Light and temperature cooperate to regulate the circadian locomotor rhythm of wild type and period mutants of Drosophila melanogaster

Developmental temperature affects thermal dependence of locomotor activity in Drosophila

In their natural environments, animals have to cope with fluctuations in numerous abiotic and biotic factors, and phenotypic plasticity can facilitate survival under such variable conditions. However, organisms may differ substantially in the ability to adjust their phenotypes in response to external factors. Here, we investigated how developmental temperature affects the thermal performance curve for locomotor activity in adult fruit flies (Drosophila melanogaster). We examined the thermal dependence of spontaneous activity in individuals originating from two natural populations (from tropical (India) and temperate climate zone (Slovakia)) that developed at three different temperatures (19 °C, 25 °C, and 29 °C). Firstly, we found that developmental temperature has a significant impact on overall activity - flies that developed at high temperature (29 °C) were, on average, less active than individuals that developed at lower temperatures. Secondly, developmental acclimation had a population-specific effect on the thermal optimum for activity. Whereas the optimal temperature was not affected by thermal conditions experienced during development in flies from India, developmental temperature shifted thermal optimum in flies from Slovakia. Thirdly, high developmental temperature broadened performance breadth in flies from the Indian population but narrowed it in individuals from the Slovak population. Finally, we did not detect a consistent effect of acclimation temperature on circadian rhythms of spontaneous activity. Altogether, our results demonstrate that developmental temperature can alter different parameters (maximum performance, thermal optimum, performance breadth) of the thermal performance curve for spontaneous activity. Since adult fruit flies are highly vagile, this sensitivity of locomotion to developmental conditions may be an important factor affecting fitness in changing environments.

The Role of Colony Temperature in the Entrainment of Circadian Rhythms of Honey Bee Foragers

Honey bees utilize their circadian rhythms to accurately predict the time of day. This ability allows foragers to remember the specific timing of food availability and its location for several days. Previous studies have provided strong evidence toward light/dark cycles being the primary Zeitgeber for honey bees. Work in our laboratory described large individual variation in the endogenous period length of honey bee foragers from the same colony and differences in the endogenous rhythms under different constant temperatures. In this study, we further this work by examining the temperature inside the honey bee colony. By placing temperature and light data loggers at different locations inside the colony we measured temperature at various locations within the colony. We observed significant oscillations of the temperature inside the hive, that show seasonal patterns. We then simulated the observed temperature oscillations in the laboratory and found that using the temperature cycle as a Zeitgeber, foragers present large individual differences in the phase of locomotor rhythms for temperature. Moreover, foragers successfully synchronize their locomotor rhythms to these simulated temperature cycles. Advancing the cycle by six hours, resulting in changes in the phase of activity in some foragers in the assay. The results are shown in this study highlight the importance of temperature as a potential Zeitgeber in the field. Future studies will examine the possible functional and evolutionary role of the observed phase differences of circadian rhythms.

Metabolic control of daily locomotor activity mediated by tachykinin in Drosophila

Metabolism influences locomotor behaviors, but the understanding of neural curcuit control for that is limited. Under standard light-dark cycles, Drosophila exhibits bimodal morning (M) and evening (E) locomotor activities that are controlled by clock neurons. Here, we showed that a high-nutrient diet progressively extended M activity but not E activity. Drosophila tachykinin (DTk) and Tachykinin-like receptor at 86C (TkR86C)-mediated signaling was required for the extension of M activity. DTk neurons were anatomically and functionally connected to the posterior dorsal neuron 1s (DN1_ps) in the clock neuronal network. The activation of DTk neurons reduced intracellular Ca²⁺ levels in DN1_ps suggesting an inhibitory connection. The contacts between DN1_ps and DTk neurons increased gradually over time in flies fed a high-sucrose diet, consistent with the locomotor behavior. DN1_ps have been implicated in integrating environmental sensory inputs (e.g., light and temperature) to control daily locomotor behavior. This study revealed that DN1_ps also coordinated nutrient information through DTk signaling to shape daily locomotor behavior.

