Wendy A. Smith
Gail S. Begley, Erin J. Cram, Frederick Charles Davis, Richard C. Deth
Date of Award
Doctor of Philosophy
Department or Academic Unit
College of Arts and Sciences. Department of Biology.
biology, tobacco hornworm, Manduca sexta
Tobacco hornworm - Development - Endocrine aspects, Tobacco hornworm - Endocrinology, Tobacco hornworm - Nutrition - Requirements
The tobacco hornworm, Manduca sexta, has been a useful model for studying insect endocrinology. In M. sexta the majority of growth occurs during the fifth (final) larval stage. This growth is concurrent with endocrine changes that lead to a metamorphic molt. Past research has focused on the interplay of three growth hormones: juvenile hormone (JH), rothoracicotropic hormone (PTTH) and ecdysone, in regulating metamorphosis. The prothoracic glands, source of ecdysteroid molting hormones in insects, play a crucial role in coordinating growth. Insulin/insulin-like growth factor (IGF) signaling has been shown to regulate growth and secretion of steroidogenic tissues in both vertebrates and invertebrates, but it had yet to be well-characterized in M. sexta. In this dissertation I examined the role of insulin-directed growth in prothoracic glands during both normal larval development and in response to nutrient deprivation. In particular, I predicted that in response to a reduction in nutrients, the prothoracic glands would stop growing and ecdysone secretion would be reduced. Changes were assessed in actin abundance and in two nutritionally sensitive transcripts, insulin receptor and 4EBP. Actin abundance in prothoracic glands increased in the first four days of this stage, demonstrating significant growth. Actin decreased in glands from starved larvae, indicating a cessation of growth. Both insulin receptor and 4EBP transcript were present during normal larval development. As expected, both insulin receptor and 4EBP transcript increased in response to starvation. This research was the first demonstration of insulin signaling in prothoracic glands of fifth instar M. sexta larvae, and revealed nutritional sensitivity of the prothoracic glands. Finally, ecdysone assays were conducted to determine the impact of nutrition on glandular secretory capacity. Starved larvae were unable to secrete as much ecdysone as fed controls. Injections of insulin were unable to rescue the ability of the glands to secrete high levels of ecdysone. This demonstrated that although insulin signaling was correlated with normal growth of the glands, insulin is not sufficient to impact the ability of the glands to secrete ecdysone in the absence of nutrients.
Amy L. Walsh
Walsh, Amy L., "Nutritional sensitivity of fifth instar prothoracic glands in the Tobacco hornworm, manduca sexta" (2009). Biology Dissertations. Paper 14. http://hdl.handle.net/2047/d20000155
Click button above to open, or right-click to save.