The regulation of body size in the tobacco hornworm, Manduca sexta

The regulation of growth and body size occurs at both the genetic and physiological level. Physiological studies on growth control in insects have focused primarily on the role of the endocrine system in the events that lead to the final size of an insect---the length of the feeding period, changes in growth rates, etc. Genetic studies have put aside this systemic regulation in favor of identifying genes that upon mutation lead to aberrant growth. These genetic studies have provided insight into the roles of cell growth and proliferation in developing tissues. However, it has become clear that to understand the processes through which an organ, or an organism, attains its final appropriate size will require a synthesis of both molecular/genetic as well as physiological perspectives. Moreover, these studies should be done in one model organism that is amenable to both types of manipulation.;In this dissertation, I examine the regulation of body size at multiple levels in the tobacco hornworm, Manduca sexta (Lepidoptera: Sphingidae). I review the major lines of research on growth and body size in insects and propose a model for the regulation of growth and body size in the Lepidoptera. I describe an evolutionary shift in body size and find that changes in 3 physiological processes that regulate body size account for >95% of the observed increase in body size. I examine the physiological regulation of body size in Manduca in greater detail by determining the contribution of JH-esterase to the clearance of juvenile hormone using radioenzymatic assays and real-time PCR combined with starvation and chemical inhibition. From these experiments it is clear that JH-esterase plays an important role in determining the length of the larval feeding period. Finally, I identify an insulin homolog in Manduca using degenerate PCR and 3 ' and 5'-RACE. Real-time PCR is used to show that the transcript abundance of the homolog fluctuates in response to the hemolymph carbohydrate concentration. Comparative homology modeling of the deduced protein sequence reveals conserved regions of the sequence that construct the likely receptor recognition surface.