Light input pathways to the circadian clock of insects with an emphasis on the fruit fly Drosophila melanogaster

Deciphering a Beetle Clock: Individual and Sex-Dependent Variation in Daily Activity Patterns

Circadian clocks are inherent to most organisms, including cryptozoic animals that seldom encounter direct light, and regulate their daily activity cycles. A conserved suite of clock genes underpins these rhythms. In this study, we explore the circadian behaviors of the red flour beetle Tribolium castaneum, a significant pest impacting stored grain globally. We report on how daily light and temperature cues synchronize distinct activity patterns in these beetles, characterized by reduced morning activity and increased evening activity, anticipating the respective environmental transitions. Although less robust, rhythmicity in locomotor activity is maintained in constant dark and constant light conditions. Notably, we observed more robust rhythmic behaviors in males than females with individual variation exceeding those previously reported for other insect species. RNA interference targeting the Clock gene weakened locomotor activity rhythms. Our findings demonstrate the existence of a circadian clock and of clock-controlled behaviors in T. castaneum. Furthermore, they highlight substantial individual differences in circadian activity, laying the groundwork for future research on the relevance of individual variation in circadian rhythms in an ecological and evolutionary context.

Reorganization of circadian activity and the pacemaker circuit under novel light regimes

Many environmental features are cyclic, with predictable changes across the day, seasons and latitudes. Additionally, anthropogenic, artificial-light-induced changes in photoperiod or shiftwork-driven novel light/dark cycles also occur. Endogenous timekeepers or circadian clocks help organisms cope with such changes. The remarkable plasticity of clocks is evident in the waveforms of behavioural and molecular rhythms they govern. Despite detailed mechanistic insights into the functioning of the circadian clock, practical means to manipulate activity waveform are lacking. Previous studies using a nocturnal rodent model showed that novel light regimes caused locomotor activity to bifurcate such that mice showed two bouts of activity restricted to the dimly lit phases. Here, we explore the generalizability of these findings and leverage the genetic toolkit of Drosophila melanogaster to obtain mechanistic insights into this unique phenomenon. We find that dim scotopic illumination of specific durations induces circadian photoreceptor CRYPTOCHROME-dependent activity bifurcation in male flies. We show circadian reorganization of the pacemaker circuit, wherein the 'evening' neurons regulate the timing of both bouts of activity under novel light regimes. Our findings indicate that such environmental regimes can be exploited to design light cycles, which can ease the circadian waveform into synchronizing with challenging conditions.

Day-night gene expression reveals circadian gene disco as a candidate for diel-niche evolution in moths

Temporal ecological niche partitioning is an underappreciated driver of speciation. While insects have long been models for circadian biology, the genes and circuits that allow adaptive changes in diel-niches remain poorly understood. We compared gene expression in closely related day- and night-active non-model wild silk moths, with otherwise similar ecologies. Using an ortholog-based pipeline to compare RNA-Seq patterns across two moth species, we find over 25 pairs of gene orthologs showing differential expression. Notably, the gene disco, involved in circadian control, optic lobe and clock neuron development in Drosophila, shows robust adult circadian mRNA cycling in moth heads. Disco is highly conserved in moths and has additional zinc-finger domains with specific nocturnal and diurnal mutations. We propose disco as a candidate gene for the diversification of temporal diel-niche in moths.

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.

Circadian and daily rhythms of disease vector mosquitoes

Mosquitoes express a rich repertoire of daily 24-hour rhythms in biochemistry, physiology, and behavior. The nocturnal Anopheles and Culex and diurnal Aedes mosquitoes are major vectors of human disease, transmitting parasites and arboviruses, such as malaria and dengue. In this review, we explore the role that 24-hour diel and circadian rhythms play in shaping the temporal life of the mosquito. We focus on recent advances in our understanding of behavioral rhythms, focusing on locomotor/flight activity, host-seeking, biting/blood feeding, and mating. We examine the molecular circadian clock, photocycle, and light signals, which in combination shape the mosquito 24-hour temporal program. We address species- and sex-specific differences and highlight important selective pressures from dynamically changing environments. This work also provides new insights into disease transmission, insect control, and future experimental design.

OpsinLW2 serves as a circadian photoreceptor in the entrainment of circadian locomotor rhythm of a firebrat

Photic entrainment is an essential function of the circadian clock, which enables organisms to set the appropriate timing of daily behavioral and physiological events. Recent studies have shown that the mechanisms of the circadian clock and photic entrainment vary among insect species. This study aimed to elucidate the circadian photoreceptors necessary for photic entrainment in firebrats Thermobia domestica, one of the most primitive apterygote insects. A homology search of publicly available RNA sequence (RNA-seq) data from T. domestica exhibited a cryptochrome 2 (cry2) gene and three opsin genes, opsin long wavelength 1 (opLW1), opLW2, and opUV, as candidate circadian photoreceptors. We examined the possible involvement of these genes in photic entrainment of firebrat locomotor rhythms. Firebrats had the highest entrainability to the light-dark cycle of green light. Treatment with dsRNA of the candidate genes strongly downregulated the respective targeted genes, and in the case of opsin genes, other untargeted genes were occasionally downregulated to various degrees. Under constant light, most control firebrats became arrhythmic, whereas a fraction of those treated with double RNAi of the two opLWs remained rhythmic. Behavioral experiments revealed that the transient cycles necessary for re-entrainment to shifted light cycles were lengthened when opLW2 expression was reduced. These results suggest that opLW2 is involved in the photic entrainment of circadian rhythm in firebrats.

Connectome-driven neural inventory of a complete visual system

Vision provides animals with detailed information about their surroundings, conveying diverse features such as color, form, and movement across the visual scene. Computing these parallel spatial features requires a large and diverse network of neurons, such that in animals as distant as flies and humans, visual regions comprise half the brain's volume. These visual brain regions often reveal remarkable structure-function relationships, with neurons organized along spatial maps with shapes that directly relate to their roles in visual processing. To unravel the stunning diversity of a complex visual system, a careful mapping of the neural architecture matched to tools for targeted exploration of that circuitry is essential. Here, we report a new connectome of the right optic lobe from a male Drosophila central nervous system FIB-SEM volume and a comprehensive inventory of the fly's visual neurons. We developed a computational framework to quantify the anatomy of visual neurons, establishing a basis for interpreting how their shapes relate to spatial vision. By integrating this analysis with connectivity information, neurotransmitter identity, and expert curation, we classified the ~53,000 neurons into 727 types, about half of which are systematically described and named for the first time. Finally, we share an extensive collection of split-GAL4 lines matched to our neuron type catalog. Together, this comprehensive set of tools and data unlock new possibilities for systematic investigations of vision in Drosophila, a foundation for a deeper understanding of sensory processing.

