Functional circadian clock genes are essential for the overwintering diapause of the Northern house mosquito, Culex pipiens

Correlation between circadian and photoperiodic latitudinal clines in Drosophila littoralis

Insects can survive harsh conditions, including Arctic winters, by entering a hormonally induced state of dormancy, known as diapause. Diapause is triggered by environmental cues such as shortening of the photoperiod (lengthening of the night). The time of entry into diapause depends on the latitude of the insects' habitat, and this applies even within a species: populations living at higher latitudes enter diapause earlier in the year than populations living at lower latitudes. A long-standing question in biology is whether the internal circadian clock, which governs daily behaviour and serves as a reference clock to measure night length, shows similar latitudinal adaptations. To address this question, we examined the onset of diapause and various behavioural and molecular parameters of the circadian clock in the cosmopolitan fly, Drosophila littoralis, a species distributed throughout Europe from the Black Sea (41° N) to Arctic regions (69° N). We found that all clock parameters examined showed the same correlation with latitude as the critical night length for diapause induction. We conclude that the circadian clock has adapted to the latitude and that this may result in the observed latitudinal differences in the onset of diapause.

A Head-Specific Transcriptomic Study Reveals Key Regulatory Pathways for Winter Diapause in the Mosquito Culex pipiens

The primary vector of the West Nile virus, Culex pipiens, undergoes reproductive dormancy during the adverse winter season. While our current understanding has mainly focused on cellular signals and phenotypic shifts occurring at a global scale during diapause, information on tissue-specific transcriptomic changes remains limited. This knowledge gap is a major challenge in interpreting the regulatory mechanisms at the tissue level. To address this, the present work utilized RNA-seq technology to investigate the transcriptional changes in the head that house the brain and crucial endocrinal organs such as corpora allata. We obtained RNA samples from the heads of diapausing and nondiapausing female mosquitoes at two specific time intervals, ZT0 and ZT16, and then subjected them to sequencing. Our results revealed differences in differentially expressed genes between diapause and non-diapause at ZT0 and ZT16, highlighting the phenotypic and diel variations in gene expression. We also selected twelve genes associated with the diapause phenotype and examined the transcript abundance at six different time points over 24 h. qRT-PCR analysis showed similar up- and downregulation of transcripts between the diapause and nondiapause phenotypes thus validating the results of RNA-seq. In summary, our study identified new genes with phenotypic and diel differentiation in their expression, potentially linking photoperiod to seasonal reproductive dormancy in insects. The newly presented information will significantly advance our understanding of head-specific genes crucial for insect diapause.

Genome-wide identification of PAR domain protein 1 (PDP1) targets through ChIP-seq reveals the regulation of diapause-specific characteristics in Culex pipiens

Insects use seasonal diapause as an alternative strategy to endure adverse seasons. This developmental trajectory is induced by environmental cues like short-day lengths in late summer and early fall, but how insects measure day length is unknown. The circadian clock has been implicated in regulating photoperiodic or seasonal responses in many insects, including the Northern house mosquito, Culex pipiens, which enters adult diapause. To investigate the potential control of diapause by circadian control, we employed ChIP-sequencing to identify the downstream targets of a circadian transcription factor, PAR domain protein 1 (PDP1), that contribute to the hallmark features of diapause. We identified the nearest genes in a 10 kb region of the anticipated PDP1 binding sites, listed prospective targets and searched for PDP1-specific binding sites. By examining the functional relevance to diapause-specific behaviours and modifications such as metabolic pathways, lifespan extension, cell cycle regulation and stress tolerance, eight genes were selected as targets and validated using ChIP-qPCR. In addition, qRT-PCR demonstrated that the mRNA abundance of PDP1 targets increased in the heads of diapausing females during the middle of the scotophase (ZT17) compared with the early photophase (ZT1), in agreement with the peak and trough of PDP1 abundance. Thus, our investigation uncovered the mechanism by which PDP1 might generate a diapause phenotype in insects.

Light pollution disrupts seasonal reproductive phenotypes and reduces lifespan in the West Nile vector, Culex pipiens

Females of the Northern House mosquito, Culex pipiens, are important disease vectors as they transmit pathogens including West Nile virus. These females survive the winter by entering diapause, a state of dormancy, characterized by the accumulation of lipids, cessation of blood-feeding, and reproductive arrest. Diapause is cued by photoperiod, so as days become short in late summer and early fall, female Cx. pipiens prepare to overwinter and disease transmission decreases. We previously demonstrated that Artificial light at night (ALAN) causes female Cx. pipiens to avert diapause and continue to blood-feed when reared under short-day conditions. Additionally, light pollution alters seasonal differences in mosquito activity and nutrient reserves. However, it is unclear how exposure to ALAN affects blood-feeding and fecundity in long-day reared females, as well as the survival of Cx. pipiens exposed under both short and long-day conditions. In this study, we hypothesized that females exposed to ALAN in long-day conditions would have a lower proclivity to blood-feed, reduced fecundity, and reduced survival. Results from our lab-based experiments demonstrate that females exposed to ALAN in long-day conditions were less likely to blood-feed but were more fecund than long-day reared females that were not exposed to ALAN, and that ALAN exposure did not affect lifespan of long-day reared females. Additionally, we hypothesized ALAN exposure under short-day conditions would reduce survival, and our data supports this hypothesis. Overall, our results demonstrate that ALAN is an important urban stressor that has the potential to affect reproduction and lifespan in mosquitoes, and therefore has the potential to create evolutionary tradeoffs.

Knock down to level up: Reframing RNAi for invertebrate ecophysiology

Comparative ecophysiologists strive to understand physiological problems in non-model organisms, but molecular tools such as RNA interference (RNAi) are under-used in our field. Here, we provide a framework for invertebrate ecophysiologists to use RNAi to answer questions focused on physiological processes, rather than as a tool to investigate gene function. We specifically focus on non-model invertebrates, in which the use of other genetic tools (e.g., genetic knockout lines) is less likely. We argue that because RNAi elicits a temporary manipulation of gene expression, and resources to carry out RNAi are technically and financially accessible, it is an effective tool for invertebrate ecophysiologists. We cover the terminology and basic mechanisms of RNA interference as an accessible introduction for "non-molecular" physiologists, include a suggested workflow for identifying RNAi gene targets and validating biologically relevant gene knockdowns, and present a hypothesis-testing framework for using RNAi to answer common questions in the realm of invertebrate ecophysiology. This review encourages invertebrate ecophysiologists to use these tools and workflows to explore physiological processes and bridge genotypes to phenotypes in their animal(s) of interest.

Differentially expressed microRNAs in brains of adult females may regulate the maternal block of diapause in Sarcophaga bullata

The maternal regulation of diapause is one type of phenotypic plasticity where the experience of the mother leads to changes in the phenotype of her offspring that impact how well-suited they will be to their future environment. Sarcophaga bullata females with a diapause history produce offspring that cannot enter diapause even if they are reared in a diapause inducing environment. Accumulating evidence suggests that microRNAs regulate diapause and, possibly, maternal regulation of diapause. We found significant differences in the abundances of several microRNAs (miR-125-5p, miR-124-3p, miR-31-5p, and miR-277-3p) in brains dissected from adult female S. bullata that had experienced diapause compared to females with no diapause history. We also found moderate differences in the mRNA expression of the circadian-clock related genes, clock, clockwork orange, and period. MiR-124-3p and miR-31-5p are part of a gene network that includes these circadian clock-related genes. Taken together our results suggest the maternal block of diapause in S. bullata is regulated, at least in part, by a network that includes microRNAs and the circadian clock.

