Circadian rhythms of locomotor activity in Drosophila

The E3 ubiquitin ligase CTRIP controls CLOCK levels and PERIOD oscillations in Drosophila

In the Drosophila circadian clock, the CLOCK/CYCLE complex activates the period and timeless genes that negatively feedback on CLOCK/CYCLE activity. The 24-h pace of this cycle depends on the stability of the clock proteins. RING-domain E3 ubiquitin ligases have been shown to destabilize PERIOD or TIMELESS. Here we identify a clock function for the circadian trip (ctrip) gene, which encodes a HECT-domain E3 ubiquitin ligase. ctrip expression in the brain is mostly restricted to clock neurons and its downregulation leads to long-period activity rhythms in constant darkness. This altered behaviour is associated with high CLOCK levels and persistence of phosphorylated PERIOD during the subjective day. The control of CLOCK protein levels does not require PERIOD. Thus, CTRIP seems to regulate the pace of the oscillator by controlling the stability of both the activator and the repressor of the feedback loop.

A role for blind DN2 clock neurons in temperature entrainment of the Drosophila larval brain

Circadian clocks synchronize to the solar day by sensing the diurnal changes in light and temperature. In adult Drosophila, the brain clock that controls rest-activity rhythms relies on neurons showing Period oscillations. Nine of these neurons are present in each larval brain hemisphere. They can receive light inputs through Cryptochrome (CRY) and the visual system, but temperature input pathways are unknown. Here, we investigate how the larval clock network responds to light and temperature. We focused on the CRY-negative dorsal neurons (DN2s), in which light-dark (LD) cycles set molecular oscillations almost in antiphase to all other clock neurons. We first showed that the phasing of the DN2s in LD depends on the pigment-dispersing factor (PDF) neuropeptide in four lateral neurons (LNs), and on the PDF receptor in the DN2s. In the absence of PDF signaling, these cells appear blind, but still synchronize to temperature cycles. Period oscillations in the DN2s were stronger in thermocycles than in LD, but with a very similar phase. Conversely, the oscillations of LNs were weaker in thermocycles than in LD, and were phase-shifted in synchrony with the DN2s, whereas the phase of the three other clock neurons was advanced by a few hours. In the absence of any other functional clock neurons, the PDF-positive LNs were entrained by LD cycles but not by temperature cycles. Our results show that the larval clock neurons respond very differently to light and temperature, and strongly suggest that the CRY-negative DN2s play a prominent role in the temperature entrainment of the network.

Edge effects in the directionally biased distribution of Choristoneura rosaceana (Lepidoptera: Tortricidae) in apple orchards

Edge effect tests have been used in a number of studies on obliquebanded leafroller, Choristoneura rosaceana (Harris), to test for evidence of mated female immigration into pheromone-treated orchards. This type of test compares obliquebanded leafroller presence or activity around the perimeter of an orchard against presence or activity in the interior. Higher numbers detected around the edges of an orchard would indicate higher levels of flight activity at the edge, a pattern that could be generated by high levels of immigration. Recent work has shown that the spatial distribution of recaptured obliquebanded leafroller adults released from a single location can be directionally biased, which could obscure the ability to detect an edge effect. To test this theory, data from an orchard study conducted in 1991 that found no significant edge effect was reanalyzed. When we accounted for the directional bias in the distribution of first-generation mated female moths, we found an edge effect with significantly more mated females captured in the edge traps than in the center or mid-interior traps. No edge effect was found when the directional bias was ignored. In addition, second-generation males and mated females both showed a significant edge effect that had not been detected in the original analysis, which had combined both first- and second-generation data.

The clockwork orange Drosophila protein functions as both an activator and a repressor of clock gene expression

The Drosophila clock relies on transcriptional feedback loops that generate daily oscillations of the clock gene expression at mRNA and protein levels. In the evening, the CLOCK (CLK) and CYCLE (CYC) basic helix-loop-helix (bHLH) PAS-domain transcription factors activate the expression of the period (per) and timeless (tim) genes. Posttranslational modifications delay the accumulation of PER and TIM, which inhibit CLK/CYC activity in the late night. We show here that a null mutant of the clockwork orange (cwo) gene encoding a bHLH orange-domain putative transcription factor displays long-period activity rhythms. cwo loss of function increases cwo mRNA levels but reduces mRNA peak levels of the 4 described CLK/CYC targets, inducing an almost complete loss of their cycling. In addition, the absence of CWO induces alterations of PER and CLK phosphorylation cycles. Our results indicate that, in vivo, CWO modulates clock gene expression through both repressor and activator transcriptional functions.