Circadian rhythms in the Drosophila eye may regulate adaptation of vision to light intensity

The sensitivity of the eye at night would lead to complete saturation of the eye during the day. Therefore, the sensitivity of the eye must be down-regulated during the day to maintain visual acuity. In the Drosophila eye, the opening of TRP and TRPL channels leads to an influx of Ca++ that triggers down-regulation of further responses to light, including the movement of the TRPL channel and Gα proteins out of signaling complexes found in actin-mediated microvillar extensions of the photoreceptor cells (the rhabdomere). The eye also exhibits a light entrained-circadian rhythm, and we have recently observed that one component of this rhythm (BDBT) becomes undetectable by antibodies after exposure to light even though immunoblot analyses still detect it in the eye. BDBT is necessary for normal circadian rhythms, and in several circadian and visual mutants this eye-specific oscillation of detection is lost. Many phototransduction signaling proteins (e.g., Rhodopsin, TRP channels and Gα) also become undetectable shortly after light exposure, most likely due to a light-induced compaction of the rhabdomeric microvilli. The circadian protein BDBT might be involved in light-induced changes in the rhabdomere, and if so this could indicate that circadian clocks contribute to the daily adaptations of the eye to light. Likewise, circadian oscillations of clock proteins are observed in photoreceptors of the mammalian eye and produce a circadian oscillation in the ERG. Disruption of circadian rhythms in the eyes of mammals causes neurodegeneration in the eye, demonstrating the importance of the rhythms for normal eye function.

Circadian entrainment to red-light Zeitgebers and action spectrum for entrainment in the jewel wasp Nasonia vitripennis

Light is the most important environmental cue for the circadian system of most organisms to stay synchronized to daily environmental changes. Like many other insects, the wasp Nasonia vitripennis has trichromatic compound eye-based colour vision and is sensitive to the light spectrum ranging from UV to green. We recently described a red-sensitive, ocelli-based photoreceptor, but its contribution to circadian entrainment remains unclear. In this study, we investigated the possibility of Nasonia circadian light entrainment under long-wavelength red LED light-dark cycles and characterized the strength of red light as a potential Zeitgeber. Additionally, we measured the possibility of entrainment under various light intensities (from 5·1012 to 4·1015 photons·cm-2·s-1) and a broader range of wavelengths (455-656 nm) to construct corresponding action spectra for characterizing all circadian photoreceptors involved in photic entrainment. We also conducted electroretinogram (ERG) recordings for each wavelength in the compound eyes. Our findings demonstrate that Nasonia can entrain under red light dark cycles, and the sensory pathway underlying the red-light Zeitgeber response may reside in the ocelli. Combined with findings from previous research, we pose that blue- and green-sensitive rhodopsin photoreceptor cells function as the major circadian photoreceptors in both circadian entrainment by light-dark cycles and circadian phase shifts by light pulses, whereas the red-sensitive photoreceptor cell requires higher light intensity for its role in circadian entrainment by light-dark cycles.

A Closer Look at Histamine in Drosophila

The present work intends to provide a closer look at histamine in Drosophila. This choice is motivated firstly because Drosophila has proven over the years to be a very simple, but powerful, model organism abundantly assisting scientists in explaining not only normal functions, but also derangements that occur in higher organisms, not excluding humans. Secondly, because histamine has been demonstrated to be a pleiotropic master molecule in pharmacology and immunology, with increasingly recognized roles also in the nervous system. Indeed, it interacts with various neurotransmitters and controls functions such as learning, memory, circadian rhythm, satiety, energy balance, nociception, and motor circuits, not excluding several pathological conditions. In view of this, our review is focused on the knowledge that the use of Drosophila has added to the already vast histaminergic field. In particular, we have described histamine's actions on photoreceptors sustaining the visual system and synchronizing circadian rhythms, but also on temperature preference, courtship behavior, and mechanosensory transmission. In addition, we have highlighted the pathophysiological consequences of mutations on genes involved in histamine metabolism and signaling. By promoting critical discussion and further research, our aim is to emphasize and renew the importance of histaminergic research in biomedicine through the exploitation of Drosophila, hopefully extending the scientific debate to the academic, industry, and general public audiences.

Bumblebee nest departures under low light conditions at sunrise and sunset

Only a few diurnal animals, such as bumblebees, extend their activity into the time around sunrise and sunset when illumination levels are low. Low light impairs viewing conditions and increases sensory costs, but whether diurnal insects use low light as a cue to make behavioural decisions is uncertain. To investigate how they decide to initiate foraging at these times of day, we observed bumblebee nest-departure behaviours inside a flight net, under naturally changing light conditions. In brighter light bees did not attempt to return to the nest and departed with minimal delay, as expected. In low light the probability of non-departures increased, as a small number of bees attempted to return after spending time on the departure platform. Additionally, in lower illumination bees spent more time on the platform before flying away, up to 68 s. Our results suggest that bees may assess light conditions once outside the colony to inform the decision to depart. These findings give novel insights into how behavioural decisions are made at the start and the end of a foraging day in diurnal animals when the limits of their vision impose additional costs on foraging efficiency.

An update and review of arthropod vector sensory systems: Potential targets for behavioural manipulation by parasites and other disease agents

For over a century, vector ecology has been a mainstay of vector-borne disease control. Much of this research has focused on the sensory ecology of blood-feeding arthropods (black flies, mosquitoes, ticks, etc.) with terrestrial vertebrate hosts. Of particular interest are the cues and sensory systems that drive host seeking and host feeding behaviours as they are critical for a vector to locate and feed from a host. An important yet overlooked component of arthropod vector ecology are the phenotypic changes observed in infected vectors that increase disease transmission. While our fundamental understanding of sensory mechanisms in disease vectors has drastically increased due to recent advances in genome engineering, for example, the advent of CRISPR-Cas9, and high-throughput "big data" approaches (genomics, proteomics, transcriptomics, etc.), we still do not know if and how parasites manipulate vector behaviour. Here, we review the latest research on arthropod vector sensory systems and propose key mechanisms that disease agents may alter to increase transmission.

