Future Insecticides Targeting Genes Involved in the Regulation of Molting and Metamorphosis

Abstract

Because of their rigid exoskeleton, insects need to undergo periodic molts during which they shed their old cuticle and produce a new one to accommodate further growth. A number of hormones play critical roles in the initiation and coordination of the processes of molting and metamorphosis. Studies on several model insects such as Bombyx mori, Hyalophora cecropia, Manduca sexta and Drosophila melanogaster, have led to the identification of key hormones and the elucidation of their roles in molting and metamorphosis. Best known for their roles in these two processes are the steroid hormone ecdysone1 and the sesquiterpenoid, juvenile hormone (JH). However, these developmental events also involve a number of relevant peptide hormones, including prothoracicotropic hormone (PTTH), eclosion hormone (EH), pre-ecdysis triggering hormone (PETH), ecdysis triggering hormone (ETH), crustacean cardioactive peptide (CCAP) and bursicon (BU). When a larva reaches a critical weight, PTTH launches the molting process by inducing ecdysone secretion in prothoracic gland cells. Although ecdysone triggers and coordinates the molting process, the type of molt is determined by JH. In the presence of JH, “status quo” is maintained and ecdysone can only induce a larval-larval molt. In the absence of JH, however, ecdysone induces a metamorphic molt leading to the morphological changes seen in the transition from larva to pupa, and from pupa to adult (Riddiford 1994). With respect to the neuropeptides EH, PETH, ETH, and CCAP, they control the behavioral aspects of ecdysis whereas BU regulates tanning and sclerotization of the cuticle. In this chapter we first summarize recent advances in our understanding of the biosynthesis and functions of key hormones involved in molting and metamorphosis. In the second half of the chapter, we discuss probable target sites for the rational design of insecticides, including biosynthetic enzymes, receptors and transcription factors, and propose various ways of exploiting these target sites for pest management. Our coverage will be restricted to the aspects of molting and metamorphosis that are relevant to finding target sites