Ecdysis behaviors and circadian rhythm of ecdysis in the stick insect, Carausius morosus

Rhythms in insect olfactory systems: underlying mechanisms and outstanding questions

Olfaction is a critical sensory modality for invertebrates, and it mediates a wide range of behaviors and physiological processes. Like most living organisms, insects live in rhythmic environments: the succession of nights and days is accompanied by cyclic variations in light intensity and temperature, as well as in the availability of resources and the activity of predators. Responding to olfactory cues in the proper temporal context is thus highly adaptive and allows for the efficient allocation of energy resources. Given the agricultural or epidemiological importance of some insect species, understanding olfactory rhythms is critical for the development of effective control strategies. Although the vinegar fly Drosophila melanogaster has been a classical model for the study of olfaction and circadian rhythms, recent studies focusing on non-model species have expanded our understanding of insect olfactory rhythms. Additionally, recent evidence revealing receptor co-expression by sensory neurons has brought about an ongoing paradigm shift in our understanding of insect olfaction, making it timely to review the state of our knowledge on olfactory rhythms and identify critical future directions for the field. In this Review, we discuss the multiple biological scales at which insect olfactory rhythms are being analyzed, and identify outstanding questions.

Neuromodulation and the toolkit for behavioural evolution: can ecdysis shed light on an old problem?

The geneticist Thomas Dobzhansky famously declared: 'Nothing in biology makes sense except in the light of evolution'. A key evolutionary adaptation of Metazoa is directed movement, which has been elaborated into a spectacularly varied number of behaviours in animal clades. The mechanisms by which animal behaviours have evolved, however, remain unresolved. This is due, in part, to the indirect control of behaviour by the genome, which provides the components for both building and operating the brain circuits that generate behaviour. These brain circuits are adapted to respond flexibly to environmental contingencies and physiological needs and can change as a function of experience. The resulting plasticity of behavioural expression makes it difficult to characterize homologous elements of behaviour and to track their evolution. Here, we evaluate progress in identifying the genetic substrates of behavioural evolution and suggest that examining adaptive changes in neuromodulatory signalling may be a particularly productive focus for future studies. We propose that the behavioural sequences used by ecdysozoans to moult are an attractive model for studying the role of neuromodulation in behavioural evolution.

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

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.

Knockdown of Na,K-ATPase β-subunits in Oncopeltus fasciatus induces molting problems and alterations in tracheal morphology

The ubiquitously expressed transmembrane enzyme Na,K-ATPase (NKA) is vital in maintaining functionality of cells. The association of α- and β-subunits is believed to be essential for forming a functional enzyme. In the large milkweed bug Oncopeltus fasciatus four α1-paralogs and four β-subunits exist that can associate into NKA complexes. This diversity raises the question of possible tissue-specific distribution and function. While the α1-subunits are known to modulate cardenolide-resistance and ion-transport efficiency, the functional importance of the β-subunits needed further investigation. We here characterize all four different β-subunits at the cellular, tissue, and whole organismal scales. A knockdown of different β-subunits heavily interferes with molting success resulting in strongly hampered phenotypes. The failure of ecdysis might be related to disrupted septate junction (SJ) formation, also reflected in β2-suppression-induced alteration in tracheal morphology. Our data further suggest the existence of isolated β-subunits forming homomeric or β-heteromeric complexes. This possible standalone and structure-specific distribution of the β-subunits predicts further, yet unknown pump-independent functions. The different effects caused by β knockdowns highlight the importance of the various β-subunits to fulfill tissue-specific requirements.