Crickets in the spotlight: exploring the impact of light on circadian behavior

Crickets serve as a well-established model organism in biological research spanning various fields, such as behavior, physiology, neurobiology, and ecology. Cricket circadian behavior was first reported over a century ago and prompted a wealth of studies delving into their chronobiology. Circadian rhythms have been described in relation to fundamental cricket behaviors, encompassing stridulation and locomotion, but also in hormonal secretion and gene expression. Here we review how changes in illumination patterns and light intensity differentially impact the different cricket behaviors as well as circadian gene expression. We further describe the cricket's circadian pacemaker. Ample anatomical manipulations support the location of a major circadian pacemaker in the cricket optic lobes and another in the central brain, possibly interconnected via signaling of the neuropeptide PDF. The cricket circadian machinery comprises a molecular cascade based on two major transcriptional/translational negative feedback loops, deviating somewhat from the canonical model of Drosophila and emphasizing the significance of exploring alternative models. Finally, the nocturnal nature of crickets has provided a unique avenue for investigating the repercussions of artificial light at night on cricket behavior and ecology, underscoring the critical role played by natural light cycles in synchronizing cricket behaviors and populations, further supporting the use of the cricket model in the study of the effects of light on insects. Some gaps in our knowledge and challenges for future studies are discussed.

Loss of functional cryptochrome 1 reduces robustness of 24-hour behavioral rhythms in monarch butterflies

Light is one of the strongest cues for entrainment of circadian clocks. While some insect species rely only on visual input, others like Drosophila melanogaster use both the visual system and the deep-brain blue-light photoreceptor cryptochrome for entraining circadian rhythms. Here, we used the monarch butterfly Danaus plexippus (dp), which possesses a light-sensitive cryptochrome 1 (dpCry1), to test the conservation of mechanisms of clock entrainment. We showed that loss of functional dpCry1 reduced the amplitude and altered the phase of adult eclosion rhythms, and disrupted brain molecular circadian rhythms. Robust rhythms could be restored by entrainment to temperature cycles, indicating a likely functional core circadian clock in dpCry1 mutants. We also showed that rhythmic flight activity was less robust in dpCry1 mutants, and that visual impairment in dpNinaB1 mutants impacted flight suppression at night. Our data suggest that dpCRY1 is a major photoreceptor for light-entrainment of the monarch circadian clock.

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.

Circadian Rhythms of Locomotor Activity Mediated by Cryptochrome 2 and Period 1 Genes in the Termites Reticulitermes chinensis and Odontotermes formosanus

Locomotor activity rhythms are crucial for foraging, mating and predator avoidance in insects. Although the circadian rhythms of activity have been studied in several termite species, the molecular mechanisms of circadian rhythms in termites are still unclear. In this study, we found that two termite species, R. chinensis and O. formosanus, exhibited clear circadian rhythms of locomotor activity in constant darkness along with rhythmically expressed core clock genes, Cry2 and Per1. The knockdown of Cry2 or Per1 expression in the two termite species disrupted the circadian rhythms of locomotor activity and markedly reduced locomotor activity in constant darkness, which demonstrates that Cry2 and Per1 can mediate the circadian rhythms of locomotor activity in termites in constant darkness. We suggest that locomotor activity in subterranean termites is controlled by the circadian clock.

The influences of green light on locomotion, growth and reproduction in the brown planthopper Nilaparvata lugens

BACKGROUND

Light stimulation at a specific wavelength triggers various responses in insects and can be used for pest control. To develop efficient and ecofriendly photophysical pest control methods, the effects of green light on locomotion, growth (molting and eclosion) and reproduction in Nilaparvata lugens (Stål) (BPH), a major rice pest, were studied. Transcriptomics and transmission electron microscopy (TEM) were used to investigate the mechanisms involved.

RESULTS

BPH adults showed disrupted daily locomotion patterns following green light treatment at night and exhibited abnormal locomotion peaks. Total 6-day locomotion of brachypterous adults was significantly greater than in the control group. The durations of growth stages 1-4 were all shorter under green light treatment than in the control, whereas the time from fourth molting to eclosion (stage 5) was significantly longer. When BPH adults under green light treatment began laying eggs, the egg hatching ratio (36.69%) was significantly lower than in the control (47.49%). Moreover, in contrast to the control, BPH molting and eclosion events tended to happen more at night. Transcriptome analysis proved that green light significantly affected the expression of genes involved in cuticular proteins, chitin deacetylase and chitinase, which are related to cuticular development. TEM observations confirmed abnormal cuticular development in nymph and adult BPHs (endocuticle, exocuticle and pore canals) under green light treatment.

CONCLUSION

Green light treatment at night notably affected locomotion, growth and reproduction in BPH, thus providing a novel idea for controlling this pest. © 2023 Society of Chemical Industry.

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.

Organismal landscape of clock cells and circadian gene expression in Drosophila

Background

Circadian rhythms time physiological and behavioral processes to 24-hour cycles. It is generally assumed that most cells contain self-sustained circadian clocks that drive circadian rhythms in gene expression that ultimately generating circadian rhythms in physiology. While those clocks supposedly act cell autonomously, current work suggests that in Drosophila some of them can be adjusted by the brain circadian pacemaker through neuropeptides, like the Pigment Dispersing Factor (PDF). Despite these findings and the ample knowledge of the molecular clockwork, it is still unknown how circadian gene expression in Drosophila is achieved across the body.

Results

Here, we used single-cell and bulk RNAseq data to identify cells within the fly that express core-clock components. Surprisingly, we found that less than a third of the cell types in the fly express core-clock genes. Moreover, we identified Lamina wild field (Lawf) and Ponx-neuro positive (Poxn) neurons as putative new circadian neurons. In addition, we found several cell types that do not express core clock components but are highly enriched for cyclically expressed mRNAs. Strikingly, these cell types express the PDF receptor (Pdfr), suggesting that PDF drives rhythmic gene expression in many cell types in flies. Other cell types express both core circadian clock components and Pdfr, suggesting that in these cells, PDF regulates the phase of rhythmic gene expression.

Conclusions

Together, our data suggest three different mechanisms generate cyclic daily gene expression in cells and tissues: canonical endogenous canonical molecular clock, PDF signaling-driven expression, or a combination of both.