The Drosophila circadian pacemaker circuit: Pas De Deux or Tarantella?

Molecular genetic analysis of the fruit fly Drosophila melanogaster has revolutionized our understanding of the transcription/translation loop mechanisms underlying the circadian molecular oscillator. More recently, Drosophila has been used to understand how different neuronal groups within the circadian pacemaker circuit interact to regulate the overall behavior of the fly in response to daily cyclic environmental cues as well as seasonal changes. Our present understanding of circadian timekeeping at the molecular and circuit level is discussed with a critical evaluation of the strengths and weaknesses of present models. Two models for circadian neural circuits are compared: one that posits that two anatomically distinct oscillators control the synchronization to the two major daily morning and evening transitions, versus a distributed network model that posits that many cell-autonomous oscillators are coordinated in a complex fashion and respond via plastic mechanisms to changes in environmental cues.

The nocturnal oviposition behaviour of blowflies (Diptera: Calliphoridae) in Central Europe and its forensic implications

Numerous factors may cause delayed colonisation of a corpse by blowflies, leading to a discrepancy between the entomologically determined post-mortem interval (PMI) and the time of death. Blowflies, for example, are considered to be inactive at night, however, published observations are contradictory. In the present study, several field experiments and one type of indoor experiment were conducted in summer of 2004 and 2005 in order to investigate the nocturnal ovipositional behaviour of blowflies. In the field, two types of bait, dead hedgehogs and fresh beef liver, were placed at night in different urban and rural locations in Frankfurt and in Munich, Germany. For the indoor-experiments beef liver was placed in small plastic boxes containing caged Lucilia sericata females in the evening and left overnight. At night, no ovipositon was observed in the field (n=51, T=10-24 degrees C). Nocturnal oviposition in complete darkness occurred in the plastic boxes in two of six cases (T=25 degrees C). Considering the behavioural and physiological characteristics of flies we suggest that nocturnal oviposition of blowflies appears to be unlikely under natural conditions in Central Europe but may occur under certain circumstances, such as unusual high nightly temperatures and the presence of gravid flies with an appropriate arousal threshold.

Light activates output from evening neurons and inhibits output from morning neurons in the Drosophila circadian clock

Animal circadian clocks are based on multiple oscillators whose interactions allow the daily control of complex behaviors. The Drosophila brain contains a circadian clock that controls rest–activity rhythms and relies upon different groups of PERIOD (PER)–expressing neurons. Two distinct oscillators have been functionally characterized under light-dark cycles. Lateral neurons (LNs) that express the pigment-dispersing factor (PDF) drive morning activity, whereas PDF-negative LNs are required for the evening activity. In constant darkness, several lines of evidence indicate that the LN morning oscillator (LN-MO) drives the activity rhythms, whereas the LN evening oscillator (LN-EO) does not. Since mutants devoid of functional CRYPTOCHROME (CRY), as opposed to wild-type flies, are rhythmic in constant light, we analyzed transgenic flies expressing PER or CRY in the LN-MO or LN-EO. We show that, under constant light conditions and reduced CRY function, the LN evening oscillator drives robust activity rhythms, whereas the LN morning oscillator does not. Remarkably, light acts by inhibiting the LN-MO behavioral output and activating the LN-EO behavioral output. Finally, we show that PDF signaling is not required for robust activity rhythms in constant light as opposed to its requirement in constant darkness, further supporting the minor contribution of the morning cells to the behavior in the presence of light. We therefore propose that day–night cycles alternatively activate behavioral outputs of the Drosophila evening and morning lateral neurons.

Living organisms have evolved circadian clocks that anticipate daily changes in their environment. Their clockwork is fully endogenous, but can be reset by external cues. (Light is the most efficient cue.) The circadian neuronal network of the fruit fly (Drosophila) brain perceives light through the visual system and a dedicated photoreceptor molecule, cryptochrome. Flies exhibit a bimodal locomotor activity pattern that peaks at dawn and dusk in light–dark conditions. These morning and evening activity bouts are controlled by two distinct neuronal clocks in the fly brain. By using flies with a deficient cryptochrome pathway, we have uncovered an unexpected role for light in the circadian system. In addition to synchronizing the two oscillators to solar time, light also controls their behavioral output. The morning oscillator can periodically rouse the fly when in constant darkness, but not in constant light, whereas the evening oscillator can do the same in constant light, but not in constant darkness. This suggests the existence of a light-dependent switch between oscillators that appears to require the visual system. Such a mechanism likely contributes to better separate the active periods of the fly at dawn and dusk, and may help the animal to adapt to seasonal changes in day length.