Lee and colleagues report the effect of a high-sucrose diet on Drosophila locomotor activity via DTk-TkR86C neuropeptide signalling. This signalling pattern appears to involve a circadian element, with pacemaker neuron involvement having a possible time-of-day effect on locomotor behaviour.

Integration of Circadian Clock Information in the Drosophila Circadian Neuronal Network

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

The voltage-gated potassium channel Shaker promotes sleep via thermosensitive GABA transmission

Genes and neural circuits coordinately regulate animal sleep. However, it remains elusive how these endogenous factors shape sleep upon environmental changes. Here, we demonstrate that Shaker (Sh)-expressing GABAergic neurons projecting onto dorsal fan-shaped body (dFSB) regulate temperature-adaptive sleep behaviors in Drosophila. Loss of Sh function suppressed sleep at low temperature whereas light and high temperature cooperatively gated Sh effects on sleep. Sh depletion in GABAergic neurons partially phenocopied Sh mutants. Furthermore, the ionotropic GABA receptor, Resistant to dieldrin (Rdl), in dFSB neurons acted downstream of Sh and antagonized its sleep-promoting effects. In fact, Rdl inhibited the intracellular cAMP signaling of constitutively active dopaminergic synapses onto dFSB at low temperature. High temperature silenced GABAergic synapses onto dFSB, thereby potentiating the wake-promoting dopamine transmission. We propose that temperature-dependent switching between these two synaptic transmission modalities may adaptively tune the neural property of dFSB neurons to temperature shifts and reorganize sleep architecture for animal fitness.

Ji-hyung Kim and Yoonhee Ki et al. show that low temperatures suppress sleep in Drosophila by increasing GABA transmission in Shaker-expressing GABAergic neurons projecting onto the dorsal fan-shaped body, while high temperatures potentiate dopamine-induced arousal by reducing GABA transmission. This study highlights a role for Shaker in sleep modulation via a temperature-dependent switch in GABA signaling.

Temperature Entrainment of Circadian Locomotor and Transcriptional Rhythms in the Cricket, Gryllus bimaculatus

Most animals exhibit circadian rhythms in various physiological and behavioral functions regulated by circadian clock that resides in brain and in many peripheral tissues. Temperature cycle is an important time cue for entrainment, even in mammals, since the daily change in body temperature is thought to be used for phase regulation of clocks in peripheral tissues. However, little is known about the mechanisms by which temperature resets the clock. In the present study, we investigated the effect of temperature on circadian activity rhythm and clock gene transcription by using the cricket, Gryllus bimaculatus. We show that temperature cycle can entrain both behavioral and transcriptional rhythms of clock genes, such as period, timeless, cryptochrome2 and cycle in the circadian pacemaker tissue, optic lobe. Under temperature cycle, phase of evening peak of locomotor activity occurred 1 h before the warm-to-cold phase transition, which is associated with earlier peaks of mRNA expression rhythm of the clock genes than that under light/dark cycles. When the temperature cycle was advanced by 6 h, behavioral rhythms re-entrained to newly phased temperature cycle after ∼16 transient cycles. The mRNA oscillation of period and timeless gained stable rhythm under phase advanced temperature cycles with a lesser number of transient cycles than cryptochrome2 and cycle. These results suggest that temperature cycle can entrain behavioral and molecular rhythms in cricket and clock genes vary in sensitivity to temperature. It is thus likely that clock genes play differential roles in resetting the clock with environmental temperature changes.

