Flies as models for circadian clock adaptation to environmental challenges

The Never Given 2022 Pittendrigh/Aschoff Lecture: The Clock Network in the Brain-Insights From Insects

My journey into chronobiology began in 1977 with lectures and internships with Wolfgang Engelmann and Hans Erkert at the University of Tübingen in Germany. At that time, the only known animal clock gene was Period, and the location and organization of the master circadian clock in the brain was completely unknown for the model insect Drosophila melanogaster. I was thus privileged to witness and participate in the research that led us from discovering the first clock gene to identifying the clock network in the fly brain and the putative pathways linking it to behavior and physiology. This article highlights my role in these developments and also shows how the successful use of D. melanogaster for studies of circadian rhythms has contributed to the understanding of clock networks in other animals. I also report on my experiences in the German scientific system and hope that my story will be of interest to some of you.

Clocks at a snail pace: biological rhythms in terrestrial gastropods

Biological rhythms are ubiquitous across the tree of life. Organisms must allocate their activities into moments of the day and of the season that will increase their probability of surviving and reproducing, which is done in the form of daily and annual rhythms. So far, the vast majority of studies on biological rhythms have focused on classical laboratory model species. Still, the use of non-model species is gaining traction, as part of an effort to achieve a more holistic understanding of clock/calendar mechanisms in the "real world" but this requires species that can be studied in both the lab and in nature. Terrestrial gastropods, i.e., land snails and slugs, have the potential to be exciting models for the study of biological rhythms in nature. Therefore, we provide a review of the research on biological rhythms in terrestrial gastropods, with a focus on ecology and evolution. We present the state of the art in the field while giving a historical perspective of the studies, exploring each of the main lineages of terrestrial gastropods. We also point out some interesting directions that future studies could take to fill some of the more urgent gaps in current knowledge. We hope that our contribution will renew interest in this area and spark novel projects.

Genomic Diversity Illuminates the Environmental Adaptation of Drosophila suzukii

Biological invasions carry substantial practical and scientific importance and represent natural evolutionary experiments on contemporary timescales. Here, we investigated genomic diversity and environmental adaptation of the crop pest Drosophila suzukii using whole-genome sequencing data and environmental metadata for 29 population samples from its native and invasive range. Through a multifaceted analysis of this population genomic data, we increase our understanding of the D. suzukii genome, its diversity and its evolution, and we identify an appropriate genotype-environment association pipeline for our dataset. Using this approach, we detect genetic signals of local adaptation associated with nine distinct environmental factors related to altitude, wind speed, precipitation, temperature, and human land use. We uncover unique functional signatures for each environmental variable, such as the prevalence of cuticular genes associated with annual precipitation. We also infer biological commonalities in the adaptation to diverse selective pressures, particularly in terms of the apparent contribution of nervous system evolution to enriched processes (ranging from neuron development to circadian behavior) and to top genes associated with all nine environmental variables. Our findings therefore depict a finer-scale adaptive landscape underlying the rapid invasion success of this agronomically important species.

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.

Circadian clock- and temperature-associated genes contribute to overall genomic differentiation along elevation in lichenized fungi

Circadian regulation is linked to local environmental adaptation, and many species with broad climatic niches display variation in circadian genes. Here, we hypothesize that lichenizing fungi occupying different climate zones tune their metabolism to local environmental conditions with the help of their circadian systems. We study two species of the genus Umbilicaria occupying similar climatic niches (Mediterranean and the cold temperate) in different continents. Using homology to Neurospora crassa genes, we identify gene sets associated with circadian rhythms (11 core, 39 peripheral genes) as well as temperature response (37 genes). Nucleotide diversity of these genes is significantly correlated with mean annual temperature, minimum temperature of the coldest month and mean temperature of the coldest quarter. Furthermore, we identify altitudinal clines in allele frequencies in several non-synonymous substitutions in core clock components, for example, white collar-like, frh-like and various ccg-like genes. A dN/dS approach revealed a few significant peripheral clock- and temperature-associated genes (e.g. ras-1-like, gna-1-like) that may play a role in fine-tuning the circadian clock and temperature-response machinery. An analysis of allele frequency changes demonstrated the strongest evidence for differentiation above the genomic background in the clock-associated genes in U. pustulata. These results highlight the likely relevance of the circadian clock in environmental adaptation, particularly frost tolerance, of lichens. Whether or not the fungal clock modulates the symbiotic interaction within the lichen consortium remains to be investigated. We corroborate the finding of genetic variation in clock components along altitude-not only latitude-as has been reported in other species.

