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Publication

Neuro-endocrine regulation of ecdysis in the desert locust, Schistocerca gregaria

Book - Dissertation

Moulting is a routinely occurring post-embryonic developmental event that permits arthropods to grow. During this developmental process, their rigid cuticle or exoskeleton has to be shed, in a process called ecdysis, to make way for a larger covering. The timing of these developmental transitions is dictated by periodic pulses of the ecdysteroid hormone 20-hydroxyecdysone and intricate interactions among multiple neuropeptide signalling systems. During the complex and strictly coordinated moulting process, the precise timing of ecdysteroid-regulated gene expression events creates a neuro-endocrine regulatory frame-work that underlies the physiological responses responsible for the execution of ecdysis. Current understanding of this neuro-endocrine regulation is largely based on research devoted to holometabolan model insects, while data on hemimetabolan insects are scarce. Moreover, it is clear that there are several differences in the underlying molecular mechanisms of these neuro-endocrine pathways between various holometabolan species, and between holometabolan and hemimetabolan insect species. In order to expand the available knowledge, this thesis has focused on the study of the physiological role and mode-of-action of neuro-endocrine pathways in the control of ecdysis in the desert locust, Schistocerca gregaria, a hemimetabolan species belonging to the order of Orthoptera. Moreover, this species is globally considered as one of the most devastating migratory pest insects. Therefore, a profound knowledge of the desert locust's physiology and the underlying molecular processes may also aid in the quest for more selective insecticides. Since not all animals develop at the same rate, quick and simple measurable physical characteristics were defined as distinctive morphological indicators to standardize sampling during the moult cycle of individual fifth instar nymphs. The selected indicators were subsequently applied throughout all sub-projects. Thereby, it was possible to more accurately investigate the properties and physiological roles of the eclosion hormone (EH) and crustacean cardioactive peptide (CCAP) signalling systems in the moulting process of S. gregaria. Firstly, the transcripts encoding two Schgr-EH precursors and one putative Schgr-EH receptor and the transcripts coding for one Schgr-CCAP precursor and three putative Schgr-CCAP receptors were identified. In addition, CCAP was shown to be a potent agonist for these predicted CCAP receptors in cell-based functional assays. Subsequently, the functional conservation of EH and CCAP and their receptors as master regulators of ecdysis of S. gregaria were demonstrated by RNA interference (RNAi). Depletion of EH or CCAP precursor and/or receptor transcripts led to lethal phenotypes, being unable to complete ecdysis. Furthermore, initial evidence was obtained suggesting the existence of a compensatory mechanism by the myoinhibiting peptide (MIP) signalling system in CCAP knockdown nymphs prior to the moult. In addition, unlike the situation in some holometabolan species, no significant changes in AKH-I/II transcript levels were observed upon depletion of the S. gregraria CCAP precursor or receptor transcripts. Finally, our results unequivocally show the existence of a complex interaction between the ecdysteroid and neuropeptidergic signalling pathways, in which the CCAP signalling system plays a critical role. Depletion of the ecdysone receptor complex transcripts, Schgr-EcR/Schgr-RXR, led to significantly decreased transcript levels of the three CCAP receptors in the prothoracic glands. In addition, when compared to control locusts, depletion of the Schgr-CCAP signalling system components resulted in significantly elevated transcript levels of Halloween genes, such as Schgr-Spook, Schgr-Disembodied and Schgr-Shade, prior to the moult. Furthermore, prothoracic gland explants exhibited significantly decreased secretion of ecdysteroids in the presence of CCAP. Hence, these data indicate that CCAP acts as a prothoracicostatic peptide limiting ecdysteroid production and/or release from the prothoracic gland at the end of the moult cycle. The study of the functional interactions between neuronal and endocrine signalling systems encourages further experimental research to establish a framework of the neuro-endocrine regulatory network controlling ecdysis in S. gregaria.
Publication year:2021
Accessibility:Open