suggesting a Geographic Mosaic of Divergence. In Chapter Two, I examined genomic heterogeneity in divergence in aphids on alfalfa and clover across North America using amplified fragment length polymorphisms (AFLPs). The degree of genetic differentiation varied greatly among markers, suggesting that divergent natural selection drives aphid divergence in all geographic locations. Three of the same genetic markers were identified as evolving under divergent selection in the eastern and western regions, and additional divergent markers were identified in the East. In Chapter Three, I investigated population structure of aphids in North America, France, and Sweden using AFLPs. Aphids on the same host plant were genetically similar across many parts of their range, so the evolution of host plant specialization does not appear to have occurred independently in every location. While aphids on alfalfa and clover were genetically differentiated in most locations, aphids from alfalfa and clover were genetically similar in both western North America and Sweden. High gene flow from alfalfa onto clover may constrain divergence in these locations.Item Microcantilever Biosensors with Chitosan for the Detection of Nucleic Acids and Dopamine(2007-05-07) Koev, Stephan; Ghodssi, Reza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Microcantilever biosensors allow label-free detection of analytes within small sample volumes. They are, however, often limited in sensitivity or specificity due to the lack of proper bio-interface layers. This thesis presents the use of the biopolymer chitosan as a bio-interface material for microcantilevers with unique advantages. Sensors coated with chitosan were designed, fabricated, and functionalized to demonstrate two distinct applications: detection of DNA hybridization and detection of the neurotransmitter dopamine. The first demonstration resulted in signals from DNA hybridization that exceed by two orders of magnitude values previously published for sensors coated with SAM (self assembled monolayer) interface. The second application is the first reported demonstration of using microcantilevers for detection of the neurotransmitter dopamine, and it is enabled by chitosan's response to dopamine electrochemical oxidation. It was shown that this method can selectively detect dopamine from ascorbic acid, a chemical that interferes with dopamine detection in biological samples.Item A phylogenetic and biogeographic analysis of Sanguisorbeae (Rosaceae), with emphasis on the Pleistocene radiation of the high Andean genus Polylepis.(2004-06-16) Kerr, Malin Sofia; Delwiche, Charles F; Reveal, James L; Cell Biology & Molecular Genetics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A phylogenetic and biogeographic analysis of the tribe Sanguisorbeae (Rosaceae) was conducted with emphasis on the radiation of the Andean tree Polylepis. Phylogenetic analyses of coding and non-coding nuclear markers reveal a complex evolutionary history of the tribe including ancient and recent allopolyploid hybridization. Sanguisorba sensu lato is shown to be paraphyletic and split between the allopolyploid hybrid Sanguisorba and the non-hybrid Poterium and Poteridium. A monophyletic origin of the southern hemispheric subtribe Sanguisorbinae is supported, and this clade is given a phylogentic taxon name (Verruchaena). Dating analyses using the penalized likelihood method suggest that this taxon originated in the late Miocene. A biogeographic hypothesis is presented in which Verruchaena originated in the New World with subsequent transoceanic dispersals to southern Africa and Australasia. The paramo genus Polylepis most likely arose from hybridization between two Andean ancestors supporting a "vertical" rather than "horizontal" origin of this taxon.