Genomic Diversity Illuminates the Environmental Adaptation of Drosophila suzukii

Biological invasions carry substantial practical and scientific importance, and represent natural evolutionary experiments on contemporary timescales. Here, we investigated genomic diversity and environmental adaptation of the crop pest Drosophila suzukii using whole-genome sequencing data and environmental metadata for 29 population samples from its native and invasive range. Through a multifaceted analysis of this population genomic data, we increase our understanding of the D. suzukii genome, its diversity and its evolution, and we identify an appropriate genotype-environment association pipeline for our data set. Using this approach, we detect genetic signals of local adaptation associated with nine distinct environmental factors related to altitude, wind speed, precipitation, temperature, and human land use. We uncover unique functional signatures for each environmental variable, such as a prevalence of cuticular genes associated with annual precipitation. We also infer biological commonalities in the adaptation to diverse selective pressures, particularly in terms of the apparent contribution of nervous system evolution to enriched processes (ranging from neuron development to circadian behavior) and to top genes associated with all nine environmental variables. Our findings therefore depict a finer-scale adaptive landscape underlying the rapid invasion success of this agronomically important species.

Characterization of clock-related proteins and neuropeptides in Drosophila littoralis and their putative role in diapause

Insects from high latitudes spend the winter in a state of overwintering diapause, which is characterized by arrested reproduction, reduced food intake and metabolism, and increased life span. The main trigger to enter diapause is the decreasing day length in summer-autumn. It is thus assumed that the circadian clock acts as an internal sensor for measuring photoperiod and orchestrates appropriate seasonal changes in physiology and metabolism through various neurohormones. However, little is known about the neuronal organization of the circadian clock network and the neurosecretory system that controls diapause in high-latitude insects. We addressed this here by mapping the expression of clock proteins and neuropeptides/neurohormones in the high-latitude fly Drosophila littoralis. We found that the principal organization of both systems is similar to that in Drosophila melanogaster, but with some striking differences in neuropeptide expression levels and patterns. The small ventrolateral clock neurons that express pigment-dispersing factor (PDF) and short neuropeptide F (sNPF) and are most important for robust circadian rhythmicity in D. melanogaster virtually lack PDF and sNPF expression in D. littoralis. In contrast, dorsolateral clock neurons that express ion transport peptide in D. melanogaster additionally express allatostatin-C and appear suited to transfer day-length information to the neurosecretory system of D. littoralis. The lateral neurosecretory cells of D. littoralis contain more neuropeptides than D. melanogaster. Among them, the cells that coexpress corazonin, PDF, and diuretic hormone 44 appear most suited to control diapause. Our work sets the stage to investigate the roles of these diverse neuropeptides in regulating insect diapause.

Latitudinal and zoo specific zeitgebers influence circadian and circannual rhythmicity of behavior in captive giant pandas (Ailuropoda melanoleuca)

Introduction

The circadian clock influences many aspects of animal welfare including metabolism, breeding, and behavior. In most species, circadian clocks are internal clocks regulated by external environmental cues called zeitgebers. The most common zeitgebers are light/dark cycles, food, and temperature. However, within captive environments, animals can be housed at latitudes with different light/dark cycles than their natural habitat and most other zeitgebers are controlled by humans. The effects that modified zeitgebers have on captive animals' circadian and circannual rhythmicity is largely unknown. To explore this and potential welfare implications, we measured and analyzed observational behavioral data of zoo-housed giant pandas for one year utilizing live camera footage from six zoos across the world. The worldwide distribution of the zoos gives us the unique opportunity to investigate how housing giant pandas within and outside of their natural latitudinal range can affect circadian rhythmicity and behavior.

Methods

Focal sampling was completed for 11 giant pandas each month for 12 consecutive months to gain an estimate of one circannual cycle. Within each month, we estimated one daylight or 24 h cycle of activity/behavior by conducting 10-min observation sessions systemically each hour the pandas were visible.

Results

Zero-inflated negative binomial mixture models found that latitude is associated with activity levels, with pandas housed outside of their natural latitudinal range displaying less activity than those within their latitudinal range. Amount of daylight, temperature minimum, and temperature range were also associated with activity cycles, potentially acting as zeitgebers. An association between sexual-related and stereotypic behavioral cycles was found, with the circannual cycles fluctuating in synchrony throughout several points in a year.

