Circadian rhythms: from behaviour to molecules

Swarming Behavior in Anopheles gambiae (sensu lato): Current Knowledge and Future Outlook

Effective management of insect disease vectors requires a detailed understanding of their ecology and behavior. In Anopheles gambiae sensu lato (s.l.) (Diptera: Culicidae) mating occurs during swarming, but knowledge of their mating behavior under natural conditions is limited. Mosquitoes mate in flight over specific landmarks, known as swarm markers, at particular locations. Swarms consist of males; the females usually approach the swarm and depart following copulation. The number of mating pairs per swarm is closely associated with swarm size. The shape and height of swarm markers vary and may depend on the environmental conditions at the swarm's location. Male-male interactions in mosquito swarms with similar levels of attractive flight activity can offer a mating advantage to some individuals. Flight tone is used by mosquitoes to recognize the other sex and choose a desirable mate. Clarifying these and other aspects of mosquito reproductive behavior can facilitate the development of population control measures that target swarming sites. This review describes what is currently known about swarming behavior in Anopheles gambiae s.l., including swarm characteristics; mating within and outside of swarms, insemination in females, and factors affecting and stimulating swarming.

Circadian Clock Components Offer Targets for Crop Domestication and Improvement

During plant domestication and improvement, farmers select for alleles present in wild species that improve performance in new selective environments associated with cultivation and use. The selected alleles become enriched and other alleles depleted in elite cultivars. One important aspect of crop improvement is expansion of the geographic area suitable for cultivation; this frequently includes growth at higher or lower latitudes, requiring the plant to adapt to novel photoperiodic environments. Many crops exhibit photoperiodic control of flowering and altered photoperiodic sensitivity is commonly required for optimal performance at novel latitudes. Alleles of a number of circadian clock genes have been selected for their effects on photoperiodic flowering in multiple crops. The circadian clock coordinates many additional aspects of plant growth, metabolism and physiology, including responses to abiotic and biotic stresses. Many of these clock-regulated processes contribute to plant performance. Examples of selection for altered clock function in tomato demonstrate that with domestication, the phasing of the clock is delayed with respect to the light–dark cycle and the period is lengthened; this modified clock is associated with increased chlorophyll content in long days. These and other data suggest the circadian clock is an attractive target during breeding for crop improvement.

Cloning of circadian rhythmic pathway genes and perturbation of oscillation patterns in endocrine disrupting chemicals (EDCs)-exposed mangrove killifish Kryptolebias marmoratus

To investigate the effect of endocrine disrupting chemicals (EDCs) on the circadian rhythm pathway, we cloned clock and circadian rhythmic pathway-associated genes (e.g. Per2, Cry1, Cry2, and BMAL1) in the self-fertilizing mangrove killifish Kryptolebias marmoratus. The promoter region of Km-clock had 1 aryl hydrocarbon receptor element (AhRE, GTGCGTGACA) and 8 estrogen receptor (ER) half-sites, indicating that the AhRE and ER half sites would likely be associated with regulation of clock protein activity during EDCs-induced cellular stress. The Km-clock protein domains (bHLH, PAS1, PAS2) were highly conserved in five additional fish species (zebrafish, Japanese medaka, Southern platyfish, Nile tilapia, and spotted green pufferfish), suggesting that the fish clock protein may play an important role in controlling endogenous circadian rhythms. The promoter regions of Km-BMAL1, -Cry1, -Cry2, and -Per2 were found to contain several xenobiotic response elements (XREs), indicating that EDCs may be able to alter the expression of these genes. To analyze the endogenous circadian rhythm in K. marmoratus, we measured expression of Km-clock and other circadian rhythmic genes (e.g. Per2, Cry1, Cry2, and BMAL1) in different tissues, and found ubiquitous expression, although there were different patterns of transcript amplification during different developmental stages. In an estrogen (E2)-exposed group, Km-clock expression was down-regulated, however, a hydroxytamoxifen (TMX, nonsteroid estrogen antagonist)-exposed group showed an upregulated pattern of Km-clock expression, suggesting that the expression of Km-clock is closely associated with exposure to EDCs. In response to the exposure of bisphenol A (BPA) and 4-tert-octyphenol (OP), Km-clock expression was down-regulated in the pituitary/brain, muscle, and skin in both gender types (hermaphrodite and secondary male). In juvenile K. marmoratus liver tissue, expression of Km-clock and other circadian rhythmic pathway-associated genes showed a regular oscillation pattern over a period of approximately 24h during a 12L:12D cycle. However, the circadian rhythm of BPA-exposed juvenile K. marmoratus liver tissue was perturbed over a 12L:12D period. This study will aid in our understanding of how EDCs perturb endogenous circadian rhythms, particularly in BPA-exposed fish liver tissue.