Parallel clinal variation in the mid-day siesta of Drosophila melanogaster implicates continent-specific targets of natural selection

Similar to many diurnal animals, Drosophila melanogaster exhibits a mid-day siesta that is more robust as ambient temperature rises, an adaptive response aimed at minimizing exposure to heat. Mid-day siesta levels are partly regulated by the thermosensitive splicing of a small intron (termed dmpi8) found in the 3’ untranslated region (UTR) of the circadian clock gene period (per). Using the well-studied D. melanogaster latitudinal cline along the eastern coast of Australia, we show that flies from temperate populations sleep less during the day compared to those from tropical regions. We identified combinations of four single nucleotide polymorphisms (SNPs) in the 3’ UTR of per that yield several different haplotypes. The two most abundant of these haplotypes exhibit a reciprocal tropical-temperate distribution in relative frequency. Intriguingly, transgenic flies with the major tropical isoform manifest increased daytime sleep and reduced dmpi8 splicing compared to those carrying the temperate variant. Our results strongly suggest that for a major portion of D. melanogaster in Australia, thermal adaptation of daily sleep behavior included spatially varying selection on ancestrally derived polymorphisms in the per 3’ UTR that differentially control dmpi8 splicing efficiency. Prior work showed that African flies from high altitudes manifest reduced mid-day siesta levels, indicative of parallel latitudinal and altitudinal adaptation across continents. However, geographical variation in per 3’ UTR haplotypes was not observed for African flies, providing a compelling case for inter-continental variation in factors targeted by natural selection in attaining a parallel adaptation. We propose that the ability to calibrate mid-day siesta levels to better match local temperature ranges is a key adaptation contributing to the successful colonization of D. melanogaster beyond its ancestral range in the lowlands of Sub-Saharan Africa.

In warm climates many animals, including humans, exhibit a mid-day siesta, almost certainly a behavior meant to minimize the harm from prolonged exposure to the hot mid-day sun. But what about animals that adapted to cooler more temperate climates, might they have a less pronounced siesta? Indeed, we show that in the common fruit fly, Drosophila melanogaster, those from temperate regions in Australia exhibit less mid-day siesta compared to their tropical counterparts. Prior work showed that mid-day sleep levels are partially regulated by a ‘clock’ gene called period (per), which controls the timing of wake-sleep cycles in addition to other daily rhythms. We identified several DNA differences in the per gene that show geographical variation and contribute to the daytime sleep differences in flies from tropical and temperate regions via a mechanism that involves how well a temperature-sensitive intron in per is removed. A similar reduction in mid-day sleep was previously observed in African flies that adapted to the cooler temperatures found at high altitudes. Together, our findings provide a rare example where latitude and altitude lead to a similar behavioral adaptation to temperature. Moreover, the results suggest inter-continental differences in the evolutionary solutions used to attain the same thermal adaptation to cooler climates.

Lift and power in fruitflies in vertically-ascending flight

We measured the wing kinematics of fruitflies in both vertically-ascending and hovering flights and studied the aerodynamic forces and power in the two flight modes. The average ascending velocity is 0.45 m s^-1; the stroke plane angle and the stroke frequency are the same as that in hovering flight, whilst the stroke amplitude is increased by 12% and the wing angle of attack in the latter half of a down- and upstroke both increased by 10%. Flow analysis shows that during ascending, the flies experience a downward inflow which reduces the effective angle of attack considerably. This problem is overcome by the increases in the stroke amplitude and the angle of attack, which result in a larger wing drag. As a result, the power at ascending is increased by 36% over that at hovering. Two very interesting observations were made. (1) Using the same power, level-forward flight can be about four times as fast as ascending flight. (2) Power for ascending flight is the same as that for carrying a load about 27% of the insect's weight at hovering.

Two sides of a coin: ecological and chronobiological perspectives of timing in the wild

Most processes within organisms, and most interactions between organisms and their environment, have distinct time profiles. The temporal coordination of such processes is crucial across levels of biological organization, but disciplines differ widely in their approaches to study timing. Such differences are accentuated between ecologists, who are centrally concerned with a holistic view of an organism in relation to its external environment, and chronobiologists, who emphasize internal timekeeping within an organism and the mechanisms of its adjustment to the environment. We argue that ecological and chronobiological perspectives are complementary, and that studies at the intersection will enable both fields to jointly overcome obstacles that currently hinder progress. However, to achieve this integration, we first have to cross some conceptual barriers, clarifying prohibitively inaccessible terminologies. We critically assess main assumptions and concepts in either field, as well as their common interests. Both approaches intersect in their need to understand the extent and regulation of temporal plasticity, and in the concept of 'chronotype', i.e. the characteristic temporal properties of individuals which are the targets of natural and sexual selection. We then highlight promising developments, point out open questions, acknowledge difficulties and propose directions for further integration of ecological and chronobiological perspectives through Wild Clock research.This article is part of the themed issue 'Wild Clocks: integrating chronobiology and ecology to understand timekeeping in free-living animals'.