Discussion

These results indicate that changes to common zeitgebers and environmental conditions can influence circadian and circannual cycles. The widespread evolution of circadian rhythms suggests an adaptive advantage to possessing one in an environment with cyclical changes, allowing species to anticipate changes in their environment and respond accordingly. Therefore, although animals are highly adaptive, creating a captive environment that mimics the environmental conditions for which the animal has evolved can encourage naturalistic cycles that ultimately aid in promoting positive welfare states and increasing chances of successful breeding and conservation.

Neural mechanisms of parasite-induced summiting behavior in 'zombie' Drosophila

For at least two centuries, scientists have been enthralled by the "zombie" behaviors induced by mind-controlling parasites. Despite this interest, the mechanistic bases of these uncanny processes have remained mostly a mystery. Here, we leverage the Entomophthora muscae-Drosophila melanogaster "zombie fly" system to reveal the mechanistic underpinnings of summit disease, a manipulated behavior evoked by many fungal parasites. Using a high-throughput approach to measure summiting, we discovered that summiting behavior is characterized by a burst of locomotion and requires the host circadian and neurosecretory systems, specifically DN1p circadian neurons, pars intercerebralis to corpora allata projecting (PI-CA) neurons and corpora allata (CA), the latter being solely responsible for juvenile hormone (JH) synthesis and release. Using a machine learning classifier to identify summiting animals in real time, we observed that PI-CA neurons and CA appeared intact in summiting animals, despite invasion of adjacent regions of the "zombie fly" brain by E. muscae cells and extensive host tissue damage in the body cavity. The blood-brain barrier of flies late in their infection was significantly permeabilized, suggesting that factors in the hemolymph may have greater access to the central nervous system during summiting. Metabolomic analysis of hemolymph from summiting flies revealed differential abundance of several compounds compared to non-summiting flies. Transfusing the hemolymph of summiting flies into non-summiting recipients induced a burst of locomotion, demonstrating that factor(s) in the hemolymph likely cause summiting behavior. Altogether, our work reveals a neuro-mechanistic model for summiting wherein fungal cells perturb the fly's hemolymph, activating a neurohormonal pathway linking clock neurons to juvenile hormone production in the CA, ultimately inducing locomotor activity in their host.

Historical Selection, Adaptation Signatures, and Ambiguity of Introgressions in Wheat

Wheat was one of the crops domesticated in the Fertile Crescent region approximately 10,000 years ago. Despite undergoing recent polyploidization, hull-to-free-thresh transition events, and domestication bottlenecks, wheat is now grown in over 130 countries and accounts for a quarter of the world's cereal production. The main reason for its widespread success is its broad genetic diversity that allows it to thrive in different environments. To trace historical selection and hybridization signatures, genome scans were performed on two datasets: approximately 113K SNPs from 921 predominantly bread wheat accessions and approximately 110K SNPs from about 400 wheat accessions representing all ploidy levels. To identify environmental factors associated with the loci, a genome-environment association (GEA) was also performed. The genome scans on both datasets identified a highly differentiated region on chromosome 4A where accessions in the first dataset were dichotomized into a group (n = 691), comprising nearly all cultivars, wild emmer, and most landraces, and a second group (n = 230), dominated by landraces and spelt accessions. The grouping of cultivars is likely linked to their potential ancestor, bread wheat cv. Norin-10. The 4A region harbored important genes involved in adaptations to environmental conditions. The GEA detected loci associated with latitude and temperature. The genetic signatures detected in this study provide insight into the historical selection and hybridization events in the wheat genome that shaped its current genetic structure and facilitated its success in a wide spectrum of environmental conditions. The genome scans and GEA approaches applied in this study can help in screening the germplasm housed in gene banks for breeding, and for conservation purposes.

The role of learning-walk related multisensory experience in rewiring visual circuits in the desert ant brain

Efficient spatial orientation in the natural environment is crucial for the survival of most animal species. Cataglyphis desert ants possess excellent navigational skills. After far-ranging foraging excursions, the ants return to their inconspicuous nest entrance using celestial and panoramic cues. This review focuses on the question about how naïve ants acquire the necessary spatial information and adjust their visual compass systems. Naïve ants perform structured learning walks during their transition from the dark nest interior to foraging under bright sunlight. During initial learning walks, the ants perform rotational movements with nest-directed views using the earth’s magnetic field as an earthbound compass reference. Experimental manipulations demonstrate that specific sky compass cues trigger structural neuronal plasticity in visual circuits to integration centers in the central complex and mushroom bodies. During learning walks, rotation of the sky-polarization pattern is required for an increase in volume and synaptic complexes in both integration centers. In contrast, passive light exposure triggers light-spectrum (especially UV light) dependent changes in synaptic complexes upstream of the central complex. We discuss a multisensory circuit model in the ant brain for pathways mediating structural neuroplasticity at different levels following passive light exposure and multisensory experience during the performance of learning walks.