Drosophila photoreceptor systems converge in arousal neurons and confer light responsive robustness

Lateral ventral neurons (LNvs) in the fly circadian neural circuit mediate behaviors other than clock resetting, including light-activated acute arousal. Converging sensory inputs often confer functional redundancy. The LNvs have three distinct light input pathways: (1) cell autonomously expressed cryptochrome (CRY), (2) rhodopsin 7 (Rh7), and (3) synaptic inputs from the eyes and other external photoreceptors that express opsins and CRY. We explored the relative photoelectrical and behavioral input contributions of these three photoreceptor systems to determine their functional impact in flies. Patch-clamp electrophysiology measuring light evoked firing frequency (FF) was performed on large LNvs (l-LNvs) in response to UV (365 nm), violet (405 nm), blue (450 nm), or red (635 nm) LED light stimulation, testing controls versus mutants that lack photoreceptor inputs gl60j, cry-null, rh7-null, and double mutant gl60j-cry-null flies. For UV, violet, and blue short wavelength light inputs, all photoreceptor mutants show significantly attenuated action potential FF responses measured in the l-LNv. In contrast, red light FF responses are only significantly attenuated in double mutant gl60j-cry-null flies. We used a light-pulse arousal assay to compare behavioral responses to UV, violet, blue and red light of control and light input mutants, measuring the awakening arousal response of flies during subjective nighttime at two different intensities to capture potential threshold differences (10 and 400 μW/cm2). The light arousal behavioral results are similar to the electrophysiological results, showing significant attenuation of behavioral light responses for mutants compared to control. These results show that the different LNv convergent photoreceptor systems are integrated and together confer functional redundancy for light evoked behavioral arousal.

One genome, multiple phenotypes: decoding the evolution and mechanisms of environmentally induced developmental plasticity in insects

Plasticity in developmental processes gives rise to remarkable environmentally induced phenotypes. Some of the most striking and well-studied examples of developmental plasticity are seen in insects. For example, beetle horn size responds to nutritional state, butterfly eyespots are enlarged in response to temperature and humidity, and environmental cues also give rise to the queen and worker castes of eusocial insects. These phenotypes arise from essentially identical genomes in response to an environmental cue during development. Developmental plasticity is taxonomically widespread, affects individual fitness, and may act as a rapid-response mechanism allowing individuals to adapt to changing environments. Despite the importance and prevalence of developmental plasticity, there remains scant mechanistic understanding of how it works or evolves. In this review, we use key examples to discuss what is known about developmental plasticity in insects and identify fundamental gaps in the current knowledge. We highlight the importance of working towards a fully integrated understanding of developmental plasticity in a diverse range of species. Furthermore, we advocate for the use of comparative studies in an evo-devo framework to address how developmental plasticity works and how it evolves.

Deep sequencing of microRNAs reveals circadian-dependent microRNA expression in the eyestalks of the Chinese mitten crab Eriocheir sinensis

MicroRNAs (miRNAs) are small endogenous non-coding RNAs. In crustaceans, miRNAs might be involved in the regulation of circadian rhythms. Many physiological functions of crustaceans including immunity and hormone secretion exhibit circadian rhythms, but it remains unclear whether specific miRNAs contribute to the alteration of crustacean physiological processes under circadian rhythms. This study investigated the mechanisms of miRNA regulation of circadian rhythms in the Chinese mitten crab (Eriocheir sinensis), one of China's most important aquaculture species. We obtained eyestalks from crab specimens at four time points (6:00; 12:00; 18:00; 24:00) during a 24-h period. We identified 725 mature miRNAs, with 23 known miRNAs differentially expressed depending on the time of day. The Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses revealed that the putative target genes for differentially expressed miRNAs were significantly enriched in the immune response and endocrine-related pathways. Numerous putative target genes are involved in the circadian-related pathways and enriched on circadian-control genes. These results suggest that the expression of miRNAs regulates some specific physiological functions in E. sinensis under circadian cycles. We also profiled various putative target genes enriched under the circadian-related pathway. This study performed miRNA expression in the eyestalks of E. sinensis during a 24-h daily cycle, providing insights into the molecular mechanism underlying crustacean circadian rhythms and suggesting miRNAs' role in studying crustacean physiology should not be overlooked.

Exposure to a nocturnal light pulse simultaneously and differentially affects stridulation and locomotion behaviors in crickets

It is crucial for living organisms to be in synchrony with their environment and to anticipate circadian and annual changes. The circadian clock is responsible for entraining organisms' activity to the day-night rhythmicity. Artificial light at night (ALAN) was shown to obstruct the natural light cycle, leading to desynchronized behavioral patterns. Our knowledge of the mechanisms behind these adverse effects of ALAN, however, is far from complete. Here we monitored the stridulation and locomotion behavior of male field crickets (Gryllus bimaculatus), raised under light:dark conditions, before, during, and after exposure to a nocturnal 3-h pulse of different ALAN intensities. The experimental insects were then placed under a constant light regime (of different intensities); their behavior was continuously monitored; and the period of their daily activity rhythms was calculated. The light pulse treatment induced a simultaneous negative (suppressing stridulation) and positive (inducing locomotion) effect, manifested in significant changes in the average level of the specific activity on the night of the pulse compared to the preceding and the following nights. The transition to constant light conditions led to significant changes in the period of the circadian rhythms. Both effects were light-intensity-dependent, indicating the importance of dark nights for both individual and population synchronization.

Circadian rhythm entrainment of the jewel wasp, Nasonia vitripennis, by antagonistic interactions of multiple spectral inputs

Circadian light entrainment in some insects is regulated by blue-light-sensitive cryptochrome (CRY) protein that is expressed in the clock neurons, but this is not the case in hymenopterans. The hymenopteran clock does contain CRY, but it appears to be light-insensitive. Therefore, we investigated the role of retinal photoreceptors in the photic entrainment of the jewel wasp Nasonia vitripennis. Application of monochromatic light stimuli at different light intensities caused phase shifts in the wasp's circadian activity from which an action spectrum with three distinct peaks was derived. Electrophysiological recordings from the compound eyes and ocelli revealed the presence of three photoreceptor classes, with peak sensitivities at 340 nm (ultraviolet), 450 nm (blue) and 530 nm (green). An additional photoreceptor class in the ocelli with sensitivity maximum at 560-580 nm (red) was found. Whereas a simple sum of photoreceptor spectral sensitivities could not explain the action spectrum of the circadian phase shifts, modelling of the action spectrum indicates antagonistic interactions between pairs of spectral photoreceptors, residing in the compound eyes and the ocelli. Our findings imply that the photic entrainment mechanism in N. vitripennis encompasses the neural pathways for measuring the absolute luminance as well as the circuits mediating colour opponency.

