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Effects of Sterol Structure on Insect Herbivore

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Effects of Sterol Structure on Insect Herbivore EFFECTS OF STEROL STRUCTURE ON INSECT HERBIVORE PHYSIOLOGY, BIOCHEMISTRY AND MOLECULAR BIOLOGY A Dissertation by XIANGFENG JING Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY December 2011 Major Subject: Entomology Effects of Sterol Structure on Insect Herbivore Physiology, Biochemistry and Molecular Biology Copyright 2011 Xiangfeng Jing EFFECTS OF STEROL STRUCTURE ON INSECT HERBIVORE PHYSIOLOGY, BIOCHEMISTRY AND MOLECULAR BIOLOGY A Dissertation by XIANGFENG JING Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Approved by: Chair of Committee, Spencer T. Behmer Committee Members, Frederick T. Davies Micky D. Eubanks Keyan Zhu-Salzman Head of Department, David Ragsdale December 2011 Major Subject: Entomology iii ABSTRACT Effects of Sterol Structure on Insect Herbivore Physiology, Biochemistry and Molecular Biology. (December 2011) Xiangfeng Jing, B.A., Shandong Agricultural University; M.S., Huazhong Agricultural University Chair of Advisory Committee: Dr. Spencer T. Behmer Sterols serve two important biological functions in animals - they act as cellular membrane components, and as the precursor to steroid hormones. Insects require a dietary source of sterol because they cannot synthesize sterols de novo . Cholesterol is the most common sterol in plant-feeding insects, but because plants contain very little cholesterol, plant-feeding insects must convert plant sterols into cholesterol. In this dissertation I investigate the effect of common and novel plant sterols and steroids found in a transgenic tobacco line on several caterpillar species. I also explore the metabolism of these sterols and steroids, and use a microarray approach to identify genes involved in sterol use and metabolism in plant-feeding insects. I also study cholesterol homeostasis using a grasshopper species. Modified tobacco plants containing a novel sterol profile negatively affected performance of three different caterpillar species, especially in the second generation. Insects reared on modified plants contained less total sterols and cholesterol than those on control plants having a normal sterol profile. Similar results were found using iv artificial diets containing atypical steroids, e.g., cholestanol and cholestan-3-one, identified in the tobacco plants that were fed to my experimental caterpillars. More importantly, the sterol/steroid ratio, but not their absolute amount in the diets, determined the negative effects. Caterpillar species could convert stigmasterol, a common plant sterol, into cholesterol. They could also convert cholestan-3-one into cholestanol and epicholestanol, although this ability varied among different species. A microarray study that focused on gene expression in midgut tissue indicated that stigmasterol, cholestanol and cholestan-3-one could induce different gene expression level, and that cholestan-3- one caused the largest pool of genes to be regulated. The genes possibly involved in the metabolism of stigmasterol and cholestan-3-one were reported. These findings are important in directing further research on the potential application of plant sterol modification to control pests in agricultural systems. Insect herbivores could behaviorally regulate the intake of several nutrients, but they could not regulate their sterol intake. They did, however, practice cholesterol homeostasis, by post-ingestively regulating tissue sterol levels, even when feeding on diets with high cholesterol content. Collectively, the results from this dissertation provide unique insight into cholesterol regulation, which is difficult to achieve in mammals that are capable of synthesizing their own sterols. v ACKNOWLEDGEMENTS I heartily thank my advisor, Spencer T. Behmer, for his assistance, guidance and patience during my research. Without his support in various aspects, I would not have been able to finish this dissertation. His philosophy of doing research and insight in science will influence me in my future career. I sincerely thank Robert J. Grebenok at Canisius College, NY, for his assistance in my research and for the use of his lab. I sincerely thank Keyan Zhu-Salzman, committee member, who allowed me to use her lab resources. I also sincerely thank Heiko Vogel at the Max-Planck Institute of Chemical Ecology in Jena, Germany, for his assistance with the molecular research conducted in this disseration. Each gave me perceptive guidance in my research and important suggestions from experimental design to manuscript writing. I also sincerely thank Micky D. Eubanks and Frederick T. Davies, committee members, for the improvement of my knowledge in insect ecology, especially insect- plant interaction, and plant physiology which are important for my knowledge as a researcher. Travis Gates provided excellent technical assistance for my research, and the advice and discussion from the members of the Behmer lab, including Steve Cook, Paul Lenhart and Marion Le Gall, were valuable in developing my research. Joshua Jones gave me important advice in my English writing. Finally, I would like to thank my family members. Their love and encouragement made it possible to finish this dissertation. vi TABLE OF CONTENTS Page ABSTRACT .............................................................................................................. iii ACKNOWLEDGEMENTS ...................................................................................... v TABLE OF CONTENTS .......................................................................................... vi LIST OF FIGURES ................................................................................................... viii LIST OF TABLES .................................................................................................... xi CHAPTER I INTRODUCTION ................................................................................ 1 II PLANT STEROLS AND HOST PLANT SUITABILITY FOR GENERALIST AND SPECIALIST CATERPILLARS ...................... 7 2.1 Introduction .............................................................................. 7 2.2 Materials and Methods ............................................................. 10 2.3 Results ...................................................................................... 17 2.4 Discussion ................................................................................ 24 III BALANCE MATTERS: HOW THE RATIO OF DIETARY STEROIDS AFFECTS CATERPILLAR DEVELOPMENT, GROWTH AND REPRODUCTION ................................................... 32 3.1 Introduction .............................................................................. 32 3.2 Materials and Methods ............................................................. 34 3.3 Results ...................................................................................... 42 3.4 Discussion ................................................................................ 60 vii CHAPTER Page IV STEROL AND STEROID METABOLISM PLUS ABSORPTION IN GENERALIST AND SPECIALIST CATERPILLARS ......................................................... 68 4.1 Introduction .............................................................................. 68 4.2 Materials and Methods ............................................................. 70 4.3 Results ...................................................................................... 78 4.4 Discussion ................................................................................ 88 V DIETARY STEROLS/STEROIDS AND THE GENERALIST CATERPILLAR HELICOVERPA ZEA : PHYSIOLOGY, BIOCHEMISTRY AND DIFFERENTIAL GENE EXPRESSION IN THE MIDGUT ....................................................... 98 5.1 Introduction .............................................................................. 98 5.2 Materials and Methods ............................................................. 100 5.3 Results ...................................................................................... 107 5.4 Discussion ................................................................................ 112 VI CHOLESTEROL HOMEOSTASIS IN A GENERALIST GRASSHOPPER, SCHISTOCERCA AMERICANA ............................ 120 6.1 Introduction .............................................................................. 120 6.2 Materials and Methods ............................................................. 122 6.3 Results ...................................................................................... 127 6.4 Discussion ................................................................................ 135 VII CONCLUSION .................................................................................... 140 REFERENCES .......................................................................................................... 143 APPENDIX ............................................................................................................... 154 VITA ......................................................................................................................... 156 viii LIST OF FIGURES FIGURE Page 1.1 Cholesterol and other sterols/steroids of interest ....................................... 2 2.1 Cholesterol and 5 typical sterols/steroids in our modified plants .............. 8 2.2 Larval and pupal performance of H. virescens , S. exigua , and M. sexta on control and modified
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