The Effects of Sex-Ratio and Density on Locomotor Activity in the House Fly, Musca domestica

Although locomotor activity is involved in almost all behavioral traits, there is a lack of knowledge on what factors affect it. This study examined the effects of sex—ratio and density on the circadian rhythm of locomotor activity of adult Musca domestica L. (Diptera: Muscidae) using an infra—red light system. Sex—ratio significantly affected locomotor activity, increasing with the percentage of males in the vials. In accordance with other studies, males were more active than females, but the circadian rhythm of the two sexes was not constant over time and changed during the light period. There was also an effect of density on locomotor activity, where males at intermediate densities showed higher activity. Further, the predictability of the locomotor activity, estimated as the degree of autocorrelation of the activity data, increased with the number of males present in the vials both with and without the presence of females. Overall, this study demonstrates that locomotor activity in M. domestica is affected by sex—ratio and density. Furthermore, the predictability of locomotor activity is affected by both sex—ratio, density, and circadian rhythm. These results add to our understanding of the behavioral interactions between houseflies and highlight the importance of these factors when designing behavioral experiments using M. domestica.

Age, but not experience, affects courtship gene expression in male Drosophila melanogaster

Mutation screens in model organisms have helped identify the foundation of many fundamental organismal phenotypes. An emerging question in evolutionary and behavioral biology is the extent to which these “developmental” genes contribute to the subtle individual variation that characterizes natural populations. A related question is whether individual differences arise from static differences in gene expression that arose during previous life stages, or whether they are due to dynamic regulation of expression during the life stage under investigation. Here, we address these questions using genes that have been discovered to control the development of normal courtship behavior in male Drosophila melanogaster. We examined whether these genes have static or dynamic expression in the heads of adult male flies of different ages and with different levels of social experience. We found that 16 genes of the 25 genes examined were statically expressed, and 9 genes were dynamically expressed with changes related to adult age. No genes exhibited rapid dynamic expression changes due to social experience or age*experience interaction. We therefore conclude that a majority of fly “courtship” genes are statically expressed, while a minority are regulated in adults with respect to age, but not with respect to relevant social experience. These results are consistent with those from a recent microarray analysis that found none of the canonical courtship genes changed expression in male flies after brief exposure to females.

Time and development

The role that biological timers play in gametogenesis and development is reviewed through use of selected examples. Some general features of biological timers are also reviewed, and two types of timing mechanism are discussed in more detail: circadian rhythms and cell-cycle-based timers. In particular, the recent evidence that oscillatory ion channel activity may play an important role in timing mechanisms is summarized. The activity and properties of an oscillatory K(+) channel present during preimplantation mouse development are described, and preliminary results from its neutralization are discussed.

The period gene in two species of tephritid fruit fly differentiated by mating behaviour

The period gene is important for the generation and maintenance of biological rhythms. It served as an ideal candidate for the investigation of the mating time isolation between two sibling Queensland fruit fly species, Bactrocera tryoni and Bactrocera neohumeralis. We have isolated the homologues of the period gene in the two species, and show that their putative amino acid sequences are identical. No length polymorphism was detected in the Thr-Gly repeat region. per mRNA expression, assayed in light-dark diurnal conditions, displayed circadian oscillation in both the head and abdomen of B. tryoni and B. neohumeralis, with the same cycling phase. An alternatively spliced intron was identified in the 3' untranslated region. The effect of temperature on the splicing and mRNA expression was examined.