Effect of Narrowband UV-B Irradiation on the Growth Performance of House Crickets

Indoor co-cultivation systems can answer to the need for sustainable and resilient food production systems. Rearing organisms under light-emitting diodes (LEDs) irradiation provides the possibility to control and shape the emitted light spectra. UV-B-irradiation (280-315 nm) can positively affect the nutritional composition of different plants and other organisms, whereas information on edible insects is scarce. To evaluate the potential effect of the photosynthetically active radiation (PAR) and LED-emitting LEDs on the rearing and nutritional quality of edible insects, house crickets (Acheta domesticus) were reared from the age of 21 days under controlled LED spectra, with an additional UV-B (0.08 W/m2) dose of 1.15 KJm2 d-1 (illuminated over a period for 4 h per day) for 34 days. UV-B exposure showed no harm to the weight of the crickets and significantly increased their survival by ca. 10% under narrowband UV-B treatment. The nutritional composition including proteins, fat and chitin contents of the insects was not affected by the UV-B light and reached values of 60.03 ± 10.41, 22.38 ± 2.12 and 9.33 ± 1.21%, respectively, under the LED irradiation. Therefore, house crickets can grow under LED irradiation with a positive effect of narrowband UV-B application on their survival.

Transcriptional Response of Circadian Clock Genes to an ‘Artificial Light at Night’ Pulse in the Cricket Gryllus bimaculatus

Light is the major signal entraining the circadian clock that regulates physiological and behavioral rhythms in most organisms, including insects. Artificial light at night (ALAN) disrupts the natural light–dark cycle and negatively impacts animals at various levels. We simulated ALAN using dim light stimuli and tested their impact on gene expression in the cricket Gryllus bimaculatus, a model of insect physiology and chronobiology. At night, adult light–dark-regime-raised crickets were exposed for 30 min to a light pulse of 2–40 lx. The relative expression of five circadian-clock-associated genes was compared using qPCR. A dim ALAN pulse elicited tissue-dependent differential expression in some of these genes. The strongest effect was observed in the brain and in the optic lobe, the cricket’s circadian pacemaker. The expression of opsin-Long Wave (opLW) was upregulated, as well as cryptochrome1-2 (cry) and period (per). Our findings demonstrate that even a dim ALAN exposure may affect insects at the molecular level, underscoring the impact of ALAN on the circadian clock system.

An extra-clock ultradian brain oscillator sustains circadian timekeeping

The master circadian clock generates 24-hour rhythms to orchestrate daily behavior, even running freely under constant conditions. Traditionally, the master clock is considered self-sufficient in sustaining free-running timekeeping via its cell-autonomous molecular clocks and interneuronal communications within the circadian neural network. Here, we find a set of bona fide ultradian oscillators in the Drosophila brain that support free-running timekeeping, despite being located outside the master clock circuit and lacking clock gene expression. These extra-clock electrical oscillators (xCEOs) generate cell-autonomous ultradian bursts, pacing widespread burst firing and promoting rhythmic resting membrane potentials in clock neurons via parallel monosynaptic connections. Silencing xCEOs disrupts daily electrical rhythms in clock neurons and impairs cycling of neuropeptide pigment dispersing factor, leading to the loss of free-running locomotor rhythms. Together, we conclude that the master clock is not self-sufficient to sustain free-running behavior rhythms but requires additional endogenous inputs to the clock from the extra-clock ultradian brain oscillators.

Timeless Plays an Important Role in Compound Eye-Dependent Photic Entrainment of the Circadian Rhythm in the Cricket Gryllus bimaculatus

The light cycle is the most powerful Zeitgeber entraining the circadian clock in most organisms. Insects use CRYPTOCHROMEs (CRYs) and/or the compound eye for the light perception necessary for photic entrainment. The molecular mechanism underlying CRY-dependent entrainment is well understood, while that of the compound eye-dependent entrainment remains to be elucidated. Using molecular and behavioral experiments, we investigated the role of timeless (tim) in the photic entrainment mechanism in the cricket Gryllus bimaculatus. RNA interference of tim (tim^RNAi) disrupted the entrainment or prolonged the transients for resynchronization to phase-delayed light-dark cycles. The treatment reduced the magnitude of phase delay caused by delayed light-off, but augmented advance shifts caused by light exposure at late night. TIM protein levels showed daily cycling with an increase during the night and reduction by light exposure at both early and late night. These results suggest that tim plays a critical role in the entrainment to delayed light cycles.

PHASE: An Open-Source Program for the Analysis of Drosophila Phase, Activity, and Sleep Under Entrainment

The problem of entrainment is central to circadian biology. In this regard, Drosophila has been an important model system. Owing to the simplicity of its nervous system and the availability of powerful genetic tools, the system has shed significant light on the molecular and neural underpinnings of entrainment. However, much remains to be learned regarding the molecular and physiological mechanisms underlying this important phenomenon. Under cyclic light/dark conditions, Drosophila melanogaster displays crepuscular patterns of locomotor activity with one peak anticipating dawn and the other anticipating dusk. These peaks are characterized through an estimation of their phase relative to the environmental light cycle and the extent of their anticipation of light transitions. In Drosophila chronobiology, estimations of phases are often subjective, and anticipation indices vary significantly between studies. Though there is increasing interest in building flexible analysis tools in the field, none incorporates objective measures of Drosophila activity peaks in combination with the analysis of fly activity/sleep in the same program. To this end, we have developed PHASE, a MATLAB-based program that is simple and easy to use and (i) supports the visualization and analysis of activity and sleep under entrainment, (ii) allows analysis of both activity and sleep parameters within user-defined windows within a diurnal cycle, (iii) uses a smoothing filter for the objective identification of peaks of activity (and therefore can be used to quantitatively characterize them), and (iv) offers a series of analyses for the assessment of behavioral anticipation of environmental transitions.

Dietary restriction and the transcription factor clock delay eye aging to extend lifespan in Drosophila Melanogaster

Many vital processes in the eye are under circadian regulation, and circadian dysfunction has emerged as a potential driver of eye aging. Dietary restriction is one of the most robust lifespan-extending therapies and amplifies circadian rhythms with age. Herein, we demonstrate that dietary restriction extends lifespan in Drosophila melanogaster by promoting circadian homeostatic processes that protect the visual system from age- and light-associated damage. Altering the positive limb core molecular clock transcription factor, CLOCK, or CLOCK-output genes, accelerates visual senescence, induces a systemic immune response, and shortens lifespan. Flies subjected to dietary restriction are protected from the lifespan-shortening effects of photoreceptor activation. Inversely, photoreceptor inactivation, achieved via mutating rhodopsin or housing flies in constant darkness, primarily extends the lifespan of flies reared on a high-nutrient diet. Our findings establish the eye as a diet-sensitive modulator of lifespan and indicates that vision is an antagonistically pleiotropic process that contributes to organismal aging.

