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 HEMIPTERAN INSECTS AS MODELS FOR UNDERSTANDING SEGMENTATION(2018) Chen, Mengyao; Pick, Leslie; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Although segmentation is highly conserved in arthropods, diverse mechanisms underlie segmentation. Pair-rule genes (PRGs) are a group of genes controlling segmentation in Drosophila melanogaster, a holometabolous insect. While Drosophila are long-germ insects, most insects add segments sequentially. Studying the role of PRGs in sequentially-segmenting species will provide a deeper understanding in terms of developmental biology. Here, I studied two such insects: Halyomorpha halys and Oncopeltus fasciatus, hemimetabolous insects in a sister order to Holometabola. I annotated segmentation genes in the Halyomorpha genome and tested its response to RNA interference which I showed to be effective in this species for the first time. I further showed that three orthologs of Drosophila PRGs are present in the Oncopeltus genome and are expressed during stages at which segments are specified. Surprisingly, only one of these orthologs is expressed in a PR-pattern, indicating that PRG expression and function have changed during insect evolution.Item INVESTIGATING PAIR-RULE GENE ORTHOLOGS IN AN INTERMEDIATE GERM BEETLE, DERMESTES MACULATUS(2017) Xiang, Jie; Pick, Leslie; Entomology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Insects share a body plan based on repeating segments. Segmentation has been well characterized in Drosophila melanogaster, in which segments are established by a genetic hierarchy including gap, pair-rule and segment polarity genes. Pair-rule genes (PRGs) are a key class of segmentation genes as they are the first cohort of genes expressed in a periodic pattern. Segments are established simultaneously in Drosophila in early embryos, while most other insects add segments sequentially as the embryo elongates. Our goal is to understand molecular mechanisms controlling segment formation and to determine the extent of their conservation during evolution. Here, we established the hide beetle Dermestes maculatus, an intermediate germ developer, as a new model system for studying segmentation patterning. We first established a lab colony and studied early embryogenesis in Dermestes. All nine PRG orthologs were isolated using degenerate PCR and RACE, and their expression patterns were examined with in situ hybridization. Except for opa, all Dermestes PRG orthologs are expressed in PR-like striped patterns. Gene functions were tested using RNA interference (RNAi). We examined both hatched and unhatched larvae to uncover defects with different severities. Both Dmac-prd and -slp knockdown resulted in typical PR defects, suggesting that they are “core” PR genes. Dmac-eve, -run and -odd have dual roles in germ band elongation and in PR segmentation, as severe knockdown caused anterior-only, asegmental embryos while moderate knockdown resulted in PR-like defects. Elongated but asegmental germ bands resulted from Dmac-prd and -slp double knockdown, suggesting decoupling of germ band elongation and PR segmentation. Extensive cell death prefigured the cuticle patterns after knockdowns, seen long ago for Drosophila PR phenotypes, although disrupted cell mitosis was also observed after Dmac-eve knockdown. We propose that PRGs have retained basic roles in PR segmentation during the transition from short-to-long germ development and share evolutionary conserved functions in promoting cell viability. Finally, I also present detailed protocols on Dermestes lab rearing, embryo collection and fixation, in situ hybridization and RNAi. The technical information described here will provide useful information for other genetic studies in this new model system.Item Regeneration, Fission and the Evolution of Developmental Novelty in Naid Annelids(2012) Zattara, Eduardo Enrique; Bely, Alexandra E; Behavior, Ecology, Evolution and Systematics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Regeneration of lost structures and asexual reproduction by fission are post-embryonic trajectories related at the evolutionary and developmental levels. Their phylogenetic distribution within Metazoa has led to the hypothesis that fission can evolve by co-opting regenerative abilities. Fission has evolved multiple times within Annelida, including independent origins at the base of the Pristininae and Naidinae lineages of naid worms. Naids are thus a great system to study the evolution of developmental trajectories of regeneration and fission and their mutual physiological interactions. I made a comparative study of morphogenesis during regeneration and fission in a representative species, Pristina leidyi Smith (Pristininae), to test the hypothesis that both trajectories are closely linked by common origin, yet have undergone functional divergence; results show that regeneration and fission share numerous, sometimes exclusive developmental processes, but also present a number of differences spread out along their trajectories. I also examined cell proliferation and growth patterns in P. leidyi to characterize the resource allocation strategies it uses to integrate multiple developmental trajectories. I found evidence for a non-linear antero-posterior gradient in proliferation potential and clear interactions between regeneration and fission that strongly depend on fission stage and what body part is lost; similar interactions have been described for naidine annelids and turbellarian flatworms representing independent origins of fission, indicating convergence of fission-associated allocation strategies. I then extended the fission-regeneration comparative study in P. leidyi to additional annelids, describing and comparing regeneration and fission in another pristinine, seven naidine and one outgroup species, and found very similar regeneration trajectories among all of them, along with striking levels of convergence of paratomic fission trajectories. Despite similarities, the two paratomic clades presented a distinctive mode of central nervous system development. Finally, I developed novel protocols for dynamic studies of the cellular basis of regeneration, laying groundwork for future comparisons at that level. Altogether, these results strongly support that fission originated multiple times by co-option of regenerative abilities; furthermore, convergence of fission trajectories and resource allocation strategies suggests that similar developmental capabilities, functional constraints and ecophysiological contexts can channel evolutionary trajectories into parallel paths, both in close and distant lineages.