In fruit flies, light not only resets the circadian clock to solar time, but also enables the signaling from one oscillator while disabling the signaling from the other.

Analysis of locomotor activity rhythms in Drosophila

The genetic, molecular and anatomical dissection of the circadian clock in Drosophila and other higher organisms relies on the quantification of rhythmic phenotypes. Here, we introduce the methods currently in use in our laboratories for the analysis of fly locomotor activity rhythms. This phenotype provides a relatively simple, automated, efficient, reliable and robust output for the circadian clock. Thus it is not surprising that it is the preferred readout for measuring rhythmicity under a variety of conditions for most fly clock laboratories. The procedure requires at least 10 days of data collection and several days for analysis. In this protocol we advise on fly maintenance and on experimental design when studying the genetics of behavioral traits. We describe the setup for studying locomotor activity rhythms in the fruit fly and we introduce the statistical methods in use in our laboratories for the analysis of periodic data.

5-HT2 receptors in Drosophila are expressed in the brain and modulate aspects of circadian behaviors

Dysregulation of 5-HT(2) receptor function has been strongly implicated in many neuropsychiatric disorders, including schizophrenia. At present, the molecular mechanisms linking 5-HT(2) receptor activation to behaviors is not well understood. In efforts to elucidate these processes, the fruit fly, Drosophila melanogaster, is proposed to serve as a powerful genetically tractable model organism to study 5-HT(2) receptor function. Data are presented here on the expression of the fly ortholog of the mammalian 5-HT(2) receptor, 5-HT(2)Dro, in the larval and adult brain of the fly, and on the involvement of these circuits in certain circadian behaviors. In the adult brain, 5-HT(2)Dro is expressed in the protocerebrum and ellipsoid body, areas believed to participate in higher order behaviors including learning, locomotion, and sensory perception. In the third instar larva, 5-HT(2)Dro receptor expression is detected in a specific pattern that markedly changes from early to late third instar. To probe the function of this receptor we have examined the effects of the 5-HT(2) receptor-specific agonist DOI in wild type and 5-HT(2)Dro hypomorphic flies on circadian behaviors. DOI was found to increase early day activity, eliminate anticipatory behavior, and reduce viability. The effects of DOI were significantly diminished in a 5-HT(2)Dro hypomorphic strain. Identifying the 5-HT(2)Dro receptor circuitry and behaviors they mediate are significant steps towards developing this model system to study conserved molecular mechanisms underlying behaviors mediated by 5-HT(2) receptors in mammalian systems.

Influences of octopamine and juvenile hormone on locomotor behavior and period gene expression in the honeybee, Apis mellifera

Octopamine (OA) and juvenile hormone (JH) are implicated in the regulation of age-based division of labor in the honeybee, Apis mellifera. We tested the hypothesis that these two neuroendocrine signals influence task-associated plasticity in circadian and diurnal rhythms, and in brain expression of the clock gene period (per). Treatment with OA, OA antagonist (epinastine), or both, did not affect the age at onset of circadian rhythmicity or the free running period in constant darkness (DD). Young bees orally treated with OA in light-dark (LD) illumination regime for 6 days followed by DD showed reduced alpha (the period between the daily onset and offset of activity) during the first 4 days in LD and the first 4 days in DD. Oral treatment with OA, epinastine, or both, but not manipulations of JH levels, caused increased average daily levels and aberrant patterns of brain per mRNA oscillation in young bees. These results suggest that OA and JH do not influence the development or function of the central pacemaker but rather that OA influences the brain expression of a clock gene and characteristics of locomotor behavior that are not thought to be under direct control of the circadian pacemaker.