Two light sensors decode moonlight versus sunlight to adjust a plastic circadian/circalunidian clock to moon phase

Many species synchronize their physiology and behavior to specific hours. It is commonly assumed that sunlight acts as the main entrainment signal for ∼24-h clocks. However, the moon provides similarly regular time information. Consistently, a growing number of studies have reported correlations between diel behavior and lunidian cycles. Yet, mechanistic insight into the possible influences of the moon on ∼24-h timers remains scarce. We have explored the marine bristleworm Platynereis dumerilii to investigate the role of moonlight in the timing of daily behavior. We uncover that moonlight, besides its role in monthly timing, also schedules the exact hour of nocturnal swarming onset to the nights’ darkest times. Our work reveals that extended moonlight impacts on a plastic clock that exhibits <24 h (moonlit) or >24 h (no moon) periodicity. Abundance, light sensitivity, and genetic requirement indicate that the Platynereis light receptor molecule r-Opsin1 serves as a receptor that senses moonrise, whereas the cryptochrome protein L-Cry is required to discriminate the proper valence of nocturnal light as either moonlight or sunlight. Comparative experiments in Drosophila suggest that cryptochrome’s principle requirement for light valence interpretation is conserved. Its exact biochemical properties differ, however, between species with dissimilar timing ecology. Our work advances the molecular understanding of lunar impact on fundamental rhythmic processes, including those of marine mass spawners endangered by anthropogenic change.

Investigation of the aging clock's intermittent-light responses uncovers selective deficits to green millisecond flashes

The central pacemaker of flies, rodents, and humans generates less robust circadian output signals across normative aging. It is not well understood how changes in light sensitivity might contribute to this phenomenon. In the present study, we summarize results from an extended data series (n = 5681) showing that the locomotor activity rhythm of aged Drosophila can phase-shift normally to intermittently spaced episodes of bright polychromatic light exposure (600 lx) but that deficits emerge in response to 8, 16, and 120-millisecond flashes of narrowband blue (λ_m, 452 nm) and green (λ_m, 525 nm) LED light. For blue, phase-resetting of the activity rhythm of older flies is not as energy efficient as it is in younger flies at the fastest flash-exposures tested (8 milliseconds), suggesting there might be different floors of light duration necessary to incur photohabituation in each age group. For green, the responses of older flies are universally crippled relative to those of younger flies across the slate of protocols we tested. The difference in green flash photosensitivity is one of the most salient age-related phenotypes that has been documented in the circadian phase-shifting literature thus far. These data provide further impetus for investigations on pacemaker aging and how it might relate to changes in the circadian system's responses to particular sequences of light exposure tuned for wavelength, intensity, duration, and tempo.

Roles of peripheral clocks: lessons from the fly

To adapt to and anticipate rhythmic changes in the environment such as daily light-dark and temperature cycles, internal timekeeping mechanisms called biological clocks evolved in a diverse set of organisms, from unicellular bacteria to humans. These biological clocks play critical roles in organisms' fitness and survival by temporally aligning physiological and behavioral processes to the external cues. The central clock is located in a small subset of neurons in the brain and drives daily activity rhythms, whereas most peripheral tissues harbor their own clock systems, which generate metabolic and physiological rhythms. Since the discovery of Drosophila melanogaster clock mutants in the early 1970s, the fruit fly has become an extensively studied model organism to investigate the mechanism and functions of circadian clocks. In this review, we primarily focus on D. melanogaster to survey key discoveries and progresses made over the past two decades in our understanding of peripheral clocks. We discuss physiological roles and molecular mechanisms of peripheral clocks in several different peripheral tissues of the fly.

Time-course RNASeq of Camponotus floridanus forager and nurse ant brains indicate links between plasticity in the biological clock and behavioral division of labor

Background

Circadian clocks allow organisms to anticipate daily fluctuations in their environment by driving rhythms in physiology and behavior. Inter-organismal differences in daily rhythms, called chronotypes, exist and can shift with age. In ants, age, caste-related behavior and chronotype appear to be linked. Brood-tending nurse ants are usually younger individuals and show “around-the-clock” activity. With age or in the absence of brood, nurses transition into foraging ants that show daily rhythms in activity. Ants can adaptively shift between these behavioral castes and caste-associated chronotypes depending on social context. We investigated how changes in daily gene expression could be contributing to such behavioral plasticity in Camponotus floridanus carpenter ants by combining time-course behavioral assays and RNA-Sequencing of forager and nurse brains.

Results

We found that nurse brains have three times fewer 24 h oscillating genes than foragers. However, several hundred genes that oscillated every 24 h in forager brains showed robust 8 h oscillations in nurses, including the core clock genes Period and Shaggy. These differentially rhythmic genes consisted of several components of the circadian entrainment and output pathway, including genes said to be involved in regulating insect locomotory behavior. We also found that Vitellogenin, known to regulate division of labor in social insects, showed robust 24 h oscillations in nurse brains but not in foragers. Finally, we found significant overlap between genes differentially expressed between the two ant castes and genes that show ultradian rhythms in daily expression.

Conclusion

This study provides a first look at the chronobiological differences in gene expression between forager and nurse ant brains. This endeavor allowed us to identify a putative molecular mechanism underlying plastic timekeeping: several components of the ant circadian clock and its output can seemingly oscillate at different harmonics of the circadian rhythm. We propose that such chronobiological plasticity has evolved to allow for distinct regulatory networks that underlie behavioral castes, while supporting swift caste transitions in response to colony demands. Behavioral division of labor is common among social insects. The links between chronobiological and behavioral plasticity that we found in C. floridanus, thus, likely represent a more general phenomenon that warrants further investigation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08282-x.