Transcriptome-Wide Evaluation Characterization of microRNAs and Assessment of Their Functional Roles as Regulators of Diapause in Ostrinia furnacalis Larvae (Lepidoptera: Crambidae)

microRNAs (miRNAs) function as vital regulators of diapause in insects through their ability to post-transcriptionally suppress target gene expression. In this study, the miRNA of Ostrinia furnacalis, an economically important global crop pest species, was characterized. For the included analyses, 9 small RNA libraries were constructed using O. furnacalis larvae in different diapause states (non-diapause, ND; diapause, D; diapause-termination, DT). The results identified 583 total miRNAs, of which 256 had previously been identified, whereas 327 were novel. Furthermore, comparison analysis revealed that 119 and 27 miRNAs were differentially expressed in the D vs. ND and DT vs. D, respectively. Moreover, the expression patterns of their miRNAs were also analyzed. GO and KEGG analysis of the target genes of differentially expressed miRNAs highlighted the importance of these miRNAs as diapause regulators in O. furnacalis, especially through metabolic processes, endocrine processes, 20-hydroxyecdysone, and circadian clock signaling pathways. In summary, this study highlighted the involvement of specific miRNAs in the control of diapause in O. furnacalis. To the best of our knowledge, this is the first study to identify miRNA expression patterns in O. furnacalis, thereby providing reference and novel evidence enhancing our current understanding of how small RNAs influence insect diapause.

Transcription profiling reveals tissue-specific metabolic pathways in the fat body and ovary of the diapausing mosquito Culex pipiens

The northern house mosquito, Culex pipiens, employs diapause as an essential survival strategy during winter, inducing important phenotypic changes such as enhanced stress tolerance, lipid accumulation, and extended longevity. During diapause, the cessation of reproductive development represents another distinctive phenotypic change, underlining the need for adjusted modulation of gene expressions within the ovary. Although considerable advancements in screening gene expression profiles in diapausing and non-diapausing mosquitoes, there remains a gap in tissue-specific transcriptomic profiling that could elucidate the complicated formation of diverse diapause features in Cx. pipiens. Here, we filled this gap by utilizing RNA sequencing, providing a detailed examination of gene expression patterns in the fat body and ovary during diapause compared to non-diapause conditions. Functional annotation of upregulated genes identified associations with carbohydrate metabolism, stress tolerance, immunity, and epigenetic regulation. The validation of candidate genes using quantitative real-time PCR verified the differentially expressed genes identified in diapausing mosquitoes. Our findings contribute novel insights into potential regulators during diapause in Cx. pipiens, thereby opening possible avenues for developing innovative vector control strategies.

Knockout of cryptochrome 1 disrupts circadian rhythm and photoperiodic diapause induction in the silkworm, Bombyx mori

Most insects enter diapause, a state of physiological dormancy crucial for enduring harsh seasons, with photoperiod serving as the primary cue for its induction, ensuring proper seasonal timing of the process. Although the involvement of the circadian clock in the photoperiodic time measurement has been demonstrated through knockdown or knockout of clock genes, the involvement of clock gene cryptochrome 1 (cry1), which functions as a photoreceptor implicated in photoentrainment of the circadian clock across various insect species, remains unclear. In bivoltine strains of the silkworm, Bombyx mori, embryonic diapause is maternally controlled and affected by environmental conditions experienced by mother moths during embryonic and larval stages. Previous research highlighted the role of core clock genes, including period (per), timeless (tim), Clock (Clk) and cycle (cyc), in photoperiodic diapause induction in B. mori. In this study, we focused on the involvement of cry1 gene in B. mori photoperiodism. Phylogenetic analysis and conserved domain identification confirmed the presence of both Drosophila-type cry (cry1) and mammalian-type cry (cry2) genes in the B. mori genome, akin to other lepidopterans. Temporal expression analysis revealed higher cry1 gene expression during the photophase and lower expression during the scotophase, with knockouts of core clock genes (per, tim, Clk and cyc) disrupting this temporal expression pattern. Using CRISPR/Cas9-mediated genome editing, we established a cry1 knockout strain in p50T, a bivoltine strain exhibiting clear photoperiodism during both embryonic and larval stages. Although the wild-type strain displayed circadian rhythm in eclosion under continuous darkness, the cry1 knockout strain exhibited arrhythmic eclosion, implicating B. mori cry1 in the circadian clock feedback loop governing behavior rhythms. Females of the cry1 knockout strain failed to control photoperiodic diapause induction during both embryonic and larval stages, mirroring the diapause phenotype of the wild-type individuals reared under constant darkness, indicating that B. mori CRY1 contributes to photoperiodic time measurement as a photoreceptor. Furthermore, photoperiodic diapause induction during the larval stage was abolished in a cry1/tim double-knockout strain, suggesting that photic information received by CRY1 is relayed to the circadian clock. Overall, this study represents the first evidence of cry1 involvement in insect photoperiodism, specifically in diapause induction.

Drosophila ezoana uses morning and evening oscillators to adjust its rhythmic activity to different daylengths but only the morning oscillator to measure night length for photoperiodic responses

Animals living at high latitudes are exposed to prominent seasonal changes to which they need to adapt to survive. By applying Zeitgeber cycles of different periods and photoperiods we show here that high-latitude D. ezoana flies possess evening oscillators and highly damped morning oscillators that help them adapting their activity rhythms to long photoperiods. In addition, the damped morning oscillators are involved in timing diapause. The flies measure night length and use external coincidence for timing diapause. We discuss the clock protein TIMELESS (d-TIM) as the molecular correlate and the small ventrolateral clock neurons (s-LNvs) as the anatomical correlates of the components measuring night length.

The circadian and photoperiodic clock of the pea aphid

The pea aphid, Acyrthosiphon pisum, is a paradigmatic photoperiodic species that exhibits a remarkable annual life cycle, which is tightly coupled to the seasonal changes in day length. During spring and summer, characterised by longer days, aphid populations consist exclusively of viviparous females that reproduce parthenogenetically. When autumn comes and the days shorten, aphids switch their reproductive mode and generate males and oviparous sexual females, which mate and produce cold-resistant eggs that overwinter and survive the unfavourable season. While the photoperiodic responses have been well described, the nature of the timing mechanisms which underlie day length discrimination are still not completely understood. Experiments from the 1960's suggested that aphids rely on an 'hourglass' clock measuring the elapsed time during the dark night by accumulating a biochemical factor, which reaches a critical threshold at a certain night length and triggers the switch in reproduction mode. However, the photoperiodic responses of aphids can also be attributed to a strongly dampened circadian clock. Recent studies have uncovered the molecular components and the location of the circadian clock in the brain of the pea aphid and revealed that it is well connected to the neurohormonal system controlling aphid reproduction. We provide an overview of the putative mechanisms of photoperiodic control in aphids, from the photoreceptors involved in this process to the circadian clock and the neuroendocrine system.

Revealing the profound influence of diapause on gene expression: Insights from the annual transcriptome of the copepod Calanus finmarchicus

Annual rhythms are observed in living organisms with numerous ecological implications. In the zooplanktonic copepod Calanus finmarchicus, such rhythms are crucial regarding its phenology, body lipid accumulation, and global carbon storage. Climate change drives annual biological rhythms out of phase with the prevailing environmental conditions with yet unknown but potentially catastrophic consequences. However, the molecular dynamics underlying phenology are still poorly described. In a rhythmic analysis of C. finmarchicus annual gene expression, results reveal that more than 90% of the transcriptome shows significant annual rhythms, with abrupt and dramatic upheaval between the active and diapause life cycle states. This work explores the implication of the circadian clock in the annual timing, which may control epigenetic mechanisms to profoundly modulate gene expression in response to calendar time. Results also suggest an increased light sensitivity during diapause that would ensure the photoperiodic entrainment of the endogenous annual clock.

Integration of photoperiodic and temperature cues by the circadian clock to regulate insect seasonal adaptations

Organisms adapt to unfavorable seasonal conditions to survive. These seasonal adaptations rely on the correct interpretation of environmental cues such as photoperiod, and temperature. Genetic studies in several organisms, including the genetic powerhouse Drosophila melanogaster, indicate that circadian clock components, such as period and timeless, are involved in photoperiodic-dependent seasonal adaptations, but our understanding of this process is far from complete. In particular, the role of temperature as a key factor to complement photoperiodic response is not well understood. The development of new sequencing technologies has proven extremely useful in understanding the plastic changes that the clock and other cellular components undergo in different environmental conditions, including changes in gene expression and alternative splicing. This article discusses the integration of photoperiod and temperature for seasonal biology as well as downstream molecular and cellular pathways involved in the regulation of physiological adaptations that occur with changing seasons. We focus our discussion on the current understanding of the involvement of the molecular clock and the circadian clock neuronal circuits in these adaptations in D. melanogaster.