Characterization of Molting Process during the Different Developmental Stages of the Diamondback Moth Plutella xylostella

The molting process of the lepidopteran insects is observed for many species. However, the detailed description of the morphological transformation and behavioral sequence during molting are rarely provided and visualized. Here, we described the molting process of the diamondback moth Plutella xylostella by providing the duration and photographic details of staging criteria of each stage using stereo microscopy and a digital video camera. We divided the morphological transformation of egg development and hatching into five stages, the larval–larval ecdysis and the larval–pupal metamorphosis into five stages, the pupal development and eclosion into three stages, and the post-eclosion behavior into four stages. Several new characters in the molting process that were not previously described in other lepidopteran insects were found, i.e., the larvae contracted anterior-posteriorly then dorsal-ventrally during pre-ecdysis, and the antennae waved backward then forward in the post-eclosion behavior. Our findings will deepen the knowledge of the molting biology of lepidopteran insects and facilitate the study of the underlying mechanisms.

Activity of Ajuga iva Extracts Against the African Cotton Leafworm Spodoptera littoralis

Control of the crop pest African cotton leafworm, Spodoptera littoralis (Boisduval), by chemical insecticides has led to serious resistance problems. Ajuga plants contain phytoecdysteroids (arthropod steroid hormone analogs regulating metamorphosis) and clerodanes (diterpenoids exhibiting antifeedant activity). We analyzed these compounds in leaf extracts of the Israeli Ajuga iva L. by liquid chromatography time-of-flight mass spectrometry (LC-TOF-MS) and thin-layer chromatography (TLC), and their efficiency at reducing S.littoralis fitness. First and third instars of S. littoralis were fed castor bean leaves (Ricinus communis) smeared with an aqueous suspension of dried methanolic crude extract of A. iva phytoecdysteroids and clerodanes. Mortality, larval weight gain, relative growth rate and survival were compared to feeding on control leaves. We used '4',6-diamidino-2-phenylindole (DAPI, a fluorescent stain) and phalloidin staining to localize A. iva crude leaf extract activity in the insect gut. Ajuga iva crude leaf extract (50, 100 and 250 µg/µL) significantly increased mortality of first-instar S. littoralis (36%, 70%, and 87%, respectively) compared to controls (6%). Third-instar larval weight gain decreased significantly (by 52%, 44% and 30%, respectively), as did relative growth rate (-0.05 g/g per day compared to the relevant controls), ultimately resulting in few survivors. Crude leaf extract (250 µg/µL) reduced gut size, with relocation of nuclei and abnormal actin-filament organization. Ajug iva extract has potential for alternative, environmentally safe insect-pest control.

Prediction and expression analysis of G protein-coupled receptors in the laboratory stick insect, Carausius morosus

G protein-coupled receptors (GPCRs) are 7-transmembrane proteins that transduce various extracellular signals into intracellular pathways. They are the major target of neuropeptides, which regulate the development, feeding behavior, mating behavior, circadian rhythm, and many other physiological functions of insects. In the present study, we performed RNA sequencing and de novo transcriptome assembly to uncover the GPCRs expressed in the stick insect Carausius morosus. The transcript assemblies were predicted for the presence of 7-transmembrane GPCR domains. As a result, 430 putative GPCR transcripts were obtained and 43 of these revealed full-length sequences with highly significant similarity to known GPCR sequences in the databases. Thirteen different GPCRs were chosen for tissue expression analysis. Some of these receptors, such as calcitonin, inotocin, and tyramine receptors, showed specific expression in some of the tissues. Additionally, GPCR prediction yielded a novel uncharacterized GPCR sequence, which was specifically expressed in the central nervous system and ganglia. Previously, the only information about the anatomy of the stick insect was on its gastrointestinal system. This study provides complete anatomical information about the adult insect.

