Theses and Dissertations from UMD
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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 give thesis/dissertation in DRUM
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Item Behavioral and Neuroendocrine Correlates of Sex Change in the Gilthead Seabream (Sparus aurata)(2009) Reyes-Tomassini, Jose J.; Zohar, Yonathan; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Sequential hermaphroditism is the most radical form of environmental sex determination observed in fish: functional adult males or females retain their ability to change sex even as adults. Among the factors that affect sex change in these species, the least understood is the social environment. Here, I studied the influences of social context on sex change in the Gilthead Seabream, Sparus aurata, by using the individual‟s dominance rank as an indicator of social status. To understand the role that the brain might play in sex change, I also studied the two main neuroendocrine factors that serve as the sexually differentiated axes of neural plasticity in most teleost species: AVT and GnRH. To do this, I first developed a set of tools designed to address the challenges associated with observing the behavior of aquacultured species. Using these tools, I provide the first in-depth study of seabream captive behavior, including the results of size-matched and sex-matched paired encounters. I found that females are more aggressive than males, but this difference is influenced by gonadal developmental status. I also showed that small but young males are more aggressive than bigger but older females. I cloned the AVT mRNA in seabream, and validated a quantitative assay to measure total brain AVT levels together with GnRH-1, GnRH-2, and GnRH-3 levels. I found that AVT and GnRH-3 levels rise during the onset of the hypothesized sex-change window, and drop to pre-quiescent levels until spawning, when all of these factors seem to increase their expression levels again. I also show for the first time, that GnRH-2 and dominance rank are strongly correlated in seabream during the spawning season but not during quiescence. GnRH-1 was strongly correlated to rank during quiescence but not during spawning. Finally, neither dominance rank nor size were a good predictor of the outcome of sex change, which seems to contradict what has been documented in sequential hermaphrodite reef fishes. I provide a model that accounts for this apparent contradiction and conclude that the Gilthead seabream remains true to the size-advantage hypothesis of sex allocation theory, if size and dominance are seen as proximate, rather than ultimate, factors.Item The Gonadotropin Releasing Hormone-3 System in Zebrafish: Early Development and Regulation(2008-12-15) Abraham, Eytan; Zohar, Yonathan; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The objective of this study was to expand our understanding of the early development of forebrain Gonadotropin Releasing Hormone (GnRH) neurons in vertebrates in general and in fish in particular. The correct migration during early development of the hypophysiotropic GnRH neurons from the olfactory region to the hypothalamus is crucial for normal gonadal development and reproduction. We developed a Tg(GnRH3:EGFP) zebrafish line in which EGFP is specifically expressed in GnRH3 neurons. Using this line, we have studied in detail the early spatiotemporal development of the GnRH3 system in vivo. In addition, we have studied various factors, including GnRH3, Netrins and Hedgehog to better understand some of the mechanisms that mediate this complex axophilic neuron migration event. Lastly, we have conducted targeted GnRH3 neuron ablation experiments in view of determining the embryonic origin of POA-hypothalamic GnRH3 neurons and the effect of lack of GnRH3 neurons in the CNS. Our findings show that: 1) GnRH neurons first differentiate and express GnRH3 at 24-26 hours post fertilization (hpf) and immediately thereafter begin to extend fibers. 2) GnRH3 neurons project a complex network of fibers, prior the GnRH3 soma migration, to various CNS regions, and to the pituitary. 3) GnRH3 soma begin migrating towards the hypothalamus at 3 days post fertilization (dpf), passing through the terminal nerve (TN), lateral telencephalon, and reaching the hypothalamus by 12 dpf. 4) expression of GnRH3 itself is necessary for the normal early differentiation and fiber extensions of GnRH3 neurons. 5) Netrin1a is directly involved as a chemoattractant in GnRH3 fiber organization and subsequently, in GnRH3 soma migration to the hypothalamus. 6). Netrin2 is required for normal early ZF embryogenesis. 7). Sonic hedgehog a does not serve as a specific factor in the development of the GnRH3 system. 8). GnRH3 neuron regeneration capacity is temporally limited. 9). Successful ablation of olfactory GnRH3 neurons during development results in lack of GnRH3 neurons in the entire sexually mature brain as well as abnormal gonadal development and inability to reproduce. This study expands our understanding vis-à-vis the early events that occur during GnRH3 system development and that regulate this complex process. In a broader sense these findings augment current knowledge regarding the regulation of long range tangential neuron migration during development.Item Gonadal and steroid feedback regulation of the hypothalamus-pituitary axis in striped bass (Morone saxatilis)(2006-09-18) Klenke, Ulrike; Zohar, Yonathan; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The objective of the present study was to expand our understanding of the mechanisms of gonadal steroid feedback regulation of the hypothalamus-pituitary (HP) axis during several reproductive stages (juvenile, pubertal, adult) throughout the life cycle of the striped bass. Towards this end, we investigated effects of bilateral gonadectomy and steroid replacement on the endocrine correlates of the HP axis in vivo. We also developed a brain-slice culture method and utilized pituitary cell cultures to investigate direct effects of estrogen on these correlates at the level of the brain and the pituitary in vitro. Our findings indicate that: 1) During their development, the gonads play an important role in providing feedback to the HP axis. These feedback patterns change during the transformation from the juvenile to the adult and throughout the adult reproductive cycle. The pathways involved use both non-steroidal and steroidal pathways as regulatory mechanisms. 2) Gonads, through their steroids, become more involved in regulating the HP axis during reproductive development and their main feedback target appears to be gene transcription in the pituitary. 3) The observed changes in gonadal feedback throughout the adult reproductive cycle probably reflect the physiological requirements of gametogenesis. 4) The responsiveness of the HP axis towards steroids initially appears during puberty and further increases in adult females. In adults, steroids solely affect the pituitary in early stages of gametogenesis, while in later stages GnRH expression in the brain is also regulated by steroids. However, the nature of the feedback is dependent on estrogenic and/or androgenic pathways. 5) Our in vitro studies showed that estrogens act directly at the levels of the brain and the pituitary in female adult fish. Based on these findings, it appears that the activity along the endocrine reproductive web of striped bass intensifies with age, and that prior cycles of oocyte development may prime the HP axis to respond faster and more vigorously in subsequent years. This study has provided an improved resolution and a broader perspective on mechanisms involved in gonadal steroid feedback regulation of GnRH neural activity and its targets at the level of the pituitary in striped bass.Item STUDIES ON REPRODUCTIVE BIOLOGY AND ENDOCRINOLOGY IN A PRIMITIVE TELEOSTEI, THE AMERICAN SHAD (Alosa sapidissima)(2004-07-07) Abraham, Eytan; Zohar, Yonathan; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)American shad is an anadromous fish that displays asynchronous ovarian development. In an effort to enhance our understanding of the reproductive cycle of this primitive Teleostei species, and provide better management tools for the Chesapeake fishery, we have conducted studies of its gonadal and hormonal cycles. We developed various assays to measure reproductive endocrine factors including: 17,20b-dihydroxy-4-pregnen-3-one (DHP) and GnRH. Fish were collected from the Susquehanna River during their spawning migration. One group of fish was sacrificed on site to assess reproductive parameters of wild shad. A second group was transported to a Maryland State hatchery and treated with gonadotropin-releasing hormone agonist (GnRHa) using several delivery systems. These treatments were followed during a two-week period by measurement of various hormonal levels. In addition, fecundity and fertilization of the hatchery groups were measured daily. Our results shed light on the reproductive physiology and endocrinology of the American shad and lay the foundation for usage of GnRHa to induce shad spawning in captivity.