Consuming royal jelly alters several phenotypes associated with overwintering dormancy in mosquitoes

Introduction

Females of the Northern house mosquito, Culex pipiens, enter an overwintering dormancy, or diapause, in response to short day lengths and low environmental temperatures that is characterized by small egg follicles and high starvation resistance. During diapause, Culex pipiens Major Royal Jelly Protein 1 ortholog (CpMRJP1) is upregulated in females of Cx. pipiens. This protein is highly abundant in royal jelly, a substance produced by honey bees (Apis mellifera), that is fed to future queens throughout larval development and induces the queen phenotype (e.g., high reproductive activity and longer lifespan). However, the role of CpMRJP1 in Cx. pipiens is unknown.

Methods

We first conducted a phylogenetic analysis to determine how the sequence of CpMRJP1 compares with other species. We then investigated how supplementing the diets of both diapausing and nondiapausing females of Cx. pipiens with royal jelly affects egg follicle length, fat content, protein content, starvation resistance, and metabolic profile.

Results

We found that feeding royal jelly to females reared in long-day, diapause-averting conditions significantly reduced the egg follicle lengths and switched their metabolic profiles to be similar to diapausing females. In contrast, feeding royal jelly to females reared in short-day, diapause-inducing conditions significantly reduced lifespan and switched their metabolic profile to be similar nondiapausing mosquitoes. Moreover, RNAi directed against CpMRJPI significantly increased egg follicle length of short-day reared females, suggesting that these females averted diapause.

Discussion

Taken together, our data show that consuming royal jelly reverses several key seasonal phenotypes of Cx. pipiens and that these responses are likely mediated in part by CpMRJP1.

Neuropeptidergic regulation of insect diapause by the circadian clock

Diapause is an endocrine-mediated strategy used by insects to survive seasons of adverse environmental conditions. Insects living in temperate zones are regularly exposed to such conditions in the form of winter. To survive winter, they must prepare for it long before it arrives. A reliable indicator of impending winter is the shortening of day length. To measure day length, insects need their circadian clock as internal time reference. In this article, I provide an overview of the current state of knowledge on the neuropeptides that link the clock to the diapause inducing hormonal brain centers.

Daphnia uses its circadian clock for short-day recognition in environmental sex determination

Some organisms have developed a mechanism called environmental sex determination (ESD), which allows environmental cues, rather than sex chromosomes or genes, to determine offspring sex.1,2,3,4 ESD is advantageous to optimize sex ratios according to environmental conditions, enhancing reproductive success.5,6 However, the process by which organisms perceive and translate diverse environmental signals into offspring sex remains unclear. Here, we analyzed the environmental perception mechanism in the crustacean, Daphnia pulex, a seasonal (photoperiodic) ESD arthropod, capable of producing females under long days and males under short days.7,8,9,10 Through breeding experiments, we found that their circadian clock likely contributes to perception of day length. To explore this further, we created a genetically modified daphnid by knocking out the clock gene, period, using genome editing. Knockout disrupted the daphnid's ability to sustain diel vertical migration (DVM) under constant darkness, driven by the circadian clock, and leading them to produce females regardless of day length. Additionally, when exposed to an analog of juvenile hormone (JH), an endocrine factor synthesized in mothers during male production, or subjected to unfavorable conditions of high density and low food availability, these knockout daphnids produced males regardless of day length, like wild-type daphnids. Based on these findings, we propose that recognizing short days via the circadian clock is the initial step in sex determination. This recognition subsequently triggers male production by signaling the endocrine system, specifically via the JH signal. Establishment of a connection between these two processes may be the crucial element in evolution of ESD in Daphnia.

Photoperiodism of Diapause Induction in the Silkworm, Bombyx mori

The silkworm Bombyx mori exhibits a photoperiodic response (PR) for embryonic diapause induction. This article provides a comprehensive review of literature on the silkworm PR, starting from early works on population to recent studies uncovering the molecular mechanism. Makita Kogure (1933) conducted extensive research on the PR, presenting a pioneering paper on insect photoperiodism. In the 1970s and 80s, artificial diets were developed, and the influence of nutrition on PR was well documented. The photoperiodic photoreceptor has been investigated from organ to molecular level in the silkworm. Culture experiments demonstrated that the photoperiodic induction can be programmed in an isolated brain (Br)-subesophageal ganglion (SG) complex with corpora cardiaca (CC)-corpora allata (CA). The requirement of dietary vitamin A for PR suggests the involvement of opsin pigment in the photoperiodic reception, and a cDNA encoding an opsin (Boceropsin) was cloned from the brain. The effector system concerning the production and secretion of diapause hormone (DH) has also been extensively investigated in the silkworm. DH is produced in a pair of posterior cells of SG, transported to CC by nervi corporis cardiaci, and ultimately released into the hemolymph. Possible involvement of GABAergic and corazonin (Crz) signal pathways was suggested in the control of DH secretion. Knockout (KO) experiments of GABA transporter (GAT) and circadian clock genes demonstrated that GAT plays a crucial role in PR through circadian control. A model outlining the PR mechanism, from maternal photoperiodic light reception to DH secretion, has been proposed.

Transcriptome-based analysis reveals a crucial role of the 20E/HR3 pathway in the diapause of Pieris rapae

Pieris rapae is among the most damaging pests globally, and diapause makes it highly resistant to environmental stresses, playing a crucial role in the survival and reproduction of P. rapae while exacerbating the challenges of pest management and control. However, the mechanisms of its diapause regulation remain poorly understood. This research used RNA sequencing to profile the transcriptomes of three diapause phases (induction and preparation, initiation, maintenance) and synchronous nondiapause phases in P. rapae. During each comparison phase, 759, 1045, and 4721 genes were found to be differentially expressed. Among these, seven clock genes and seven pivotal hormone synthesis and metabolism genes were identified as having differential expression patterns in diapause type and nondiapause type. The weighted gene co-expression network analysis (WGCNA) revealed the red and blue modules as pivotal for diapause initiation, while the grey module was identified to be crucial to diapause maintenance. Meanwhile, the hub genes HDAC11, METLL16D, Dyw-like, GST, and so on, were identified within these hub modules. Moreover, an ecdysone downstream nuclear receptor gene, HR3, was found to be a shared transcription factor across all three phases. RNA interference of HR3 resulted in delayed pupal development, indicating its involvement in regulating pupal dipause in P. rapae. The further hormone assays revealed that the 20-hydroxyecdysone (20E) titer in diapause type pupae was lower than that in nondiapause type pupae, which exhibited a similar trend to HR3. When 20E was injected into diapause pupae, the HR3 expression levels were improved, and the pupal diapause were broken. These results indicate that the 20E/HR3 pathway is a critical pathway for the diapause regulation of P. rapae, and perturbing this pathway by ecdysone treatment or RNAi would result in the disruption of diapause. These findings provide initial insights into the molecular mechanisms of P. rapae diapause and suggest the potential use of ecdysone analogs and HR3 RNAi pesticides, which specifically target to diapause, as a means of pest control in P. rapae.

Light pollution disrupts circadian clock gene expression in two mosquito vectors during their overwintering dormancy

Artificial light at night (ALAN) is an increasingly important form of environmental disturbance as it alters Light:Dark cycles that regulate daily and seasonal changes in physiology and phenology. The Northern house mosquito (Culex pipiens) and the tiger mosquito (Aedes albopictus) enter an overwintering dormancy known as diapause that is cued by short days. These two species differ in diapause strategy: Cx. pipiens diapause as adult females while Ae. albopictus enter a maternally-programmed, egg diapause. Previous studies found that ALAN inhibits diapause in both species, but the mechanism is unknown. As the circadian clock is implicated in the regulation of diapause in many insects, we examined whether exposure to ALAN altered the daily expression of core circadian cloc genes (cycle, Clock, period, timeless, cryptochrome 1, cryptochrome 2, and Par domain protein 1) in these two species when reared under short-day, diapause-inducing conditions. We found that exposure to ALAN altered the abundance of several clock genes in adult females of both species, but that clock gene rhythmicity was maintained for most genes. ALAN also had little effect on clock gene abundance in mature oocytes that were dissected from female Ae. albopictus that were reared under short day conditions. Our findings indicate that ALAN may inhibit diapause initiation through the circadian clock in two medically-important mosquitoes.