Reproductive Ecology of Drosophila obscura: A Cold Adapted Species

The study of insect reproductive ecology is essential to determine species distributions and fate under changing environments. Species adapted to harsh environments are good examples to investigate the reproductive mechanisms that allow them to cope with the challenging conditions. We here focus on studying for the first time the reproductive ecology of a cold-adapted Drosophila obscura (Diptera: Drosophilidae) strain collected in Finland (subarctic climate region). We tested several reproductive traits such as fertility and fecundity to observe the onset of reproduction and gauge when sexual maturity is reached in both males and females. We combined these measures with an analysis of changes of their reproductive organs shortly after eclosion. We found that males matured several days before females and that this process was underpinned by female egg maturation and male accessory gland growth, while sperm was already present in two-day old males. This delayed maturation is not observed to the same extent in other closely related species and might be a signature of exposure to harsh environments. Whether this delay is an adaptation to cope with variation in resource availability or prolonged unfavorable temperatures is though not clear. Finally, our study adds to the set of reproductive mechanisms used by cold adapted species and the information presented here contributes to understanding the breadth of Drosophila reproductive ecology.

Nucleotide Variation in Drosophila cryptochrome Is Linked to Circadian Clock Function: An Association Analysis

Cryptochrome (CRY) is a conserved protein associated with the circadian clock in a broad range of organisms, including plants, insects, and mammals. In Drosophila, cry is a pleiotropic gene that encodes a blue light-dedicated circadian photoreceptor, as well as an electromagnetic field sensor and a geotaxis behavior regulator. We have generated a panel of nearly-isogenic strains that originated from various wild populations and which carry different natural alleles of cry. Sequencing of these alleles revealed substantial polymorphism, the functional role of which was elusive. To link this natural molecular diversity to gene function, we relied on association mapping. Such analysis revealed two major haplogroups consisting of six linked nucleotides associated with circadian phase (haplotypes All1/All2). We also generated a maximum-likelihood gene-tree that uncovered an additional pair of haplogroups (B1/B2). Behavioral analysis of the different haplotypes indicated significant effect on circadian phase and period, as well on the amount of activity and sleep. The data also suggested substantial epistasis between the All and B haplogroups. Intriguingly, circadian photosensitivity, assessed by light-pulse experiments, did not differ between the genotypes. Using CRISPR-mediated transgenic flies, we verified the effect of B1/B2 polymorphism on circadian phase. The transgenic flies also exhibited substantially different levels of cry transcription. We, moreover, analyzed the geographical distribution of the B1/B2 haplotypes, focusing on a 12 bp insertion/deletion polymorphism that differentiates the two haplotypes. Analysis of cry sequences in wild populations across Europe revealed a geographical cline of B1/B2 indel frequency, which correlated with seasonal bioclimatic variables. This spatial distribution of cry polymorphism reinforces the functional importance of these haplotypes in the circadian system and local adaptation.

Adaptation and ecological speciation in seasonally varying environments at high latitudes: Drosophila virilis group

Living in high latitudes and altitudes sets specific requirements on species’ ability to forecast seasonal changes and to respond to them in an appropriate way. Adaptation into diverse environmental conditions can also lead to ecological speciation through habitat isolation or by inducing changes in traits that influence assortative mating. In this review, we explain how the unique time-measuring systems of Drosophila virilis group species have enabled the species to occupy high latitudes and how the traits involved in species reproduction and survival exhibit strong linkage with latitudinally varying photoperiodic and climatic conditions. We also describe variation in reproductive barriers between the populations of two species with overlapping distributions and show how local adaptation and the reinforcement of prezygotic barriers have created partial reproductive isolation between conspecific populations. Finally, we consider the role of species-specific chromosomal inversions and the X chromosome in the development of reproductive barriers between diverging lineages.