Mid-day siesta in natural populations of D. melanogaster from Africa exhibits an altitudinal cline and is regulated by splicing of a thermosensitive intron in the period clock gene

Background

Many diurnal animals exhibit a mid-day ‘siesta’, generally thought to be an adaptive response aimed at minimizing exposure to heat on warm days, suggesting that in regions with cooler climates mid-day siestas might be a less prominent feature of animal behavior. Drosophila melanogaster exhibits thermal plasticity in its mid-day siesta that is partly governed by the thermosensitive splicing of the 3’-terminal intron (termed dmpi8) from the key circadian clock gene period (per). For example, decreases in temperature lead to progressively more efficient splicing, which increasingly favors activity over sleep during the mid-day. In this study we sought to determine if the adaptation of D. melanogaster from its ancestral range in the lowlands of tropical Africa to the cooler temperatures found at high altitudes involved changes in mid-day sleep behavior and/or dmpi8 splicing efficiency.

Results

Using natural populations of Drosophila melanogaster from different altitudes in tropical Africa we show that flies from high elevations have a reduced mid-day siesta and less consolidated sleep. We identified a single nucleotide polymorphism (SNP) in the per 3’ untranslated region that has strong effects on dmpi8 splicing and mid-day sleep levels in both low and high altitude flies. Intriguingly, high altitude flies with a particular variant of this SNP exhibit increased dmpi8 splicing efficiency compared to their low altitude counterparts, consistent with reduced mid-day siesta. Thus, a boost in dmpi8 splicing efficiency appears to have played a prominent but not universal role in how African flies adapted to the cooler temperatures at high altitude.

Conclusions

Our findings point towards mid-day sleep behavior as a key evolutionary target in the thermal adaptation of animals, and provide a genetic framework for investigating daytime sleep in diurnal animals which appears to be driven by mechanisms distinct from those underlying nighttime sleep.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-017-0880-8) contains supplementary material, which is available to authorized users.

Cycles of circadian illuminance are sufficient to entrain and maintain circadian locomotor rhythms in Drosophila

Light at night disrupts the circadian clock and causes serious health problems in the modern world. Here, we show that newly developed four-package light-emitting diodes (LEDs) can provide harmless lighting at night. To quantify the effects of light on the circadian clock, we employed the concept of circadian illuminance (CIL). CIL represents the amount of light weighted toward the wavelengths to which the circadian clock is most sensitive, whereas visual illuminance (VIL) represents the total amount of visible light. Exposure to 12 h:12 h cycles of white LED light with high and low CIL values but a constant VIL value (conditions hereafter referred to as CH/CL) can entrain behavioral and molecular circadian rhythms in flies. Moreover, flies re-entrain to phase shift in the CH/CL cycle. Core-clock proteins are required for the rhythmic behaviors seen with this LED lighting scheme. Taken together, this study provides a guide for designing healthful white LED lights for use at night, and proposes the use of the CIL value for estimating the harmful effects of any light source on organismal health.