Identification of CYCLE targets that contribute diverse features of circadian rhythms in the mosquito Culex pipiens

Culex pipiens demonstrates robust circadian rhythms in adult eclosion, flight activity, mating, and development. These rhythmic patterns are believed to be controlled by the endogenous light-entrainable circadian clock that consists of positive and negative regulators working in a transcription-translation feedback loop. Moreover, these mosquitoes undergo seasonal diapause in exposure to the short photoperiod of late summer or early fall. However, the exact genetic and cellular mechanism behind the clock gene-mediated activity pattern, seasonal time measurement, and subsequent diapause initiation still need to be unraveled. To determine the possible linkage between clock genes and downstream processes, here we employed ChIP-sequencing to identify the direct targets of one of the core clock proteins, Cycle (CYC). The nearest genes with peaks mapping to their 1Kb upstream region of the transcription start site were extracted and scanned for consensus E box sequences, resulting in a dataset comprising the target genes possibly regulated by CYC. Based on the highest fold enrichment and functional relevance, we identified genes relating to five gene categories of potential interest, including peptide/receptors, neurotransmission, olfaction, immunity, and reproductive growth. Of these, we validated fourteen genes with ChIP-qPCR and qRT-PCR. These genes showed a significantly high expression in dusk compared to dawn in concert with the activity level of the CYC transcription factor and are thus strong candidates for mediating circadian rhythmicity and possibly regulating seasonal shifts in mosquito reproductive activity.

Pigment-dispersing factor is present in circadian clock neurons of pea aphids and may mediate photoperiodic signalling to insulin-producing cells

The neuropeptide pigment-dispersing factor (PDF) plays a pivotal role in the circadian clock of most Ecdysozoa and is additionally involved in the timing of seasonal responses of several photoperiodic species. The pea aphid, Acyrthosiphon pisum, is a paradigmatic photoperiodic species with an annual life cycle tightly coupled to the seasonal changes in day length. Nevertheless, PDF could not be identified in A. pisum so far. In the present study, we identified a PDF-coding gene that has undergone significant changes in the otherwise highly conserved insect C-terminal amino acid sequence. A newly generated aphid-specific PDF antibody stained four neurons in each hemisphere of the aphid brain that co-express the clock protein Period and have projections to the pars lateralis that are highly plastic and change their appearance in a daily and seasonal manner, resembling those of the fruit fly PDF neurons. Most intriguingly, the PDF terminals overlap with dendrites of the insulin-like peptide (ILP) positive neurosecretory cells in the pars intercerebralis and with putative terminals of Cryptochrome (CRY) positive clock neurons. Since ILP has been previously shown to be crucial for seasonal adaptations and CRY might serve as a circadian photoreceptor vital for measuring day length, our results suggest that PDF plays a critical role in aphid seasonal timing.

Characterizing seasonal changes in the reproductive activity of Culex mosquitoes throughout the fall, winter, and spring in Ohio

BACKGROUND

Culex mosquitoes are the primary vectors of West Nile virus (WNV) across the USA. Understanding when these vectors are active indicates times when WNV transmission can occur. This study determined the proportion of female Culex mosquitoes that were in diapause during the fall and winter and when they terminated diapause and began blood feeding in the spring.

METHODS

Mosquitoes were collected from parks using various traps and/or aspirated from culverts in Franklin County, Ohio, from October to mid-May from 2019 to 2022. Culex mosquitoes were morphologically identified to species, and the ovaries of females were dissected to determine their diapause and parity statuses.

RESULTS

By early October 2021, roughly 95% of Culex pipiens collected in culverts were in diapause and 98% of Cx. erraticus were in diapause. Furthermore, gravid and blood-fed Culex salinarius, Cx. pipiens, and Cx. restuans were collected in late November in 2019 and 2021 in standard mosquito traps. In the winter of 2021, the proportions of non-diapausing Culex decreased within culverts. The last non-diapausing Cx. erraticus was collected in late December 2021 while the final non-diapausing Cx. pipiens was collected in mid-January 2022, both in culverts. Roughly 50% of Cx. pipiens terminated diapause by mid-March 2022, further supported by our collections of gravid females in late March in all 3 years of mosquito collection. In fact, male mosquitoes of Cx. pipiens, Cx. restuans, and Cx. territans were collected by the 1st week of May in 2022, indicating that multiple species of Culex produced a second generation that reached adulthood by this time.

CONCLUSIONS

We collected blood-fed and gravid Culex females into late November in 2 of the 3 years of our collections, indicating that it might be possible for WNV transmission to occur in late fall in temperate climates like Ohio. The persistence of non-diapausing Cx. pipiens and Cx. erraticus throughout December has important implications for the winter survival of WNV vectors and our overall understanding of diapause. Finally, determining when Culex terminate diapause in the spring may allow us to optimize mosquito management programs and reduce the spread of WNV before it is transmitted to humans.

Quantitative transcriptomic and proteomic analyses reveal the potential maintenance mechanism of female adult reproductive diapause in Chrysoperla nipponensis

BACKGROUND

The green lacewing Chrysoperla nipponensis is an important natural enemy of many insect pests and exhibits reproductive diapause to overwinter. Our previous studies showed that adult C. nipponensis enters reproductive diapause under a short-day photoperiod. However, the molecular mechanism underlying diapause maintenance in C. nipponensis is still unknown.

RESULTS

The total lipid and triglyceride content showed the reservation and degradation of energy during diapause in C. nipponensis. Thus, we performed combined transcriptomic and proteomic analyses of female reproductive diapause in C. nipponensis at three ecophysiological phases (initiation, maintenance and termination). A total of 64 388 unigenes and 5532 proteins were identified from the transcriptome and proteome. In-depth dissection of the gene-expression dynamics revealed that differentially expressed genes and proteins were predominately involved in the lipid and carbohydrate metabolic pathways, in particular fatty acid metabolism, metabolic pathways and the citrate cycle. Among of these genes, TIM, CLK, JHAMT2, PMK, HMGS, HMGR, FKBP39, Kr-h1, Phm, ECR, IR1, ILP3, ILP4, mTOR, ACC, LSD1 and LSD2 were differentially expressed in diapause and non-diapause female adults of C. nipponensis. The expression patterns of these genes were consistent with the occurrence of vitellogenesis and expression of either Vg or VgR.

CONCLUSION

Our findings indicated that diapause adult C. nipponensis accumulate energy resources to overwinter. Transcriptomic and proteomic analyses suggested candidate key genes involved in the maintenance of C. nipponensis during adult reproductive diapause. Taken together, these results provide in-depth knowledge to understand the maintenance mechanism of C. nipponensis during adult reproductive diapause. © 2023 Society of Chemical Industry.

Circadian clock genes and photoperiodic diapause in the moth Sesamia nonagrioides

Insects, like most organisms, have an internal circadian clock that oscillates with a daily rhythmicity, and a timing mechanism (photoperiodic clock) that mediates seasonal events, including diapause. It has been argued that there is a connection between the two clocks. The Mediterranean corn stalk borer moth, Sesamia nonagrioides, undergoes facultative diapause governed by photoperiod. To obtain clues to the link between the molecular mechanism of circadian and photoperiod clocks, we cloned and investigated the expression profiles of the clock genes Snper, Sntim, Sncyc and Sncry1 in the aforementioned moth species. Our previous results suggested that these genes might be implicated in the regulation of the diapause programming in S. nonagrioides. Here we studied the expression patterns of these four clock genes in larvae reared under abnormal non-24 h light-dark cycles (L10:D62 and L10:D14:L10:D62) in order to assess whether disruption of circadian clock would have any effect in the photoperiodic regulation of diapause. In the L10:D14:L10:D62 cycle abnormal expression patterns of the Sntim/Sncry1 and Snper/Sncyc, pairs were found, compared to normal 24 h light-dark photoperiods suggesting that individual clock genes are acting independently in the molecular diapause program of S. nonagrioides. Photoperiod therefore appears to be the crucial signal for the regulation of these four genes.