Memory shapes microbial populations

Correct decision making is fundamental for all living organisms to thrive under environmental changes. The patterns of environmental variation and the quality of available information define the most favourable strategy among multiple options, from randomly adopting a phenotypic state to sensing and reacting to environmental cues. Cellular memory—the ability to track and condition the time to switch to a different phenotypic state—can help withstand environmental fluctuations. How does memory manifest itself in unicellular organisms? We describe the population-wide consequences of phenotypic memory in microbes through a combination of deterministic modelling and stochastic simulations. Moving beyond binary switching models, our work highlights the need to consider a broader range of switching behaviours when describing microbial adaptive strategies. We show that memory in individual cells generates patterns at the population level coherent with overshoots and non-exponential lag times distributions experimentally observed in phenotypically heterogeneous populations. We emphasise the implications of our work in understanding antibiotic tolerance and, in general, bacterial survival under fluctuating environments.

While being genetically the same, a population of cells can show phenotypic variability even under homogeneous environments. Often advantageous under heterogeneous environments, this phenotypic heterogeneity is highly relevant in the studies of antibiotic resistance evolution and cancer resurgence. Numerous theoretical models exist applying a simple model of phenotypic switching. Experimental measurements on phenotypic heterogeneity have increased in precision over the past decade, and the simple models are inadequate to explain the new observations. In this paper, we explore the role of cellular memory as a crucial component of phenotypic switching. We see that memory helps account for the hitherto unexplained observations and fundamentally extend our understanding of phenotypic heterogeneity.

VRILLE shows high divergence among Higher Diptera flies but may retain role as transcriptional repressor of clock

In the circadian system, the clock gene vrille (vri) is an essential component of the second feedback loop, being responsible in Drosophila for the rhythmicity of the Clock (Clk) gene transcription by its repression. Here we studied vri in a fruit fly pest, the Tephritidae Anastrepha fraterculus, aimingtoinvestigate its molecular evolution and expression patterns from whole-head extracts. We used a combination of transcriptomic, genomic and gene walking strategies to sequence and characterize Afravri in male and female head transcriptomes of A. fraterculus and detected two putative isoforms that may correspond to A and D vri isoforms of Drosophila. Both isoforms produced a full-length sequence that translates to 842 amino acids. While the protein sequence showed significant divergence to orthologous sequences from other organisms, the bZIP domain was highly conserved. Molecular evolutionary analyses showed that vri in higher Diptera flies has been evolving under positive selection. A more detailed analysis showed positive selection also in Tephritidae with 29 sites evolving under positive selection in comparison with Drosophilidae. Real time expression analysis in LD and DD conditions showed cyclic expression of Afravri mRNA with oscillation opposite to AfraClk, suggesting that VRI may also behave in Anastrepha as a transcriptional repressor of Clk, providing another indication that higher Diptera might share common interlocked transcript-translation feedback loops (TTFLs) mechanisms that differ from other insects in target genes.

Multiple paths to cold tolerance: the role of environmental cues, morphological traits and the circadian clock gene vrille

Background

Tracing the association between insect cold tolerance and latitudinally and locally varying environmental conditions, as well as key morphological traits and molecular mechanisms, is essential for understanding the processes involved in adaptation. We explored these issues in two closely-related species, Drosophila montana and Drosophila flavomontana, originating from diverse climatic locations across several latitudes on the coastal and mountainous regions of North America. We also investigated the association between sequence variation in one of the key circadian clock genes, vrille, and cold tolerance in both species. Finally, we studied the impact of vrille on fly cold tolerance and cold acclimation ability by silencing it with RNA interference in D. montana.

Results

We performed a principal component analysis (PCA) on variables representing bioclimatic conditions on the study sites and used latitude as a proxy of photoperiod. PC1 separated the mountainous continental sites from the coastal ones based on temperature variability and precipitation, while PC2 arranged the sites based on summer and annual mean temperatures. Cold tolerance tests showed D. montana to be more cold-tolerant than D. flavomontana and chill coma resistance (CT_min) of this species showed an association with PC2. Chill coma recovery time (CCRT) of both species improved towards northern latitudes, and in D. flavomontana this trait was also associated with PC1. D. flavomontana flies were darkest in the coast and in the northern mountainous populations, but coloration showed no linkage with cold tolerance. Body size decreased towards cold environments in both species, but only within D. montana populations largest flies showed fastest recovery from cold. Finally, both the sequence analysis and RNAi study on vrille suggested this gene to play an essential role in D. montana cold resistance and acclimation, but not in recovery time.