The transcriptome sequencing and functional analysis of eyestalk ganglions in Chinese mitten crab (Eriocheir sinensis) treated with different photoperiods

Photoperiod plays an important role in individual growth, development, and metabolism in crustaceans. The growth and reproduction of crabs are closely related to the photoperiod. However, as of yet, there are still no transcriptomic reports of eyestalk ganglions treated under different photoperiods in the Chinese mitten crab (Eriocheir sinensis), which is a benthonic crab with high commercial value in Asia. In this study, we collected the eyestalk ganglions of crabs that were reared under different photoperiods, including a control group (L: D = 12 h: 12 h, named CC), a constant light group (L: D = 24 h: 0 h, named LL) and a constant darkness group (L: D = 0 h: 24 h, named DD). RNA sequencing was performed on these tissues in order to examine the effects of different photoperiods. The total numbers of clean reads from the CC, LL and DD groups were 48,772,584 bp, 53,943,281 bp and 53,815,178 bp, respectively. After de novo assembly, 161,380 unigenes were obtained and were matched with different databases. The DEGs were significantly enriched in phototransduction and energy metabolism pathways. Results from RT-qPCR showed that TRP channel protein (TRP) in the phototransduction pathway had a significantly higher level of expression in LL and DD groups than in the CC group. We found that the downregulation of the pyruvate dehydrogenase complex (PDC) gene and the upregulation phosphoenolpyruvate carboxykinase (PPC) gene were involved in energy metabolism processes in LL or DD. In addition, we also found that the upregulation of the expression level of the genes Gαq, pyruvate kinase (PK), NADH peroxidase (NADH) and ATPase is involved in phototransduction and energy metabolism. These results may shed some light on the molecular mechanism underlying the effect of photoperiod in physiological activity of E. sinensis.

cGMP/cGMP-dependent protein kinase pathway modulates nicotine-induced currents through the activation of α-bungarotoxin-insensitive nicotinic acetylcholine receptors from insect neurosecretory cells

Insect neurosecretory cells, called dorsal unpaired median neurons, are known to express two α-bungarotoxin-insensitive nicotinic acetylcholine receptor (nAChR) subtypes, nAChR1 and nAChR2. It was demonstrated that nAChR1 was sensitive to cAMP/cAMP-dependent protein kinase (PKA) regulation, resulting in a modulation of nicotine currents. In this study, we show that cyclic guanosine monophosphate (cGMP)/cGMP-dependent protein kinase (PKG) pathway modulates nicotine-induced currents, as increased cGMP affects the second compound of the biphasic current-voltage curve, corresponding to the nAChR2 receptors. Indeed, maintaining the guanosine triphosphate level with 100 μM guanosine triphosphate-γ-S increased nicotine currents through nAChR2. We also demonstrated that inhibition of PKG activity with 0.2 μM (8R,9S,11S)-(-)-9-methoxy-carbamyl-8-methyl-2,3,9,10-tetrahydro-8,11-epoxy-1H,8H,11H-2,7b,11a-trizadibenzo-(a,g)-cycloocta-(c,d,e)-trinden-1-one (KT5823), a PKG specific inhibitor, reduced nicotine-induced current amplitudes. KT5823 effect on nicotine currents is associated with calcium (Ca(2+) ) activity because inhibition of Ca(2+) concentration with cadmium chloride (CdCl2 ) abolished KT5823-induced inhibition mediated by nAChR2. However, specific inhibition of nitric oxide-guanylyl cyclase (GC) complex by 10 μM 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) significantly increased nicotine-induced current amplitudes on both nAChR1 and nAChR2. These results suggest that nicotine-induced currents mediated by both α-bungarotoxin-insensitive nAChR1 and nAChR2 are coupled to the cGMP/PKG pathway. We propose that nicotinic acetylcholine receptor activation induces an increase in intracellular calcium (Ca(2+) ) concentration. Elevation of intracellular Ca(2+) results in the formation of Ca(2+) -calmodulin (CaM) complex, which activates guanylyl cyclase (GC) and/or adenylyl cyclase (AC). Ca(2+) -CaM complex could activate Ca(2+) calmodulin kinase II which could directly or indirectly modulate the nicotinic response. The mechanisms by which cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) interact remain unclear. We demonstrate that nicotine-induced currents are coupled to the cGMP/PKG pathway.