Hatching rhythm and clock gene expression in the egg of the pea aphid, Acyrthosiphon pisum

Many insects exhibit diel rhythms in physiology and behavior, driven by an endogenous circadian clock. Although aphids are paradigmatic insects whose photoperiodic time measurement is based on a heavily damped circadian clock, there is a lack of empirical data on such a damped circadian clock. The present study investigated the temporal distribution of hatching and the temporal expression patterns of circadian clock genes in the pea aphid, Acyrthosiphon pisum under light-dark (LD) cycles and constant darkness (DD). Hatching occurred intensively in the early photophase, and this rhythm persisted under LD cycles, but damped under DD for a few days. Of the six clock genes analyzed, cyc showed a temporal change in expression under LD cycles, whereas this temporal change was lost under DD. These results suggest that the circadian clock of A. pisum is easily damped during the embryonic stage, supporting the heavily damped oscillator model in photoperiodic time measurement of aphids.

Molecular Mechanisms of Winter Survival

Winter provides many challenges for insects, including direct injury to tissues and energy drain due to low food availability. As a result, the geographic distribution of many species is tightly coupled to their ability to survive winter. In this review, we summarize molecular processes associated with winter survival, with a particular focus on coping with cold injury and energetic challenges. Anticipatory processes such as cold acclimation and diapause cause wholesale transcriptional reorganization that increases cold resistance and promotes cryoprotectant production and energy storage. Molecular responses to low temperature are also dynamic and include signaling events during and after a cold stressor to prevent and repair cold injury. In addition, we highlight mechanisms that are subject to selection as insects evolve to variable winter conditions. Based on current knowledge, despite common threads, molecular mechanisms of winter survival vary considerably across species, and taxonomic biases must be addressed to fully appreciate the mechanistic basis of winter survival across the insect phylogeny.

Light Pollution Disrupts Seasonal Differences in the Daily Activity and Metabolic Profiles of the Northern House Mosquito, Culex pipiens

The Northern House mosquito, Culex pipiens, is an important disease vector, and females are capable of surviving the winter in a state of overwintering diapause. This species' diapause response has been extensively studied, and recent evidence suggests that the circadian clock is involved in measuring seasonal changes in daylength to initiate the diapause response. However, differences in the circadian activity of diapausing and non-diapausing Cx. pipiens have not been thoroughly investigated. Additionally, recent findings indicate that artificial light at night (ALAN) can disrupt mosquito diapause, potentially prolonging the mosquito biting season. We compared the circadian locomotor activity of mosquitoes reared in diapause-averting, long-day conditions and diapause-inducing, short-day conditions with and without ALAN to elucidate the interplay between circadian activity, diapause, and light pollution. We also uncovered metabolic differences between mosquitoes reared under diapausing and non-diapausing photoperiods with and without ALAN by measuring the concentration of protein, fructose, glycogen, water-soluble carbohydrates, and lipids. We found that ALAN exposure altered several diapause-associated phenotypes including slightly, but not significantly, increasing activity levels in short day-reared mosquitoes; and preventing some short day-reared mosquitoes from accumulating lipids. ALAN also significantly reduced glycogen and water-soluble carbohydrate levels in long day-reared mosquitoes. Based on our findings, light pollution may decrease insect fitness by perturbing metabolism, and may also impact several phenotypes associated with insect diapause, potentially extending the mosquito biting season and preventing insects in urban environments from overwintering successfully.

Knockouts of positive and negative elements of the circadian clock disrupt photoperiodic diapause induction in the silkworm, Bombyx mori

Diapause is one of the most important traits that have sustained insects to thrive. To survive harsh seasons, most insects can arrest their development and enter diapause. The photoperiod is the signal that indicates insects the proper timing to enter diapause. Circadian clock genes are shown to be involved in photoperiodic diapause induction in various insect species. The silkworm, Bombyx mori, enters diapause at the embryonic stage. In bivoltine strains, diapause determination is under maternal control and affected by temperature and photoperiodic conditions that mothers experienced during embryonic and larval stages. Two independent studies showed that knocking out the core clock gene, period, perturb photoperiodic diapause induction in B. mori. However, whether the circadian clock as whole or individual clock genes are responsible for the photoperiodic diapause induction remains unknown. In this study, using CRISPR/Cas9 we knocked out negative (period and timeless) and positive elements (Clock and cycle) in p50T, a bivoltine strain which exhibits photoperiodic diapause induction during both embryonic and larval stages. The temporal expression patterns of clock genes changed in each core clock gene knockout strain, suggesting disruption of normal feedback loops produced by circadian clock genes. Furthermore, the ability of female moths to appropriately produce diapause or non-diapause eggs in response to photoperiod in both embryonic and larval stages was lost in all knockout strains. Our results indicate the involvement of circadian clock in photoperiodic diapause induction in B. mori.

Characterizing the relative abundance of circadian transcription factors in diapausing and nondiapausing Northern house mosquitoes

The Northern house mosquito (Culex pipiens) is a major vector of West Nile virus. To survive harsh conditions in winter adult females of Cx. pipiens enter a state of arrested reproductive development called diapause. Diapause is triggered by the short daylengths of late summer and early fall. The methods by which Cx. pipiens measures daylength are still unknown. However, it is suspected that clock genes, which provide information on daylength, may also regulate diapause. The proteins produced by these genes often cycle in abundance throughout the day in diapausing and nondiapausing insects. Two clock genes suspected to control diapause are cycle (cyc) and Par domain protein1 (Pdp1) as they encode circadian transcription factors that may regulate genes that are involved in diapause. Using Western blotting we measured the relative protein abundance of CYC and PDP1 throughout the day in the whole bodies and the heads of Cx. pipiens reared under either long-day, diapause-averting conditions or short-day, diapause-inducing conditions. We found that in whole bodies there was no significant oscillation of CYC or PDP1 abundance in both long day and short day-reared mosquitoes. In the heads of long day-reared mosquitoes both CYC and PDP1 cycled. In contrast, only PDP1 abundance showed diel differences in abundance in the heads of short day-reared mosquitoes. These data bring us one step closer to understanding the role that CYC and PDP1 may play in regulating diapause and other biological processes.

Circadian Clock Genes Regulate Temperature-Dependent Diapause Induction in Silkworm Bombyx mori

The bivoltine strain of the domestic silkworm, Bombyx mori, exhibits a facultative diapause phenotype that is determined by maternal environmental conditions during embryonic and larval development. Although a recent study implicated a circadian clock gene period (per) in circadian rhythms and photoperiod-induced diapause, the roles of other core feedback loop genes, including timeless (tim), Clock (Clk), cycle (cyc), and cryptochrome2 (cry2), have to be clarified yet. Therefore, the aim of this study was to elucidate the roles of circadian clock genes in temperature-dependent diapause induction. To achieve this, per, tim, Clk, cyc, and cry2 knockout (KO) mutants were generated, and the percentages of diapause and non-diapause eggs were determined. The results show that per, tim, Clk, cyc, and cry2 regulated temperature-induced diapause by acting upstream of cerebral γ-aminobutyric acid (GABA)ergic and diapause hormone signaling pathways. Moreover, the temporal expression of the clock genes in wild-type (wt) silkworms was significantly different from that of thermosensitive transient receptor potential ankyrin 1 (TRPA1) KO mutants during embryonic development. Overall, the findings of this study provide target genes for regulating temperature-dependent diapause induction in silkworms.