Cloning and daily expression of the timeless gene in Aedes aegypti (Diptera:Culicidae)

Molecular approaches for studying biological rhythms in insects have been well investigated in the model Drosophila melanogaster, in which a number of genes have been characterized in terms of sequence, expression, protein interactions and involvement in the control of locomotor activity and eclosion rhythms. However, only scattered information is available for insect vectors of diseases. In this paper, we report the cloning and expression analysis of the clock gene timeless in the mosquito Aedes aegypti, vector of Dengue and Yellow Fever viruses. In Drosophila, timeless has a crucial role in the control of the central pacemaker and the resetting mechanism that allows the clock to synchronize with the environment light-dark cycles. Comparison of the predicted protein sequence encoded by timeless in Ae. aegypti and D. melanogaster demonstrated high similarity in some important domains, suggesting functional conservation. Analysis of the daily expression of timeless in Ae. aegypti shows a peak in mRNA abundance around the light-dark transition.

Developmentally determined attenuation in circadian rhythms links chronobiology to social organization in bees

We investigated labor-related plasticity in the circadian clock of the bumblebee Bombus terrestris. Bumblebee workers vary enormously in body size, and we found that size, division of labor, and diurnal rhythms in activity are correlated in B. terrestris colonies. Large workers typically perform foraging activities with strong diurnal rhythms and low activity at night, whereas small bees typically care for (nurse) brood around the clock with weak or no diurnal rhythms. Under constant laboratory conditions, circadian rhythms in locomotor activity were weaker, less stable, and developed at a later age in small (nurse-size) bees compared to their larger (forager-size) sisters. Under a light:dark illumination regime, many small bees, particularly at a young age, were active during the dark phase, fewer small bees developed rhythms, and they did so later compared to large bees. Taken together these findings reveal naturally occurring attenuation or suppression in the circadian clock of small bees that is determined during pre-adult development. This deficiency in clock function, however, does not result in pathology but rather appears to be functionally significant, because it is associated with around-the-clock brood care activity and therefore apparently improves divisions of labor and colony efficiency. This in turn suggests that variation in social biology influences traits of the circadian clock.

Neural circuits underlying circadian behavior in Drosophila melanogaster

Circadian clocks include control systems for organizing daily behavior. Such a system consists of a time-keeping mechanism (the clock or pacemaker), input pathways for entraining the clock, and output pathways for producing overt rhythms in behavior and physiology. In Drosophila melanogaster, as in mammals, neural circuits play vital roles in all three functional subdivisions of the circadian system. Regarding the pacemaker, multiple clock neurons, each with cell-autonomous pacemaker capability, are coupled to each other in a network. The outputs of different sets of clock neurons in this network combine to produce the normal bimodal pattern of locomotor activity observed in Drosophila. Regarding input, multiple sensory modalities (including light, temperature, and pheromones) use their own circuitry to entrain the clock. Regarding output, distinct circuits are likely involved for controlling the timing of eclosion and for generating the locomotor activity rhythms. This review summarizes work on all of these circadian circuits, and discusses the broader utility of studying the fly's circadian system.

PER/TIM-mediated amplification, gene dosage effects and temperature compensation in an interlocking-feedback loop model of the Drosophila circadian clock

We have analysed a first-order kinetic representation of a interlocking-feedback loop model for the Drosophila circadian clock. In this model, the transcription factor Drosophila CLOCK (dCLK) which activates the clock genes period (per) and timeless (tim) is subjected to positive and negative regulations by the proteins 'PAR Domain Protein 1' (PDP1) and VRILLE (VRI), whose transcription is activated by dCLK. The PER/TIM complex binds to dCLK and in this way reduces the activity of dCLK. The results of our simulations suggest that the positive and negative feedback loops of Pdp1 and vri are essential for the overall oscillations. Although self sustained oscillations can be obtained without per/tim, the model shows that the PER/TIM complex plays an important role in amplification and stabilization of the oscillations generated by the Pdp1/vri positive/negative feedback loops. We further show that in contrast to a single (per/tim) negative feedback loop oscillator, the interlocking-feedback loop model can readily account for the effect of gene dosages of per, vri, and Pdp1 on the period length. Calculations of phase resetting on a temperature compensated version of the model shows good agreement with experimental phase response curves for high and low temperature pulses. Also, the partial losses of temperature compensation in perS and perL mutants can be described, which are related to decreased stabilities of the PER/TIM complex in perS and the stronger/more stable inhibitory complex between dCLK and PER/TIM in perL, respectively. The model shows (somewhat surprisingly) poor entrainment properties, especially under extended light/dark (L/D) cycles, which suggests that parts of the L/D tracking or sensing system are not well represented.