The pigment-dispersing factor neuronal network systematically grows in developing honey bees

The neuropeptide pigment-dispersing factor (PDF) plays a prominent role in the circadian clock of many insects including honey bees. In the honey bee brain, PDF is expressed in about 15 clock neurons per hemisphere that lie between the central brain and the optic lobes. As in other insects, the bee PDF neurons form wide arborizations in the brain, but certain differences are evident. For example, they arborize only sparsely in the accessory medulla (AME), which serves as important communication center of the circadian clock in cockroaches and flies. Furthermore, all bee PDF neurons cluster together, which makes it impossible to distinguish individual projections. Here, we investigated the developing bee PDF network and found that the first three PDF neurons arise in the third larval instar and form a dense network of varicose fibers at the base of the developing medulla that strongly resembles the AME of hemimetabolous insects. In addition, they send faint fibers toward the lateral superior protocerebrum. In last larval instar, PDF cells with larger somata appear and send fibers toward the distal medulla and the medial protocerebrum. In the dorsal part of the medulla serpentine layer, a small PDF knot evolves from which PDF fibers extend ventrally. This knot disappears during metamorphosis and the varicose arborizations in the putative AME become fainter. Instead, a new strongly stained PDF fiber hub appears in front of the lobula. Simultaneously, the number of PDF neurons increases and the PDF neuronal network in the brain gets continuously more complex.

dFRAME: A Video Recording-Based Analytical Method for Studying Feeding Rhythm in Drosophila

Animals, from insects to humans, exhibit obvious diurnal rhythmicity of feeding behavior. Serving as a genetic animal model, Drosophila has been reported to display feeding rhythms; however, related investigations are limited due to the lack of suitable and practical methods. Here, we present a video recording-based analytical method, namely, Drosophila Feeding Rhythm Analysis Method (dFRAME). Using our newly developed computer program, FlyFeeding, we extracted the movement track of individual flies and characterized their food-approaching behavior. To distinguish feeding and no-feeding events, we utilized high-magnification video recording to optimize our method by setting cut-off thresholds to eliminate the interference of no-feeding events. Furthermore, we verified that this method is applicable to both female and male flies and for all periods of the day. Using this method, we analyzed long-term feeding status of wild-type and period mutant flies. The results recaptured previously reported feeding rhythms and revealed detailed profiles of feeding patterns in these flies under either light/dark cycles or constant dark environments. Together, our dFRAME method enables a long-term, stable, reliable, and subtle analysis of feeding behavior in Drosophila. High-throughput studies in this powerful genetic animal model will gain great insights into the molecular and neural mechanisms of feeding rhythms.

The Neuropeptide PDF Is Crucial for Delaying the Phase of Drosophila's Evening Neurons Under Long Zeitgeber Periods

Circadian clocks schedule biological functions at a specific time of the day. Full comprehension of the clock function requires precise understanding of their entrainment to the environment. The phase of entrained clock is plastic, which depends on different factors such as the period of endogenous oscillator, the period of the zeitgeber cycle (T), and the proportion of light and darkness (day length). The circadian clock of fruit fly Drosophila melanogaster is able to entrain to a wide range of T-cycles and day lengths. Here, we investigated the importance of the neuropeptide Pigment-Dispersing Factor (PDF) for entrainment by systematically studying locomotor activity rhythms of Pdf ⁰ mutants and wild-type flies under different T-cycles (T22 to T32) and different day lengths (8, 12, and 16 hour [h]). Furthermore, we analysed PERIOD protein oscillations in selected groups of clock neurons in both genotypes under T24 and T32 at a day length of 16 h. As expected, we found that the phase of Drosophila’s evening activity and evening neurons advanced with increasing T in all the day lengths. This advance was much larger in Pdf ⁰ mutants (~7 h) than in wild-type flies causing (1) pronounced desynchrony between morning and evening neurons and (2) evening activity to move in the morning instead of the evening. Most interestingly, we found that the lights-off transition determines the phase of evening neurons in both genotypes and that PDF appears necessary to delay the evening neurons by ~3 h to their wild-type phase. Thus, in T32, PDF first delays the molecular cycling in the evening neurons, and then, as shown in previous studies, delays their neuronal firing rhythms to produce a total delay of ~7 h necessary for a wild-type evening activity phase. We conclude that PDF is crucial for appropriate phasing of Drosophila activity rhythm.

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.

Antibodies Against the Clock Proteins Period and Cryptochrome Reveal the Neuronal Organization of the Circadian Clock in the Pea Aphid

Circadian clocks prepare the organism to cyclic environmental changes in light, temperature, or food availability. Here, we characterized the master clock in the brain of a strongly photoperiodic insect, the aphid Acyrthosiphon pisum, immunohistochemically with antibodies against A. pisum Period (PER), Drosophila melanogaster Cryptochrome (CRY1), and crab Pigment-Dispersing Hormone (PDH). The latter antibody detects all so far known PDHs and PDFs (Pigment-Dispersing Factors), which play a dominant role in the circadian system of many arthropods. We found that, under long days, PER and CRY are expressed in a rhythmic manner in three regions of the brain: the dorsal and lateral protocerebrum and the lamina. No staining was detected with anti-PDH, suggesting that aphids lack PDF. All the CRY1-positive cells co-expressed PER and showed daily PER/CRY1 oscillations of high amplitude, while the PER oscillations of the CRY1-negative PER neurons were of considerable lower amplitude. The CRY1 oscillations were highly synchronous in all neurons, suggesting that aphid CRY1, similarly to Drosophila CRY1, is light sensitive and its oscillations are synchronized by light-dark cycles. Nevertheless, in contrast to Drosophila CRY1, aphid CRY1 was not degraded by light, but steadily increased during the day and decreased during the night. PER was always located in the nuclei of the clock neurons, while CRY was predominantly cytoplasmic and revealed the projections of the PER/CRY1-positive neurons. We traced the PER/CRY1-positive neurons through the aphid protocerebrum discovering striking similarities with the circadian clock of D. melanogaster: The CRY1 fibers innervate the dorsal and lateral protocerebrum and putatively connect the different PER-positive neurons with each other. They also run toward the pars intercerebralis, which controls hormone release via the neurohemal organ, the corpora cardiaca. In contrast to Drosophila, the CRY1-positive fibers additionally travel directly toward the corpora cardiaca and the close-by endocrine gland, corpora allata. This suggests a direct link between the circadian clock and the photoperiodic control of hormone release that can be studied in the future.

Integration of Circadian Clock Information in the Drosophila Circadian Neuronal Network

Circadian clocks are biochemical time-keeping machines that synchronize animal behavior and physiology with planetary rhythms. In Drosophila, the core components of the clock comprise a transcription/translation feedback loop and are expressed in seven neuronal clusters in the brain. Although it is increasingly evident that the clocks in each of the neuronal clusters are regulated differently, how these clocks communicate with each other across the circadian neuronal network is less clear. Here, we review the latest evidence that describes the physical connectivity of the circadian neuronal network . Using small ventral lateral neurons as a starting point, we summarize how one clock may communicate with another, highlighting the signaling pathways that are both upstream and downstream of these clocks. We propose that additional efforts are required to understand how temporal information generated in each circadian neuron is integrated across a neuronal circuit to regulate rhythmic behavior.

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.