Pigment Dispersing Factor Is a Circadian Clock Output and Regulates Photoperiodic Response in the Linden Bug, Pyrrhocoris apterus

Daily and annually cycling conditions manifested on the Earth have forced organisms to develop time-measuring devices. Circadian clocks are responsible for adjusting physiology to the daily cycles in the environment, while the anticipation of seasonal changes is governed by the photoperiodic clock. Circadian clocks are cell-autonomous and depend on the transcriptional/translational feedback loops of the conserved clock genes. The synchronization among clock centers in the brain is achieved by the modulatory function of the clock-dependent neuropeptides. In insects, the most prominent clock neuropeptide is Pigment Dispersing Factor (PDF). Photoperiodic clock measures and computes the day and/or night length and adjusts physiology accordingly to the upcoming season. The exact mechanism of the photoperiodic clock and its direct signaling molecules are unknown but, in many insects, circadian clock genes are involved in the seasonal responses. While in Drosophila, PDF signaling participates both in the circadian clock output and in diapause regulation, the weak photoperiodic response curve of D. melanogaster is a major limitation in revealing the full role of PDF in the photoperiodic clock. Here we provide the first description of PDF in the linden bug, Pyrrhocoris apterus, an organism with a robust photoperiodic response. We characterize in detail the circadian and photoperiodic phenotype of several CRISPR/Cas9-generated pdf mutants, including three null mutants and two mutants with modified PDF. Our results show that PDF acts downstream of CRY and plays a key role as a circadian clock output. Surprisingly, in contrast to the diurnal activity of wild-type bugs, pdf null mutants show predominantly nocturnal activity, which is caused by the clock-independent direct response to the light/dark switch. Moreover, we show that together with CRY, PDF is involved in the photoperiod-dependent diapause induction, however, its lack does not disrupt the photoperiodic response completely, suggesting the presence of additional clock-regulated factors. Taken together our data provide new insight into the role of PDF in the insect’s circadian and photoperiodic systems.

Maternally Instigated Diapause in Aedes albopictus: Coordinating Experience and Internal State for Survival in Variable Environments

The Asian tiger mosquito, Aedes albopictus, is one of the most dangerous invasive species in the world. Females bite mammalian hosts, including humans, to obtain blood for egg development. The ancestral range of Ae. albopictus likely spanned from India to Japan and this species has since invaded a substantial portion of the globe. Ae. albopictus can be broadly categorized into temperate and tropical populations. One key to their ability to invade diverse ecological spaces is the capacity of females to detect seasonal changes and produce stress-resistant eggs that survive harsh winters. Females living in temperate regions respond to cues that predict the onset of unfavorable environmental conditions by producing eggs that enter maternally instigated embryonic diapause, a developmentally arrested state, which allows species survival by protecting the embryos until favorable conditions return. To appropriately produce diapause eggs, the female must integrate environmental cues and internal physiological state (blood feeding and reproductive status) to allocate nutrients and regulate reproduction. There is variation in reproductive responses to environmental cues between interfertile tropical and temperate populations depending on whether females are actively producing diapause vs. non-diapause eggs and whether they originate from populations that are capable of diapause. Although diapause-inducing environmental cues and diapause eggs have been extensively characterized, little is known about how the female detects gradual environmental changes and coordinates her reproductive status with seasonal dynamics to lay diapause eggs in order to maximize offspring survival. Previous studies suggest that the circadian system is involved in detecting daylength as a critical cue. However, it is unknown which clock network components are important, how these connect to reproductive physiology, and how they may differ between behavioral states or across populations with variable diapause competence. In this review, we showcase Ae. albopictus as an emerging species for neurogenetics to study how the nervous system combines environmental conditions and internal state to optimize reproductive behavior. We review environmental cues for diapause induction, downstream pathways that control female metabolic changes and reproductive capacity, as well as diapause heterogeneity between populations with different evolutionary histories. We highlight genetic tools that can be implemented in Ae. albopictus to identify signaling molecules and cellular circuits that control diapause. The tools and discoveries made in this species could translate to a broader understanding of how environmental cues are interpreted to alter reproductive physiology in other species and how populations with similar genetic and circuit organizations diversify behavioral patterns. These approaches may yield new targets to interfere with mosquito reproductive capacity, which could be exploited to reduce mosquito populations and the burden of the pathogens they transmit.

Mapping and quantification of cryptochrome expression in the brain of the pea aphid Acyrthosiphon pisum

Aphids are paradigmatic photoperiodic animals often used to study the role of the circadian clock in the seasonal response. Previously, we described some elements of the circadian clock core (genes period and timeless) and output (melatonin, AANATs and PTTH) that could have a role in the regulation of the aphid seasonal response. More recently, we identified two opsins (C-ops and SWO4) as candidate input photoperiodic receptors. In the present report, we focus on the study of cryptochromes (cry) as photoreceptors of the circadian clock and discuss their involvement in the seasonal response. We analyse the expression of cry1 and cry2 genes in a circadian and seasonal context, and map their expression sites in the brain. We observe a robust rhythmic expression of cry2 peaking at dusk in phase with core clock genes period and timeless, while cry1 shows a weaker rhythm. Changes in cry1 and cry2 expression correlate with activation of the seasonal response, suggesting a possible link. Finally, we map the expression of cry1 and cry2 genes to clock neurons in the pars lateralis, a region essential for the photoperiodic response. Our results support a role for cry as elements of the aphid circadian clock and suggest a role in photoreception for cry1 and in clock repression for cry2.

Identification and dynamic expression profiling of circadian clock genes in Spodoptera litura provide new insights into the regulation of sex pheromone communication

Spodoptera litura is an important pest that causes significant economic damage to numerous crops worldwide. Sex pheromones (SPs) mediate sexual communication in S. litura and show a characteristic degree of rhythmic activity, occurring mainly during the scotophase; however, the specific regulatory mechanisms remain unclear. Here, we employed a genome-wide analysis to identify eight candidate circadian clock genes in S. litura. Sequence characteristics and expression patterns were analyzed. Our results demonstrated that some circadian clock genes might regulate the biosynthesis and perception of SPs by regulating the rhythmic expression of SP biosynthesis-related genes and SP perception-related genes. Interestingly, all potential genes exhibited peak expression in the scotophase, consistent with the SP could mediate courtship and mating behavior in S. litura. Our findings are helpful in elucidating the molecular mechanism by which circadian clock genes regulate sexual communication in S. litura.

Circadian clock outputs regulating insect photoperiodism: A potential role for glutamate transporter

Many temperate ectotherms survive winter by entering diapause - a state of developmental (or reproductive) suppression or arrest - in response to short autumnal day lengths. Day lengths are assessed by the circadian clock, the biological time-keeping system that governs biological rhythms with a period of approximately 24 h. However, clock output molecules controlling this photoperiodic response are largely unknown for many insects. To identify these molecules in Hemiptera, we performed RNAi knockdowns of several candidate genes in the bean bug Riptortus pedestris to determine whether their silencing affects photoperiodic regulation of ovarian development (reproductive diapause). Knockdown of diuretic hormone 31, short neuropeptide F, neuropeptide F, ion transport peptide, neuropeptide-like precursor 1, and choline acetyltransferase had no effect on ovarian development and were therefore ruled out as regulators of the photoperiodic response. However, knockdown of vesicular glutamate transporter promoted ovarian development under diapause-inducing short days, and this is the first report of the functional involvement of glutamate signalling in insect photoperiodism. Improved knockdown of this transporter (or receptor) and RNAi of other genes involved in glutamate signal transduction is required to verify its role as an output of the circadian clock.

Involvement of the Clock Gene Period in the Photoperiodism of the Silkmoth Bombyx mori

We established a knockout strain of a clock gene, period (per), by using TALEN in a bivoltine strain (Kosetsu) of Bombyx mori (Insecta, Lepidoptera), and examined the effect of per knockout on the circadian rhythm and photoperiodism. The generated per knockout allele was considered to be null, because a new stop codon was present in the insertion allele. The wild type (Kosetsu) showed clear circadian rhythms in eclosion and hatching, whereas the per knockout strain showed arrhythmic eclosion and hatching under constant darkness. In this strain, moreover, temporal expression changes of clock genes per and timeless were disrupted. The wild type showed a clear long-day response for induction of embryonic diapause: when larvae were reared under long-day and short-day conditions at 25°C, adults produced nondiapause and diapause eggs, respectively. However, the per knockout strain lost the sensitivity to photoperiod and laid nondiapause eggs under both conditions. We conclude that per plays an important role both in circadian rhythms and in photoperiodism of B. mori, indicating the involvement of the circadian clock consisting of per in the photoperiodism.

