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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Now showing 1 - 4 of 4
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    Genotypic Diversity of Common Phytophthora Species in Maryland Nurseries and Characterization of Fungicide Efficacy
    (2015) Beaulieu, Justine; Balci, Yilmaz; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The genetic diversity of P. plurivora, P. cinnamomi, P. pini, P. multivora, and P. citrophthora, five of the most common species found in Maryland ornamental nurseries and mid-Atlantic forests, was characterized using amplified fragment length polymorphism (AFLP). Representative isolates of genotypic clusters were then screened against five fungicides commonly used to manage Phytophthora. Three to six populations were identified for each species investigated with P. plurivora being the most diverse and P. cinnamomi the least. Clonal groups that originated from forest or different nurseries suggest an ongoing pathway of introduction. In addition, significant molecular variation existed for some species among nurseries an indication that unique genotypes being present in different nurseries. Insensitive isolates to fungicides were detected with P. plurivora (13), P. cinnamomi (3), and P. multivora (2). Interestingly, insensitive isolates primarily belonged to the least common genotypic clusters. Because all but two isolates were sensitive to dimethomorph and ametoctradin, the ability of these chemicals to manage Phytophthora is promising. Nevertheless, the presence of two insensitive isolates could portend general insensitivity to these chemicals as well. Results from this study provide a foundation to future population determination and fungicide sensitivity of the plant pathogenic genus Phytophthora in Maryland's ornamental nursery industry.
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    Population Genomics of Selection in the Eastern Oyster Contact Zone
    (2012) Murray, Maria C; Kocher, Thomas D; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Intraspecific clinal systems are ideal for investigating the how divergence occurs in the presence of gene flow because they represent a balance between selection and gene flow prior to speciation. High dispersal marine species with clinal variation are particularly informative to test for divergent selection because selection likely is strong enough to counteract high gene flow. The degree of population structure varies considerably among loci, such that the genome acts as a sieve allowing gene flow at neutral loci and impeding it at selected loci, creating a genomic mosaic of differentiation. In this study, I examine genomic and geographic patterns of differentiation among parapatric populations of the eastern oyster (Crassostrea virginica) along their contact zone in Florida estuaries. The planktotrophic larval phase of this species gives it the potential for regular long-distance dispersal and genetically homogeneous populations. However Florida populations at the center of its range exhibit a sharp step cline at some loci, suggesting a role for divergent selection. Using 217 AFLP loci, including seven candidate loci for differential selection between the two populations, I genotyped 1,011 spat over two seasons and 274 adults at sites along the contact zone. I examined: (1) whether genome scans can detect divergent selection in a clinal system, (2) the genomic and geographic patterns of differentiation along the cline at neutral and selected loci, and (3) regional patterns of differentiation and genotypic distributions among the life stages. Results demonstrated: (1) candidate loci for regionally divergent selection, (2) a genomic and geographic mosaic of differentiation, (3) regional and localized selection at a non-trivial portion of loci, (4) lower recruitment and some mortality in the center of the cline, and (5) strong exogenous, post-settlement viability selection against intermediate and non-native-like genotypes. While a combination of neutral and adaptive processes likely shape genomic and geographic patterns of differentiation, this study revealed evidence for divergent selection in an estuarine species with high potential for gene flow. Overall, these results point to a major role for post-zygotic, environment-dependent selection in the maintenance of the contact zone between Atlantic and Gulf-type oyster populations.
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    A study of unusual metabolic variants of Aeromonas caviae and Aeromonas hydrophila using a polyphasic taxonomic approach
    (2010) Chang, Zenas; Joseph, Sam W; Cell Biology & Molecular Genetics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Variation in acid production from carbohydrate metabolism has been identified in Aeromonas as a potential indicator for new subspecies. Therefore, pure cultures of non-lactose fermenting Aeromonas caviae, a cause of waterborne infections in humans and other vertebrates, were studied after noting a mixture of acid producing and non-acid producing colonies after four days of incubation on MacConkey agar at ambient temperature. Unusual arabinose negative strains of A. hydrophila (usually arabinose positive) were added to the project to further study the correlation between carbohydrate fermentation and taxonomy. These metabolic variants of A. caviae and A. hydrophila were studied for phenotypic differences via carbohydrate utilization assays as well as genotypic differences via FAFLP. The results suggest that the A. caviae isolates MB3 and MB7 should be considered novel subspecies, while the arabinose negative strain designated A. hydrophila subsp. dhakensis is correctly identified as a subspecies of A. hydrophila.
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    The Role of Host-Plant Species in the Differentiation of Sympatric Populations of Hymenopteran Parasitoids
    (2005-11-02) Medina, Raul Francisco; Barbosa, Pedro; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    ABSTRACT Title of Document: THE ROLE OF HOST-PLANT SPECIES IN THE DIFFERENTIATION OF SYMPATRIC POPULATIONS OF HYMENOPTERAN PARASITOIDS. Raul F. Medina, PhD, 2005 Directed By: Professor, Pedro Barbosa, Department of Entomology The biology and ecology of insect parasitoids is strongly influenced by the host-plant species on which their herbivorous hosts occur. Hymenopteran parasitoids in particular, present a series of characteristics that made them good candidates for phenotypic and genotypic differentiation. Thus, parasitoid adaptation to plant traits may promote significant phenotypic and genotypic differences among sympatric populations of parasitoids associated with different host-plant species. The present study assessed phenotypic and genotypic differentiation in two braconid parasitoids ovipositing on the same host species, the green cloverworm, Plathypena scabra Fabricius (Lepidoptera: Noctuidae) feeding on alfalfa and soybean. Developmental time, adult weight, percent parasitism and preference for host-plant odors of the generalist parasitoid Cotesia marginiventris Cresson (Hymenoptera: Braconidae) and of the specialist parasitoid Aleiodes nolophanae Ashmead (Hymenoptera: Braconidae) were compared among individuals ovipositing in green cloverworm larvae feeding on alfalfa and soybean. In addition, amplified fragment length polymorphisms (AFLP) were used to assess if genotypic differentiation between parasitoids ovipositing on green cloverworm larvae feeding on different host-plant species (i.e., alfalfa or soybean) was present. Phenotypic differentiation in adult mass, adult longevity and percent parasitism between parasitoids ovipositing green cloverworm larvae on alfalfa and on soybean were found. These phenotypic differences between parasitoids associated with different host-plant species were observed in both the generalist and the specialist parasitoids. No evidence of parasitoids showing preferences for the host-plant species from which their host fed was found in the generalist nor in the specialist parasitoid. Contrary to the expectations and predictions from the literature, these parasitoid species did not show evidence of reproductive isolation when associated with different host-plant species (i.e., alfalfa or soybean), as evidenced by the lack of genetic differentiation in AFLP profiles between parasitoids associated with alfalfa and soybean. In order to ensure that the number of wasps and the number of AFLP bands used were enough to provide an accurate assessment of genetic differentiation among wasps ovipositing hosts on different host-plant species, a method for determining the minimum number of individuals and AFLP bands to include to obtain accurate genetic profiles of hypothesized populations was proposed.