Circadian rhythms of female mating activity governed by clock genes in Drosophila

The physiological and behavioral activities of many animals are restricted to specific times of the day. The daily fluctuation in the mating activity of some insects is controlled by an endogenous clock, but the genetic mechanism that controls it remains unknown. Here we demonstrate that wild-type Drosophila melanogaster display a robust circadian rhythm in the mating activity, and that these rhythms are abolished in period- or timeless-null mutant flies (per(01) and tim(01)). Circadian rhythms were lost when rhythm mutant females were paired with wild-type males, demonstrating that female mating activity is governed by clock genes. Furthermore, we detected an antiphasic relationship in the circadian rhythms of mating activity between D. melanogaster and its sibling species Drosophila simulans. Female- and species-specific circadian rhythms in the mating activity of Drosophila seem to cause reproductive isolation.

The clock gene period of the housefly, Musca domestica, rescues behavioral rhythmicity in Drosophila melanogaster. Evidence for intermolecular coevolution?

In Drosophila, the clock gene period (per), is an integral component of the circadian clock and acts via a negative autoregulatory feedback loop. Comparative analyses of per genes in insects and mammals have revealed that they may function in similar ways. However in the giant silkmoth, Antheraea pernyi, per expression and that of the partner gene, tim, is not consistent with the negative feedback role. As an initial step in developing an alternative dipteran model to Drosophila, we have identified the per orthologue in the housefly, Musca domestica. The Musca per sequence highlights a pattern of conservation and divergence similar to other insect per genes. The PAS dimerization domain shows an unexpected phylogenetic relationship in comparison with the corresponding region of other Drosophila species, and this appears to correlate with a functional assay of the Musca per transgene in Drosophila melanogaster per-mutant hosts. A simple hypothesis based on the coevolution of the PERIOD and TIMELESS proteins with respect to the PER PAS domain can explain the behavioral data gathered from transformants.

Circadian clocks in the mammalian brain

Daily cycles in physiology and behaviour are probably a universal feature of multicellular organisms. These rhythms are predominantly driven by endogenous clocks with a periodicity approximating to one day, i.e. circadian. In mammals, the circadian clock governing activity/ rest, neuroendocrine and autonomic rhythms lies in the hypothalamus, in the suprachiasmatic nuclei (SCN). Intrinsic circadian oscillators are also present in the retina. The SCN "clockwork" is based on a cell autonomous, genetically determined mechanism. Mammalian homologues of a number of Drosophila genes which encode elements of the fly circadian mechanism have recently been identified. In Drosophila, the protein products of these genes interact in a negative feedback loop, establishing a circadian cycle in gene expression. Characterisation of the roles played by putative mammalian clock genes in the SCN, and how the emergent cellular signal imposes order over the entire neuraxis, will provide a fundamental contribution to our understanding of the molecular basis of behaviour. BioEssays 22:23-31, 2000.

Egg timers: how is developmental time measured in the early vertebrate embryo?

Eggs and early embryos appear to be programmed to undertake particular developmental decisions at characteristic times, although precisely how these decisions are timed is unknown. We discuss the possible roles and interactions during early vertebrate development of two broad categories of timers: 1) those that involve cyclic or sequential mechanisms, referred to as clocks; and 2) those that require an increase or decrease in some factor to a threshold level for progression of time, referred to as hourglass timers. It is concluded that both clock-like timers linked to various features of the cell cycle and hourglass timers are involved in early developmental timing. The possible involvement of elements of circadian clock timers is also considered. BioEssays 22:57-63, 2000.

PER and TIM inhibit the DNA binding activity of a Drosophila CLOCK-CYC/dBMAL1 heterodimer without disrupting formation of the heterodimer: a basis for circadian transcription

The Drosophila CLOCK (dCLOCK) and CYCLE (CYC) (also referred to as dBMAL1) proteins are members of the basic helix-loop-helix PAS (PER-ARNT-SIM) superfamily of transcription factors and are required for high-level expression of the circadian clock genes period (per) and timeless (tim). Several lines of evidence indicate that PER, TIM, or a PER-TIM heterodimer somehow inhibit the transcriptional activity of a putative dCLOCK-CYC complex, generating a negative-feedback loop that is a core element of the Drosophila circadian oscillator. In this report we show that PER and/or TIM inhibits the binding of a dCLOCK-CYC heterodimer to an E-box-containing DNA fragment that is present in the 5' nontranscribed region of per and acts as a circadian enhancer element. Surprisingly, inhibition of this DNA binding activity by PER, TIM, or both is not accompanied by disruption of the association between dCLOCK and CYC. The results suggest that the interaction of PER, TIM, or both with the dCLOCK-CYC heterodimer induces a conformational change or masks protein regions in the heterodimer, leading to a reduction in DNA binding activity. Together with other findings, our results strongly suggest that daily cycles in the association of PER and TIM with the dCLOCK-CYC complex probably contribute to rhythmic expression of per and tim.

