The Intricate Role of Ecdysis Triggering Hormone Signaling in Insect Development and Reproductive Regulation

SIMPLE SUMMARY: Ecdysis triggering hormone (ETH) synthesized and secreted from peripheral Inka cells on the tracheal surface play a vital role in the triggering and orchestration of ecdysis in insects and other arthropod species. Binding of ETH to the ETH receptor (ETHR) located on the peptidergic neurons in the central nervous system (CNS) facilitates synthesis of ecdysis-related neuropeptides. Although the role of ETH on ecdysis has been well-studied in various holometabolous insects, only very little information is available in hemimetabolous insects. Recent studies in hemimetabolous insects have identified and characterized diverse features of ETH precursors, ETH and ETH receptors. In addition to this, the involvement of ETH in Juvenile hormone (JH) mediated courtship short-term memory (STM) retention and long-term courtship memory regulation and retention has also been shown recently. Here, we extensively studied ETH signaling cascades in various insects and their roles in triggering diverse functions in the adults and juvenile insects including development and reproductive regulation. Understanding the intricate details of ETH signaling in diverse insect species could lead to the identification of novel targets and chemicals, as well as the development of various sustainable pest management solutions. ABSTRACT: Insect growth is interrupted by molts, during which the insect develops a new exoskeleton. The exoskeleton confers protection and undergoes shedding between each developmental stage through an evolutionarily conserved and ordered sequence of behaviors, collectively referred to as ecdysis. Ecdysis is triggered by Ecdysis triggering hormone (ETH) synthesized and secreted from peripheral Inka cells on the tracheal surface and plays a vital role in the orchestration of ecdysis in insects and possibly in other arthropod species. ETH synthesized by Inka cells then binds to ETH receptor (ETHR) present on the peptidergic neurons in the central nervous system (CNS) to facilitate synthesis of various other neuropeptides involved in ecdysis. The mechanism of ETH function on ecdysis has been well investigated in holometabolous insects such as moths Manduca sexta and Bombyx mori, fruit fly Drosophila melanogaster, the yellow fever mosquito Aedes aegypti and beetle Tribolium castaneum etc. In contrast, very little information is available about the role of ETH in sequential and gradual growth and developmental changes associated with ecdysis in hemimetabolous insects. Recent studies have identified ETH precursors and characterized functional and biochemical features of ETH and ETHR in a hemimetabolous insect, desert locust, Schistocerca gregaria. Recently, the role of ETH in Juvenile hormone (JH) mediated courtship short-term memory (STM) retention and long-term courtship memory regulation and retention have also been investigated in adult male Drosophila. Our review provides a novel synthesis of ETH signaling cascades and responses in various insects triggering diverse functions in adults and juvenile insects including their development and reproductive regulation and might allow researchers to develop sustainable pest management strategies by identifying novel compounds and targets.