Conclusions

Our study demonstrates the complexity of insect cold tolerance and emphasizes the need to trace its association with multiple environmental variables and morphological traits to identify potential agents of natural selection. It also shows that a circadian clock gene vrille is essential both for short- and long-term cold acclimation, potentially elucidating the connection between circadian clock system and cold tolerance.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01849-y.

Temperature, rainfall and wind variables underlie environmental adaptation in natural populations of Drosophila melanogaster

While several studies in a diverse set of species have shed light on the genes underlying adaptation, our knowledge on the selective pressures that explain the observed patterns lags behind. Drosophila melanogaster is a valuable organism to study environmental adaptation because this species originated in Southern Africa and has recently expanded worldwide, and also because it has a functionally well-annotated genome. In this study, we aimed to decipher which environmental variables are relevant for adaptation of D. melanogaster natural populations in Europe and North America. We analysed 36 whole-genome pool-seq samples of D. melanogaster natural populations collected in 20 European and 11 North American locations. We used the BayPass software to identify single nucleotide polymorphisms (SNPs) and transposable elements (TEs) showing signature of adaptive differentiation across populations, as well as significant associations with 59 environmental variables related to temperature, rainfall, evaporation, solar radiation, wind, daylight hours, and soil type. We found that in addition to temperature and rainfall, wind related variables are also relevant for D. melanogaster environmental adaptation. Interestingly, 23%-51% of the genes that showed significant associations with environmental variables were not found overly differentiated across populations. In addition to SNPs, we also identified 10 reference transposable element insertions associated with environmental variables. Our results showed that genome-environment association analysis can identify adaptive genetic variants that are undetected by population differentiation analysis while also allowing the identification of candidate environmental drivers of adaptation.

Amplitude of circadian rhythms becomes weaken in the north, but there is no cline in the period of rhythm in a beetle

Many species show rhythmicity in activity, from the timing of flowering in plants to that of foraging behavior in animals. The free-running periods and amplitude (sometimes called strength or power) of circadian rhythms are often used as indicators of biological clocks. Many reports have shown that these traits are highly geographically variable, and interestingly, they often show latitudinal or longitudinal clines. In many cases, the higher the latitude is, the longer the free-running circadian period (i.e., period of rhythm) in insects and plants. However, reports of positive correlations between latitude or longitude and circadian rhythm traits, including free-running periods, the power of the rhythm and locomotor activity, are limited to certain taxonomic groups. Therefore, we collected a cosmopolitan stored-product pest species, the red flour beetle Tribolium castaneum, in various parts of Japan and examined its rhythm traits, including the power and period of the rhythm, which were calculated from locomotor activity. The analysis revealed that the power was significantly lower for beetles collected in northern areas than southern areas in Japan. However, it is worth noting that the period of circadian rhythm did not show any clines; specifically, it did not vary among the sampling sites, despite the very large sample size (n = 1585). We discuss why these cline trends were observed in T. castaneum.

Intrapopulation genetic variation in the level and rhythm of daily activity in Drosophila immigrans

Genetic diversity within a population, such as polymorphisms and personality, is considered to improve population performance because such intraspecific variations have the potential to alleviate the competition for a limited resource or the risk of predation and sexual harassment at a population level. Variation in the level and rhythm of daily activity in a population could also affect population performance by directly altering ecological, social, and sexual interactions among individuals. However, it remains to be elucidated whether such intra-population variation in the level and rhythms of daily activity exists in a natural population. Here, we investigated the genetic variation in daily activity within a single natural population of Drosophila immigrans. We established 21 isofemale lines from a single natural population and measured larval activity level and the level and daily pattern of adult activity over a 24 hr period. Larval activity level significantly varied among isofemale lines. Likewise, the activity level in the adult stage significantly varied among lines. The significant variation was also found in the daily pattern of adult activity; some lines showed greater activity level in the daytime, and others showed greater activity level in the night. Our results consistently suggest that there is a genetic variation in behavioral activity in a natural population, probably contributing to shaping the population performance.

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.