Role of posttranscriptional regulation in circadian clocks: lessons from Drosophila

Incredible progress has been made in the last few years in our understanding of the molecular mechanisms underlying circadian clocks. Many of the recent insights have been gained by the isolation and characterization of novel clock mutants and their associated gene products. As might be expected based on theoretical considerations and earlier studies that indicated the importance of temporally regulated macromolecular synthesis for the manifestation of overt rhythms, daily oscillations in the levels of "clock" RNAs and proteins are a pervasive feature of these timekeeping devices. How are these molecular rhythms generated and synchronized? Recent evidence accumulated from a wide variety of model organisms, ranging from bacteria to mammals, points toward an emerging trend; at the "heart" of circadian oscillators lies a cell autonomous transcriptional feedback loop that is composed of alternatively functioning positive and negative elements. Nonetheless, it is also clear that to bring this transcriptional feedback loop to "life" requires important contributions from posttranscriptional regulatory schemes. For one thing, there must be times in the day when the activities of negative-feedback regulators are separated from the activities of the positive regulators they act on, or else the oscillatory potential of the system will be dissipated, resulting in a collection of molecules at steady state. This review mainly summarizes the role of posttranscriptional regulation in the Drosophila melanogaster time-keeping mechanism. Accumulating evidence from Drosophila and other systems suggests that posttranscriptional regulatory mechanisms increase the dynamic range of circadian transcriptional feedback loops, overlaying them with appropriately timed biochemical constraints that not only engender these loops with precise daily periods of about 24 h, but also with the ability to integrate and respond rapidly to multiple environmental cues such that their phases are aligned optimally to the prevailing external conditions.

Expression of basic helix-loop-helix/PAS genes in the mouse suprachiasmatic nucleus

The suprachiasmatic nuclei contain a circadian clock that drives rhythmicity in physiology and behavior. In mice, mutation of the Clock gene produces abnormal circadian behavior [Vitaterna M. H. et al. (1994) Science 264, 715-725]. The Clock gene encodes a protein containing basic helix-loop-helix and PAS (PER-ARNT-SIM) domains [King D. P. et al. (1997) Cell 89, 641-653]. The PAS domain may be an important structural feature of a subset of genes involved in photoreception and circadian rhythmicity. The expression and regulation of messenger RNAs encoding eight members of the basic helix-loop-helix/PAS protein superfamily were examined by in situ hybridization. Six of the genes studied (aryl hydrocarbon receptor nuclear transporter, aryl hydrocarbon receptor nuclear transporter-2, Clock, endothelial PAS-containing protein, hypoxia-inducible factor-1alpha and steroid receptor coactivator-1) were expressed in the suprachiasmatic nucleus of adult and neonatal mice. No evidence for rhythmicity of expression was observed when comparing brains collected early in the subjective day (circadian time 3) with those collected early in subjective night (circadian time 15). Neuronal PAS-containing protein-1 messenger RNA was expressed in the suprachiasmatic nucleus of adult (but not neonatal) mice, and a low-amplitude rhythm of neuronal PAS-containing protein-1 gene expression was detected in the suprachiasmatic nucleus. Neuronal PAS-containing protein-2 messenger RNA was not detected in adult or neonatal suprachiasmatic nucleus. Exposure to light at night (30 or 180 min of light, beginning at circadian time 15) did not alter the expression of any of the genes studied. The expression of multiple members of the basic helix-loop-helix/PAS family in the suprachiasmatic nucleus suggests a rich array of potential interactions relevant to the regulation of the suprachiasmatic circadian clock.