Peripheral Sensory Organs Contribute to Temperature Synchronization of the Circadian Clock in Drosophila melanogaster

Circadian clocks are cell-autonomous endogenous oscillators, generated and maintained by self-sustained 24-h rhythms of clock gene expression. In the fruit fly Drosophila melanogaster, these daily rhythms of gene expression regulate the activity of approximately 150 clock neurons in the fly brain, which are responsible for driving the daily rest/activity cycles of these insects. Despite their endogenous character, circadian clocks communicate with the environment in order to synchronize their self-sustained molecular oscillations and neuronal activity rhythms (internal time) with the daily changes of light and temperature dictated by the Earth's rotation around its axis (external time). Light and temperature changes are reliable time cues (Zeitgeber) used by many organisms to synchronize their circadian clock to the external time. In Drosophila, both light and temperature fluctuations robustly synchronize the circadian clock in the absence of the other Zeitgeber. The complex mechanisms for synchronization to the daily light-dark cycles are understood with impressive detail. In contrast, our knowledge about how the daily temperature fluctuations synchronize the fly clock is rather limited. Whereas light synchronization relies on peripheral and clock-cell autonomous photoreceptors, temperature input to the clock appears to rely mainly on sensory cells located in the peripheral nervous system of the fly. Recent studies suggest that sensory structures located in body and head appendages are able to detect temperature fluctuations and to signal this information to the brain clock. This review will summarize these studies and their implications about the mechanisms underlying temperature synchronization.

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.

Multi-regional circuits underlying visually guided decision-making in Drosophila

Visually guided decision-making requires integration of information from distributed brain areas, necessitating a brain-wide approach to examine its neural mechanisms. New tools in Drosophila melanogaster enable circuits spanning the brain to be charted with single cell-type resolution. Here, we highlight recent advances uncovering the computations and circuits that transform and integrate visual information across the brain to make behavioral choices. Visual information flows from the optic lobes to three primary central brain regions: a sensorimotor mapping area and two 'higher' centers for memory or spatial orientation. Rapid decision-making during predator evasion emerges from the spike timing dynamics in parallel sensorimotor cascades. Goal-directed decisions may occur through memory, navigation and valence processing in the central complex and mushroom bodies.

Neonicotinoids disrupt circadian rhythms and sleep in honey bees

Honey bees are critical pollinators in ecosystems and agriculture, but their numbers have significantly declined. Declines in pollinator populations are thought to be due to multiple factors including habitat loss, climate change, increased vulnerability to disease and parasites, and pesticide use. Neonicotinoid pesticides are agonists of insect nicotinic cholinergic receptors, and sub-lethal exposures are linked to reduced honey bee hive survival. Honey bees are highly dependent on circadian clocks to regulate critical behaviors, such as foraging orientation and navigation, time-memory for food sources, sleep, and learning/memory processes. Because circadian clock neurons in insects receive light input through cholinergic signaling we tested for effects of neonicotinoids on honey bee circadian rhythms and sleep. Neonicotinoid ingestion by feeding over several days results in neonicotinoid accumulation in the bee brain, disrupts circadian rhythmicity in many individual bees, shifts the timing of behavioral circadian rhythms in bees that remain rhythmic, and impairs sleep. Neonicotinoids and light input act synergistically to disrupt bee circadian behavior, and neonicotinoids directly stimulate wake-promoting clock neurons in the fruit fly brain. Neonicotinoids disrupt honey bee circadian rhythms and sleep, likely by aberrant stimulation of clock neurons, to potentially impair honey bee navigation, time-memory, and social communication.

Better Sleep at Night: How Light Influences Sleep in Drosophila

Sleep-like states have been described in Drosophila and the mechanisms and factors that generate and define sleep-wake profiles in this model organism are being thoroughly investigated. Sleep is controlled by both circadian and homeostatic mechanisms, and environmental factors such as light, temperature, and social stimuli are fundamental in shaping and confining sleep episodes into the correct time of the day. Among environmental cues, light seems to have a prominent function in modulating the timing of sleep during the 24 h and, in this review, we will discuss the role of light inputs in modulating the distribution of the fly sleep-wake cycles. This phenomenon is of growing interest in the modern society, where artificial light exposure during the night is a common trait, opening the possibility to study Drosophila as a model organism for investigating shift-work disorders.

Serotonergic modulation of visual neurons in Drosophila melanogaster

Sensory systems rely on neuromodulators, such as serotonin, to provide flexibility for information processing as stimuli vary, such as light intensity throughout the day. Serotonergic neurons broadly innervate the optic ganglia of Drosophila melanogaster, a widely used model for studying vision. It remains unclear whether serotonin modulates the physiology of interneurons in the optic ganglia. To address this question, we first mapped the expression patterns of serotonin receptors in the visual system, focusing on a subset of cells with processes in the first optic ganglion, the lamina. Serotonin receptor expression was found in several types of columnar cells in the lamina including 5-HT2B in lamina monopolar cell L2, required for spatiotemporal luminance contrast, and both 5-HT1A and 5-HT1B in T1 cells, whose function is unknown. Subcellular mapping with GFP-tagged 5-HT2B and 5-HT1A constructs indicated that these receptors localize to layer M2 of the medulla, proximal to serotonergic boutons, suggesting that the medulla neuropil is the primary site of serotonergic regulation for these neurons. Exogenous serotonin increased basal intracellular calcium in L2 terminals in layer M2 and modestly decreased the duration of visually induced calcium transients in L2 neurons following repeated dark flashes, but otherwise did not alter the calcium transients. Flies without functional 5-HT2B failed to show an increase in basal calcium in response to serotonin. 5-HT2B mutants also failed to show a change in amplitude in their response to repeated light flashes but other calcium transient parameters were relatively unaffected. While we did not detect serotonin receptor expression in L1 neurons, they, like L2, underwent serotonin-induced changes in basal calcium, presumably via interactions with other cells. These data demonstrate that serotonin modulates the physiology of interneurons involved in early visual processing in Drosophila.

Serotonergic neurons innervate the Drosophila melanogaster eye, but it was not known whether serotonin signaling could induce acute physiological responses in visual interneurons. We found serotonin receptors expressed in all neuropils of the optic lobe and identified specific neurons involved in visual information processing that express serotonin receptors. Activation of these receptors increased intracellular calcium in first order interneurons L1 and L2 and may enhance visually induced calcium transients in L2 neurons. These data support a role for the serotonergic neuromodulation of interneurons in the Drosophila visual system.

Circadian Clocks: Structural Plasticity on the Input Side

Key Drosophila clock neurons remodel their axonal arborization on a daily basis. The current view is that remodelling is part of the control of clock neuron output but new data support a major role in modulating sensory inputs.