To Ub or not to Ub: Regulation of circadian clocks by ubiquitination and deubiquitination

Circadian clocks are internal timing systems that enable organisms to adjust their behavioral and physiological rhythms to the daily changes of their environment. These clocks generate self-sustained oscillations at the cellular, tissue, and behavioral level. The rhythm-generating mechanism is based on a gene expression network with a delayed negative feedback loop that causes the transcripts to oscillate with a period of approximately 24 hr. This oscillatory nature of the proteins involved in this network necessitates that they are intrinsically unstable, with a short half-life. Hence, post-translational modifications (PTMs) are important to precisely time the presence, absence, and interactions of these proteins at appropriate times of the day. Ubiquitination and deubiquitination are counter-balancing PTMs which play a key role in this regulatory process. In this review, we take a comprehensive look at the roles played by the processes of ubiquitination and deubiquitination in the clock machinery of the most commonly studied eukaryotic models of the circadian clock: plants, fungi, fruit flies, and mammals. We present the effects exerted by ubiquitinating and deubiquitinating enzymes on the stability, but also the activity, localization, and interactions of clock proteins. Overall, these PTMs have key roles in regulating not only the pace of the circadian clocks but also their response to external cues and their control of cellular functions.

Influence of light and temperature cycles on the expression of circadian clock genes in the mussel Mytilus edulis

Clock genes and environmental cues regulate essential biological rhythms. The blue mussel, Mytilus edulis, is an ecologically and economically important intertidal bivalve undergoing seasonal reproductive rhythms. We previously identified seasonal expression differences in M. edulis clock genes. Herein, the effects of light/dark cycles, constant darkness, and daily temperature cycles on the circadian expression patterns of such genes are characterised. Clock genes Clk, Cry1, ROR/HR3, Per and Rev-erb/NR1D1, and Timeout-like, show significant mRNA expression variation, persisting in darkness indicating endogenous control. Rhythmic expression was apparent under diurnal temperature cycles in darkness for all except Rev-erb. Temperature cycles induced a significant expression difference in the non-circadian clock-associated gene aaNAT. Furthermore, Suppression Subtractive Hybridisation (SSH) was used to identify seasonal genes with potential links to molecular clock function and revealed numerous genes meriting further investigation. Understanding the relationship between environmental cues and molecular clocks is crucial in predicting the outcomes of environmental change on fundamental rhythmic processes.

Circadian and Genetic Modulation of Visually-Guided Navigation in Drosophila Larvae

Organisms possess an endogenous molecular clock which enables them to adapt to environmental rhythms and to synchronize their metabolism and behavior accordingly. Circadian rhythms govern daily oscillations in numerous physiological processes, and the underlying molecular components have been extensively described from fruit flies to mammals. Drosophila larvae have relatively simple nervous system compared to their adult counterparts, yet they both share a homologous molecular clock with mammals, governed by interlocking transcriptional feedback loops with highly conserved constituents. Larvae exhibit a robust light avoidance behavior, presumably enabling them to avoid predators and desiccation, and DNA-damage by exposure to ultraviolet light, hence are crucial for survival. Circadian rhythm has been shown to alter light-dark preference, however it remains unclear how distinct behavioral strategies are modulated by circadian time. To address this question, we investigate the larval visual navigation at different time-points of the day employing a computer-based tracking system, which allows detailed evaluation of distinct navigation strategies. Our results show that due to circadian modulation specific to light information processing, larvae avoid light most efficiently at dawn, and a functioning clock mechanism at both molecular and neuro-signaling level is necessary to conduct this modulation.

Flies as models for circadian clock adaptation to environmental challenges

Life on earth is assumed to have developed in tropical regions that are characterized by regular 24 hr cycles in irradiance and temperature that remain the same throughout the seasons. All organisms developed circadian clocks that predict these environmental cycles and prepare the organisms in advance for them. A central question in chronobiology is how endogenous clocks changed in order to anticipate very different cyclical environmental conditions such as extremely short and long photoperiods existing close to the poles. Flies of the family Drosophilidae can be found all over the world-from the tropics to subarctic regions-making them unprecedented models for studying the evolutionary processes that underlie the adaptation of circadian clocks to different latitudes. This review summarizes our current understanding of these processes. We discuss evolutionary changes in the clock genes and in the clock network in the brain of different Drosophilids that may have caused behavioural adaptations to high latitudes.