The 69 bp circadian regulatory sequence (CRS) mediates per-like developmental, spatial, and circadian expression and behavioral rescue in Drosophila

The period (per) gene is an essential component of the circadian timekeeping mechanism in Drosophila. This gene is expressed in a circadian manner, giving rise to a protein that feeds-back to regulate its own transcription. A 69 bp clock regulatory sequence (CRS) has been identified previously upstream of the period gene. The CRS confers wild-type mRNA cycling when used to drive a lacZ reporter gene in transgenic flies. To determine whether the CRS also mediates proper developmental and spatial expression and behavioral rescue, we used the CRS to drive either lacZ or per in transgenic flies. The results show that the CRS is able to activate expression in pacemaker neuron precursors in larvae and essentially all tissues that normally express per in pupae and adults. The CRS is sufficient to rescue circadian feedback loop function and behavioral rhythms in per01 flies. However, the period of locomotor activity rhythms shortens if a stronger basal promoter is used. This study shows that regulatory elements sufficient for clock-dependent and tissue-specific per expression in larvae, pupae, and adults are present in the CRS and that the period of adult locomotor activity rhythms is dependent, in part, on the overall level of per transcripts.

Mammalian circadian autoregulatory loop: a timeless ortholog and mPer1 interact and negatively regulate CLOCK-BMAL1-induced transcription

We report the cloning and mapping of mouse (mTim) and human (hTIM) orthologs of the Drosophila timeless (dtim) gene. The mammalian Tim genes are widely expressed in a variety of tissues; however, unlike Drosophila, mTim mRNA levels do not oscillate in the suprachiasmatic nucleus (SCN) or retina. Importantly, hTIM interacts with the Drosophila PERIOD (dPER) protein as well as the mouse PER1 and PER2 proteins in vitro. In Drosophila (S2) cells, hTIM and dPER interact and translocate into the nucleus. Finally, hTIM and mPER1 specifically inhibit CLOCK-BMAL1-induced transactivation of the mPer1 promoter. Taken together, these results demonstrate that mTim and hTIM are mammalian orthologs of timeless and provide a framework for a basic circadian autoregulatory loop in mammals.

Functional and evolutionary implications of natural variation in clock genes

Nearly all studies of natural variation within clock genes involve the period (per) locus, which was originally isolated in the fruit-fly. Intra- and interspecific work on per has focused mostly on a region of Thr-Gly or Ser-Gly repeats, which show rapid length and sequence evolution. The functional implications of nucleotide variation in this repetitive array have been characterised using behavioural, molecular, ecological, structural and statistical analyses. A population genetics approach to variation in per has also been useful in defining species histories within Drosophilids and, in some cases, in implicating selective processes in the evolution of the per gene. Interspecific analysis of per expression patterns reveals evolutionary alterations in this clock gene's regulation.

Activating inhibitors and inhibiting activators: a day in the life of a fly

The circadian clock keeps time through an intracellular oscillator that requires rhythmic gene expression. In Drosophila melanogaster, the core of this oscillator is composed of a circadian feedback loop in which the transcription of the period and timeless genes is repressed by their own protein products. In the past year, our understanding of clock organization and function in Drosophila has been advanced by breakthroughs that define when, where and how this feedback loop operates. These studies, along with those in other organisms, suggest that circadian feedback loops are widespread and that genes within these feedback loops are conserved between Drosophila and mammals.

Circadian rhythms: molecular basis of the clock

Much progress has been made during the past year in the molecular dissection of the circadian clock. Recently identified circadian genes in mouse, Drosophila, and cyanobacteria demonstrate the universal nature of negative feedback regulation as a circadian mechanism; furthermore, the mouse and Drosophila genes are structurally and functionally conserved. In addition, the discovery of brain-independent clocks promises to revolutionize the study of circadian biology.

A serum shock induces circadian gene expression in mammalian tissue culture cells

The treatment of cultured rat-1 fibroblasts or H35 hepatoma cells with high concentrations of serum induces the circadian expression of various genes whose transcription also oscillates in living animals. Oscillating genes include rper1 and rper2 (rat homologs of the Drosophila clock gene period), and the genes encoding the transcription factors Rev-Erb alpha, DBP, and TEF. In rat-1 fibroblasts, up to three consecutive daily oscillations with an average period length of 22.5 hr could be recorded. The temporal sequence of the various mRNA accumulation cycles is the same in cultured cells and in vivo. The serum shock of rat-1 fibroblasts also results in a transient stimulation of c-fos and rper expression and thus mimics light-induced immediate-early gene expression in the suprachiasmatic nucleus.