Systems approaches to biological rhythms in Drosophila

The chronobiological system of Drosophila is considered from the perspective of rhythm-regulated genes. These factors are enumerated and discussed not so much in terms of how the gene products are thought to act on behalf of circadian-clock mechanisms, but with special emphasis on where these molecules are manufactured within the organism. Therefore, with respect to several such cell and tissue types in the fly head, what is the "systems meaning" of a given structure's function insofar as regulation of rest-activity cycles is concerned? (Systematic oscillation of daily behavior is the principal overt phenotype analyzed in studies of Drosophila chronobiology). In turn, how do the several separate sets of clock-gene-expressing cells interact--or in some cases act in parallel--such that intricacies of the fly's sleep-wake cycles are mediated? Studying Drosophila chrono-genetics as a system-based endeavor also encompasses the fact that rhythm-related genes generate their products in many tissues beyond neural ones and during all stages of the life cycle. What, then, is the meaning of these widespread gene-expression patterns? This question is addressed with regard to circadian rhythms outside the behavioral arena, by considering other kinds of temporally based behaviors, and by contemplating how broadly systemic expression of rhythm-related genes connects with even more pleiotropic features of Drosophila biology. Thus, chronobiologically connected factors functioning within this insect comprise an increasingly salient example of gene versatility--multi-faceted usages of, and complex interactions among, entities that set up an organism's overall wherewithal to form and function. A corollary is that studying Drosophila development and adult-fly actions, even when limited to analysis of rhythm-systems phenomena, involves many of the animal's tissues and phenotypic capacities. It follows that such chronobiological experiments are technically demanding, including the necessity for investigators to possess wide-ranging expertise. Therefore, this chapter includes several different kinds of Methods set-asides. These techniques primers necessarily lack comprehensiveness, but they include certain discursive passages about why a given method can or should be applied and concerning real-world applicability of the pertinent rhythm-related technologies.

Morning and evening peaks of activity rely on different clock neurons of the Drosophila brain

In Drosophila, a 'clock' situated in the brain controls circadian rhythms of locomotor activity. This clock relies on several groups of neurons that express the Period (PER) protein, including the ventral lateral neurons (LN(v)s), which express the Pigment-dispersing factor (PDF) neuropeptide, and the PDF-negative dorsal lateral neurons (LN(d)s). In normal cycles of day and night, adult flies exhibit morning and evening peaks of activity; however, the contribution of the different clock neurons to the rest-activity pattern remains unknown. Here, we have used targeted expression of PER to restore the clock function of specific subsets of lateral neurons in arrhythmic per(0) mutant flies. We show that PER expression restricted to the LN(v)s only restores the morning activity, whereas expression of PER in both the LN(v)s and LN(d)s also restores the evening activity. This provides the first neuronal bases for 'morning' and 'evening' oscillators in the Drosophila brain. Furthermore, we show that the LN(v)s alone can generate 24 h activity rhythms in constant darkness, indicating that the morning oscillator is sufficient to drive the circadian system.

Novel features of cryptochrome-mediated photoreception in the brain circadian clock of Drosophila

In Drosophila, light affects circadian behavioral rhythms via at least two distinct mechanisms. One of them relies on the visual phototransduction cascade. The other involves a presumptive photopigment, cryptochrome (cry), expressed in lateral brain neurons that control behavioral rhythms. We show here that cry is expressed in most, if not all, larval and adult neuronal groups expressing the PERIOD (PER) protein, with the notable exception of larval dorsal neurons (DN2s) in which PER cycles in antiphase to all other known cells. Forcing cry expression in the larval DN2s gave them a normal phase of PER cycling, indicating that their unique antiphase rhythm is related to their lack of cry expression. We were able to directly monitor CRY protein in Drosophila brains in situ. It appeared highly unstable in the light, whereas in the dark, it accumulated in both the nucleus and the cytoplasm, including some neuritic projections. We also show that dorsal PER-expressing brain neurons, the adult DN1s, are the only brain neurons to coexpress the CRY protein and the photoreceptor differentiation factor GLASS. Studies of various visual system mutants and their combination with the cry(b) mutation indicated that the adult DN1s contribute significantly to the light sensitivity of the clock controlling activity rhythms, and that this contribution depends on CRY. Moreover, all CRY-independent light inputs into this central behavioral clock were found to require the visual system. Finally, we show that the photoreceptive DN1 neurons do not behave as autonomous oscillators, because their PER oscillations in constant darkness rapidly damp out in the absence of pigment-dispersing-factor signaling from the ventral lateral neurons.