Latitudinal Variation in Circadian Rhythmicity in Nasonia vitripennis

Many physiological processes of living organisms show circadian rhythms, governed by an endogenous clock. This clock has a genetic basis and is entrained by external cues, such as light and temperature. Other physiological processes exhibit seasonal rhythms, that are also responsive to light and temperature. We previously reported a natural latitudinal cline of photoperiodic diapause induction in the parasitic wasp Nasonia vitripennis in Europe and a correlated haplotype frequency for the circadian clock gene period (per). To evaluate if this correlation is reflected in circadian behaviour, we investigated the circadian locomotor activity of seven populations from the cline. We found that the proportion of rhythmic males was higher than females in constant darkness, and that mating decreased rhythmicity of both sexes. Only for virgin females, the free running period (τ) increased weakly with latitude. Wasps from the most southern locality had an overall shorter free running rhythm and earlier onset, peak, and offset of activity during the 24 h period, than wasps from the northernmost locality. We evaluated this variation in rhythmicity as a function of period haplotype frequencies in the populations and discussed its functional significance in the context of local adaptation.

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

Light is the most important Zeitgeber for entraining animal activity rhythms to the 24-h day. In all animals, the eyes are the main visual organs that are not only responsible for motion and colour (image) vision, but also transfer light information to the circadian clock in the brain. The way in which light entrains the circadian clock appears, however, variable in different species. As do vertebrates, insects possess extraretinal photoreceptors in addition to their eyes (and ocelli) that are sometimes located close to (underneath) the eyes, but sometimes even in the central brain. These extraretinal photoreceptors contribute to entrainment of their circadian clocks to different degrees. The fruit fly Drosophila melanogaster is special, because it expresses the blue light-sensitive cryptochrome (CRY) directly in its circadian clock neurons, and CRY is usually regarded as the fly’s main circadian photoreceptor. Nevertheless, recent studies show that the retinal and extraretinal eyes transfer light information to almost every clock neuron and that the eyes are similarly important for entraining the fly’s activity rhythm as in other insects, or more generally spoken in other animals. Here, I compare the light input pathways between selected insect species with a focus on Drosophila’s special case.

Life at High Latitudes Does Not Require Circadian Behavioral Rhythmicity under Constant Darkness

Nearly all organisms evolved endogenous self-sustained timekeeping mechanisms to track and anticipate cyclic changes in the environment. Circadian clocks, with a periodicity of about 24 h, allow animals to adapt to day-night cycles. Biological clocks are highly adaptive, but strong behavioral rhythms might be a disadvantage for adaptation to weakly rhythmic environments such as polar areas [1, 2]. Several high-latitude species, including Drosophila species, were found to be highly arrhythmic under constant conditions [3-6]. Furthermore, Drosophila species from subarctic regions can extend evening activity until dusk under long days. These traits depend on the clock network neurochemistry, and we previously proposed that high-latitude Drosophila species evolved specific clock adaptations to colonize polar regions [5, 7, 8]. We broadened our analysis to 3 species of the Chymomyza genus, which diverged circa 5 million years before the Drosophila radiation [9] and colonized both low and high latitudes [10, 11]. C. costata, pararufithorax, and procnemis, independently of their latitude of origin, possess the clock neuronal network of low-latitude Drosophila species, and their locomotor activity does not track dusk under long photoperiods. Nevertheless, the high-latitude C. costata becomes arrhythmic under constant darkness (DD), whereas the two low-latitude species remain rhythmic. Different mechanisms are behind the arrhythmicity in DD of C. costata and the high-latitude Drosophila ezoana, suggesting that the ability to maintain behavioral rhythms has been lost more than once during drosophilids' evolution and that it might indeed be an evolutionary adaptation for life at high latitudes.

Circadian regulation of membrane physiology in neural oscillators throughout the brain

Twenty-four-hour rhythmicity in physiology and behavior are driven by changes in neurophysiological activity that vary across the light-dark and rest-activity cycle. Although this neural code is most prominent in neurons of the primary circadian pacemaker in the suprachiasmatic nucleus (SCN) of the hypothalamus, there are many other regions in the brain where region-specific function and behavioral rhythmicity may be encoded by changes in electrical properties of those neurons. In this review, we explore the existing evidence for molecular clocks and/or neurophysiological rhythms (i.e., 24 hr) in brain regions outside the SCN. In addition, we highlight the brain regions that are ripe for future investigation into the critical role of circadian rhythmicity for local oscillators. For example, the cerebellum expresses rhythmicity in over 2,000 gene transcripts, and yet we know very little about how circadian regulation drives 24-hr changes in the neural coding responsible for motor coordination. Finally, we conclude with a discussion of how our understanding of circadian regulation of electrical properties may yield insight into disease mechanisms which may lead to novel chronotherapeutic strategies in the future.