Neuroendocrine Regulation of Reproductive Dormancy in the Fruit Fly Drosophila melanogaster: A Review of Juvenile Hormone-Dependent Regulation

Animals can adjust their physiology, helping them survive and reproduce under a wide range of environmental conditions. One of the strategies to endure unfavorable environmental conditions such as low temperature and limited food supplies is dormancy. In some insect species, this may manifest as reproductive dormancy, which causes their reproductive organs to be severely depleted under conditions unsuitable for reproduction. Reproductive dormancy in insects is induced by a reduction in juvenile hormones synthesized in the corpus allatum (pl. corpora allata; CA) in response to winter-specific environmental cues, such as low temperatures and short-day length. In recent years, significant progress has been made in the study of dormancy-inducing conditions dependent on CA control mechanisms in Drosophila melanogaster. This review summarizes dormancy control mechanisms in D. melanogaster and discusses the implications for future studies of insect dormancy, particularly focusing on juvenile hormone-dependent regulation.

Circadian Clock Gene Period Contributes to Diapause via GABAeric-Diapause Hormone Pathway in Bombyx mori

Diapause is a developmental transition in insects based on seasonal adaptation to adversity; it is regulated by a circadian clock system and the endocrine system. However, the molecular node and its mechanism underlying the effects of these systems are still unclear. Here, a mutant of Bombyx mori with the circadian clock gene Period (Per) knocked out was constructed, which dramatically changed the classic diapause-destined pathway. Per-knockout silkworms powerfully attenuated, but could not completely block, the predetermined effects of temperature and photoperiod on diapause determination, and this effect depended on the diapause hormone (DH) pathway. The impaired transcription-translation feedback loop of the circadian clock system lacking the Per gene caused direct up-regulation of the expression of GRD, a receptor of γ-aminobutyric acid (GABA), by changing expression level of Cycle. The synthesis of GABA in the tissue complex of brain-suboesophageal ganglion then increased and restricted the decomposition, which continuously promoted the GABAergic signal to play a role, and finally inhibiting (delaying) the release of DH to the hemolymph, and reducing the diapause-inducing effect of DH. The results provided an example to explain the regulatory mechanism of the circadian clock on endocrine hormones in the silkworm.

Comparative Transcriptome Analysis Reveals bmo-miR-6497-3p Regulate Circadian Clock Genes during the Embryonic Diapause Induction Process in Bivoltine Silkworm

Diapause is one of the survival strategies of insects for confronting adverse environmental conditions. Bombyx mori displays typical embryonic diapause, and offspring diapause depends on the incubation environment of the maternal embryo in the bivoltine strains of the silkworm. However, the molecular mechanisms of the diapause induction process are still poorly understood. In this study, we compared the differentially expressed miRNAs (DEmiRs) in bivoltine silkworm embryos incubated at diapause- (25 °C) and non-diapause (15 °C)-inducing temperatures during the blastokinesis (BK) and head pigmentation (HP) phases using transcriptome sequencing. There were 411 known miRNAs and 71 novel miRNAs identified during the two phases. Among those miRNAs, there were 108 and 74 DEmiRs in the BK and HP groups, respectively. By the Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis of the predicted target genes of the DEmiRs, we found that aside from metabolism, the targets were also enriched in phototransduction-fly and insect hormone biosynthesis in the BK group and the HP group, respectively. Dual luciferase reporter assay illustrated that bmo-miR-6497-3p directly regulated Bmcycle and subsequently regulated the expression of circadian genes. These results imply that microRNAs, as vitally important regulators, respond to different temperatures and participate in the diapause induction process across species.

Photoperiodic Response in the Pars Intercerebralis Neurons, Including Plast-MIP Neurons, in the Brown-Winged Green Bug, Plautia stali

Many insects in temperate regions avoid environmental adversity for reproduction, and thus enter reproductive diapause according to photoperiod. This reproductive diapause is induced by inhibition of juvenile hormone biosynthesis in the corpus allatum. Some neuropeptides that have an effect on juvenile hormone biosynthesis have been detected in insect brains. Thus, the reproductive diapause may be photoperiodically regulated by these juvenile hormones-controlling neuropeptides. However, there is limited understanding of how the neurons expressing these neuropeptides respond to the photoperiod and control the peptide release accordingly. Here, we performed electrophysiological analyses in the pars intercerebralis (PI) of Plautia stali, where juvenile hormone inhibitory neuropeptides, Plautia stali myoinhibitory peptides (Plast-MIPs) are expressed. We found that the large neurons in the PI showed very high firing activity under diapause-inducing short day conditions. Neurotracer staining revealed that all recorded neurons projected to the nervus corporis cardiaci 1, which is known to be connected to the corpus cardiacum-corpus allatum complex. Finally, we determined how many of the large PI cells expressed Plast-MIP by single cell reverse transcription PCR. About half of large PI neurons coexpressed Plast-Mip and other neuropeptides, Diuretic hormone 44 and insulin-like peptide 1. The remaining cells only expressed Diuretic hormone 44 and insulin-like peptide 1. The present results suggested that large PI neurons, including Plast-MIP neurons, have enhanced activity under short day conditions, which may increase Plast-MIP release to the corpus cardiacum-corpus allatum complex and thus contribute to reproductive diapause.

Oviposition-promoting pars intercerebralis neurons show period-dependent photoperiodic changes in their firing activity in the bean bug

Animals show photoperiodic responses in physiology and behavior to adapt to seasonal changes. Recent genetic analyses have demonstrated the significance of circadian clock genes in these responses. However, the importance of clock genes in photoperiodic responses at the cellular level and the physiological roles of the cellular responses are poorly understood. The bean bug Riptortus pedestris shows a clear photoperiodic response in its reproduction. In the bug, the pars intercerebralis (PI) is an important brain region for promoting oviposition. Here, we analyzed the role of the photoperiodic neuronal response and its relationship with clock genes, focusing on PI neurons. Large PI neurons exhibited photoperiodic firing changes, and high firing activities were primarily found under photoperiodic conditions suitable for oviposition. RNA interference-mediated knockdown of the clock gene period abolished the photoperiodic response in PI neurons, as well as the response in ovarian development. To clarify whether the photoperiodic response in the PI was dependent on ovarian development, we performed an ovariectomy experiment. Ovariectomy did not have significant effects on the firing activity of PI neurons. Finally, we identified the output molecules of the PI neurons and analyzed the relevance of the output signals in oviposition. PI neurons express multiple neuropeptides-insulin-like peptides and diuretic hormone 44-and RNA interference of these neuropeptides reduced oviposition. Our results suggest that oviposition-promoting peptidergic neurons in the PI exhibit a circadian clock-dependent photoperiodic firing response, which contributes to the photoperiodic promotion of oviposition.

Induction of diapause by clock proteins period and timeless via changes in PTTH and ecdysteroid titer in the sugar beet moth, Scrobipalpa ocellatella (Lepidoptera: Gelechiidae)

The sugar beet moth, Scrobipalpa ocellatella (Boyd), one of the most severe sugar beet pests, causes quantitative and qualitative yield losses late in the autumn. Previously, it was shown that low temperature and short-day photoperiod together cause diapause induction in pupae. Here, the interaction of the critical elements of the diapause induction, including the period (PER), timeless (TIM), prothoracicotropic hormone (PTTH), and ecdysteroid titer, were investigated. Immunohistochemistry results showed that the number of period immunoreactivity (PER-ir) and TIM-ir cells in nondiapause pupae (NDP) was lower than in the brain of the diapause pupae (DP). Moreover, the number of PER-ir and TIM-ir cells in the protocerebrum and optic lobe (OL) of NDP was lower than DP. Moreover, lower PTTH content in the brain and hemolymph of DP was confirmed by competitive enzyme-linked immunosorbent assay. Enzyme immunoassay showed a lower 20-hydroxyecdysone (20E) titer in the hemolymph of the DP compared with the NDP. Within a short-day condition, PER and TIM titers increased in the brain leading to decreasing PTTH titers in the brain and hemolymph that caused decreasing 20E titer in the hemolymph, leading to the induction of diapause. This study suggests that PER and TIM could be one of the brain factors that play an essential role in regulating diapause in S. ocellatella.