Closing the circadian loop: CLOCK-induced transcription of its own inhibitors per and tim

The circadian oscillator generates a rhythmic output with a period of about 24 hours. Despite extensive studies in several model systems, the biochemical mode of action has not yet been demonstrated for any of its components. Here, the Drosophila CLOCK protein was shown to induce transcription of the circadian rhythm genes period and timeless. dCLOCK functioned as a heterodimer with a Drosophila homolog of BMAL1. These proteins acted through an E-box sequence in the period promoter. The timeless promoter contains an 18-base pair element encompassing an E-box, which was sufficient to confer dCLOCK responsiveness to a reporter gene. PERIOD and TIMELESS proteins blocked dCLOCK's ability to transactivate their promoters via the E-box. Thus, dCLOCK drives expression of period and timeless, which in turn inhibit dCLOCK's activity and close the circadian loop.

Molecular and behavioral analysis of four period mutants in Drosophila melanogaster encompassing extreme short, novel long, and unorthodox arrhythmic types

Of the mutationally defined rhythm genes in Drosophila melanogaster, period (per) has been studied the most. We have molecularly characterized three older per mutants-perT, perClk, and per04-along with a novel long-period one (perSLIH). Each mutant is the result of a single nucleotide change. perT, perClk, and perSLIH are accounted for by amino acid substitutions; per04 is altered at a splice site acceptor and causes aberrant splicing. perSLIH exhibits a long period of 27 hr in constant darkness and entrains to light/dark (L/D) cycles with a later-than-normal evening peak of locomotion. perSLIH males are more rhythmic than females. perSLIH's clock runs faster at higher temperatures and slower at lower ones, exhibiting a temperature-compensation defect opposite to that of perLong. The per-encoded protein (PER) in the perT mutant cycles in L/D with an earlier-than-normal peak; this peak in perSLIH is later than normal, and there was a slight difference in the PER timecourse of males vs. females. PER in per04 was undetectable. Two of these mutations, perSLIH and perClk, lie within regions of PER that have not been studied previously and may define important functional domains of this clock protein.

Molecular coevolution within a Drosophila clock gene

The period (per) gene in Drosophila melanogaster provides an integral component of biological rhythmicity and encodes a protein that includes a repetitive threonine-glycine (Thr-Gly) tract. Similar repeats are found in the frq and wc2 clock genes of Neurospora crassa and in the mammalian per homologues, but their circadian functions are unknown. In Drosophilids, the length of the Thr-Gly repeat varies widely between species, and sequence comparisons have suggested that the repeat length coevolves with the immediately flanking amino acids. A functional test of the coevolution hypothesis was performed by generating several hybrid per transgenes between Drosophila pseudoobscura and D. melanogaster, whose repetitive regions differ in length by about 150 amino acids. The positions of the chimeric junctions were slightly altered in each transgene. Transformants carrying per constructs in which the repeat of one species was juxtaposed next to the flanking region of the other were almost arrhythmic or showed a striking temperature sensitivity of the circadian period. In contrast, transgenes in which the repeat and flanking regions were conspecific gave wild-type levels of circadian rescue. These results support the coevolutionary interpretation of the interspecific sequence changes in this region of the PER molecule and reveal a functional dimension to this process related to the clock's temperature compensation.

Genetic approaches to neuroethology

In neuroethology, the nervous system and behavior are analyzed in the context of the animal's natural habitat and evolutionary history. For the last 30 years the influence of genetics on neuroethology has steadily grown, particularly in Drosophila. Genetic variants reveal new properties of neurons; they help to dissect neuronal circuits and complex behavioral systems; genetics provides new methods to visualize certain brain structures and to assign behavioural functions to them; and, finally, genetic variants can be used to test ecological models. While single-gene mutations can hve highly specific behavioral effects, molecular analysis of the corresponding genes reveals that the latter normally have a much broader functional scope. The 'graininess' of a functional model of the brain, therefore, is defined by the independent regulatory units of the genes rather than by the genes themselves.

Insect societies and the molecular biology of social behavior

This article outlines the rationale for a molecular genetic study of social behavior, and explains why social insects are good models. Summaries of research on brain and behavior in two species, honey bees and fire ants, are presented to illustrate the richness of the behavioral phenomena that can be addressed with social insects and to show how they are beginning to be used to study genes that influence social behavior. We conclude by considering the problems and potential of this emerging field.