The urban-adapted underground mosquito Culex pipiens form molestus maintains exogenously influenceable circadian rhythms

Genes known to affect circadian rhythms (i.e. 'clock genes') also influence the photoperiodic induction of overwintering reproductive diapause in the northern house mosquito, Culex pipiens f. pipiens. This suggests that molecular changes in one or more clock genes could contribute to the inability to diapause in a second form of this mosquito, Culex pipiens f. molestus. Temperate populations of Cx. pipiens f. molestus inhabit underground locations generally devoid of predictable photoperiods. For this reason, there could be limited fitness consequences if the hypothesized molecular changes to its clock genes also eliminated this mosquito's ability to regulate circadian rhythms in response to photoperiod variation. Here, we demonstrate that in contrast to this prediction, underground derived Cx. pipiens f. molestus retain exogenously influenceable circadian rhythms. Nonetheless, our genetic analyses indicate that the gene Helicase domino (dom) has a nine-nucleotide, in-frame deletion specific to Cx. pipiens f. molestus. Previous work has shown that splice variants in this gene differentially influence circadian behavior in Drosophila melanogaster. We also find derived, non-synonymous single nucleotide polymorphisms (SNPs) in eight genes that may also affect circadian rhythms and/or diapause induction in Cx. pipiens f. molestus. Finally, four putative circadian genes were found to have no quantifiable expression during any examined life stage, suggesting potential regulatory effects. Collectively, our findings indicate that the distinct, but molecularly interconnected life-history traits of diapause induction and circadian rhythms are decoupled in Cx. pipiens f. molestus and suggest this taxon may be a valuable tool for exploring exogenously influenced phenotypes in mosquitoes more broadly.

Mapping PERIOD-immunoreactive cells with neurons relevant to photoperiodic response in the bean bug Riptortus pedestris

Circadian clock genes are involved in photoperiodic responses in many insects; however, there is a lack of understanding in the neural pathways that process photoperiodic information involving circadian clock cells. PERIOD-immunohistochemistry was conducted in the bean bug Riptortus pedestris to localise clock cells and their anatomical relationship with other brain neurons necessary for the photoperiodic response. PERIOD-immunoreactive cells were found in the six brain regions. In the optic lobe, two cell groups called lateral neuron lateral (LNl) and lateral neuron medial (LNm), were labelled anterior medial to the medulla and lobula, respectively. In the protocerebrum of the central brain, dorsal neuron (Prd), posterior neuron (Prp), and antennal lobe posterior neuron (pAL) were found. In the deutocerebrum, antennal lobe local neurons (ALln) were detected. Double immunohistochemistry revealed that PERIOD and serotonin were not co-localised. Furthermore, pigment-dispersing factor-immunoreactive neurons and anterior lobula neurons essential for R. pedestris photoperiodic response were not PERIOD immunopositive. LNl cells were located in the vicinity of the pigment-dispersing factor immunoreactive cells at the anterior base of the medulla. LNm cells were located close to the somata of the anterior lobula neurons. Fibres from the anterior lobula neurons and pigment-dispersing factor-immunoreactive neurons had contacts at the anterior base of the medulla. It is suggested that LNl cells work as clock cells involved in the photoperiodic response and the region at the medulla anterior base serves as a hub to receive photic and clock information relevant to the photoperiodic clock in R. pedestris.

Hello Darkness, My Old Friend: A Tutorial of Nanda-Hamner Protocols

Plants and animals use circadian and photoperiodic timekeeping mechanisms to respond to daily and seasonal changes in light:dark and appropriately coordinate their development. Although the mechanisms that may connect the circadian and photoperiodic clock are still unclear in many species, researchers have been using Nanda-Hamner protocols for decades to elucidate how seasonal time is measured and determine whether seasonal responses have a circadian basis in a given species. In this brief tutorial we describe how to design and interpret the results of Nanda-Hamner experiments, and provide suggestions on how to use both Nanda-Hamner protocols and modern molecular experiments to better understand the mechanisms of seasonal timekeeping.

Model and Non-model Insects in Chronobiology

The fruit fly Drosophila melanogaster is an established model organism in chronobiology, because genetic manipulation and breeding in the laboratory are easy. The circadian clock neuroanatomy in D. melanogaster is one of the best-known clock networks in insects and basic circadian behavior has been characterized in detail in this insect. Another model in chronobiology is the honey bee Apis mellifera, of which diurnal foraging behavior has been described already in the early twentieth century. A. mellifera hallmarks the research on the interplay between the clock and sociality and complex behaviors like sun compass navigation and time-place-learning. Nevertheless, there are aspects of clock structure and function, like for example the role of the clock in photoperiodism and diapause, which can be only insufficiently investigated in these two models. Unlike high-latitude flies such as Chymomyza costata or D. ezoana, cosmopolitan D. melanogaster flies do not display a photoperiodic diapause. Similarly, A. mellifera bees do not go into "real" diapause, but most solitary bee species exhibit an obligatory diapause. Furthermore, sociality evolved in different Hymenoptera independently, wherefore it might be misleading to study the social clock only in one social insect. Consequently, additional research on non-model insects is required to understand the circadian clock in Diptera and Hymenoptera. In this review, we introduce the two chronobiology model insects D. melanogaster and A. mellifera, compare them with other insects and show their advantages and limitations as general models for insect circadian clocks.

Circadian transcription factors differentially regulate features of the adult overwintering diapause in the Northern house mosquito, Culex pipiens

The short days of late summer and early fall are the environmental cues that most temperate insects and other animals use to predict winter's arrival. Although it is still unclear precisely how insects measure daylength, there is mounting evidence that the circadian clock regulates seasonal responses including photoperiodic diapause. Females of the Northern house mosquito, Culex pipiens, enter an adult reproductive diapause in response to short daylengths. While in this state, females divert their resources from reproduction to survival, arresting egg follicle development and increasing fat content. Here, we characterized the expression profile of two circadian transcription factors, vrille (vri) and Par domain protein 1 (Pdp1), as well as genes downstream of the clock, takeout (to) and Nocturnin (Noc) and under different seasonal conditions. We saw that while vri mRNA oscillated under both long day and short day conditions, Pdp1 expression oscillated only under long day conditions and was constitutively upregulated in diapausing females. We saw similar expression profiles for to and Noc, suggesting that PDP1 might regulate their expression or that Pdp1, to and Noc might be regulated by the same transcription factor. We suppressed vri and Pdp1 using RNA interference. dsRNA against vri provided inconsistent results, sometimes stimulating autogenous egg follicle development in both long and short day-reared females, and other times had no effect. In contrast, knocking down Pdp1 prevented short day-reared females from accumulating fat reserves, but increased expression of to and Noc. Taken together, these data suggest that the circadian transcription factors Vri and Pdp1 may independently regulate signaling pathways underlying arrested egg follicle development and fat accumulation in diapausing females of Cx. pipiens.

Melatonin in the seasonal response of the aphid Acyrthosiphon pisum

Aphids display life cycles largely determined by the photoperiod. During the warm long-day seasons, most aphid species reproduce by viviparous parthenogenesis. The shortening of the photoperiod in autumn induces a switch to sexual reproduction. Males and sexual females mate to produce overwintering resistant eggs. In addition to this full life cycle (holocycle), there are anholocyclic lineages that do not respond to changes in photoperiod and reproduce continuously by parthenogenesis. The molecular or hormonal events that trigger the seasonal response (i.e., induction of the sexual phenotypes) are still unknown. Although circadian synthesis of melatonin is known to play a key role in vertebrate photoperiodism, the involvement of the circadian clock and/or of the hormone melatonin in insect seasonal responses is not so well established. Here we show that melatonin levels in the aphid Acyrthosiphon pisum are significantly higher in holocyclic aphids reared under short days than under long days, while no differences were found between anholocyclic aphids under the same conditions. We also found that melatonin is localized in the aphid suboesophageal ganglion (SOG) and in the thoracic ganglionic mass (TGM). In analogy to vertebrates, insect-type arylalkylamine N-acetyltransferases (i-AANATs) are thought to play a key role in melatonin synthesis. We measured the expression of four i-AANAT genes identified in A. pisum and localized two of them in situ in the insect central nervous systems (CNS). Levels of expression of these genes were compatible with the quantities of melatonin observed. Moreover, like melatonin, expression of these genes was found in the SOG and the TGM.