The effects of time-varying temperature on delays in genetic networks

Delays in gene networks result from the sequential nature of protein assembly. However, it is unclear how models of gene networks that use delays should be modified when considering time-dependent changes in temperature. This is important, as delay is often used in models of genetic oscillators that can be entrained by periodic fluctuations in temperature. Here, we analytically derive the time dependence of delay distributions in response to time-varying temperature changes. We find that the resulting time-varying delay is nonlinearly dependent on parameters of the time-varying temperature such as amplitude and frequency, therefore, applying an Arrhenius scaling may result in erroneous conclusions. We use these results to examine a model of a synthetic gene oscillator with temperature compensation. We show that temperature entrainment follows from the same mechanism that results in temperature compensation. Under a common Arrhenius scaling alone, the frequency of the oscillator is sensitive to changes in the mean temperature but robust to changes in the frequency of a periodically time-varying temperature. When a mechanism for temperature compensation is included in the model, however, we show that the oscillator is entrained by periodically varying temperature even when maintaining insensitivity to the mean temperature.

Induction of Drosophila Behavioral and Molecular Circadian Rhythms by Temperature Steps in Constant Light

In constant light, where Drosophila rhythms are normally disrupted, temperature cycles induce circadian rhythms at both the molecular and behavioral level. The authors investigated the process by which the thermoperiod induces the rhythms using temperature steps. A 10 °C temperature step-up induced a single locomotor activity peak ca 9 h after the temperature transition, whereas a 10 °C step-down induced a strong activity peak ca 24 h after the transition, and the peak recurred for several cycles, suggesting that the underlying clock is reset. Arrhythmic per01, tim 01, dClkJrk, and cyc01 mutant flies failed to show the rhythm after the step-down, suggesting that per, tim, dClk, and cyc are necessary for the step-down—induced rhythm. After the step-up, per01 flies exhibited an activity peak similar to that of wild-type flies, suggesting that the peak can be induced by the step-up in absence of PER. mRNA levels of per, tim , dClk, vri, and Pdp1ε were changed in response to the temperature steps, but the changes differed depending on the direction of temperature steps, suggesting that steps-up and steps-down have different roles in the initiation of the oscillation. Probably, alternating 12-h temperature steps-up and steps-down will induce opposite changes in mRNA levels of clock genes, eventually producing stable molecular oscillations. Although TIM shows responses to temperature consistent with the changes of its mRNA, this is not the case for PER, consistent with posttranscriptional regulation. Changes of the mRNA levels were significantly altered but still observed in per 01 flies but not observed in dClkJrk flies, except for per mRNA. This suggests that dCLK is involved in the temperature-induced changes in the levels of most clock gene mRNA but that per is regulated via a different mechanism.

The Fruit Fly Drosophila melanogaster Favors Dim Light and Times Its Activity Peaks to Early Dawn and Late Dusk

The light preferences of fruit flies were tested by 2 different means. First, flies were allowed to choose between different illuminations, and their favorite resting, grooming, and feeding places were determined with an infrared-sensitive camera. Second, the activity levels of the animals during their main daily activity period were determined photoelectrically (via infrared light beams) under different light intensities. Both methods revealed that the flies prefer dim light. They rested, groomed, and fed preferentially in places with a light intensity between 5 and 10 lux, and they showed the highest activity level when the light intensity during the day was kept at 10 lux. Furthermore, when dawn and dusk were simulated by logarithmically increasing/decreasing the light intensity during a 1.5-h interval, the flies' activity maxima occurred at about 7.5 lux during early dawn and late dusk. The results suggest that fruit flies time their clocks by early dawn and late dusk and avoid bright light during the day.

Temperature cycle amplitude alters the adult eclosion time and expression pattern of the circadian clock gene period in the onion fly

Soil temperature cycles are considered to play an important role in the entrainment of circadian clocks of underground insects. However, because of the low conductivity of soil, temperature cycles are gradually dampened and the phase of the temperature cycle is delayed with increasing soil depth. The onion fly, Delia antiqua, pupates at various soil depths, and its eclosion is timed by a circadian clock. This fly is able to compensate for the depth-dependent phase delay of temperature change by advancing the eclosion time with decreasing amplitude of the temperature cycle. Therefore, pupae can eclose at the appropriate time irrespective of their location at any depth. However, the mechanism that regulates eclosion time in response to temperature amplitude is still unknown. To understand whether this mechanism involves the circadian clock or further downstream physiological processes, we examined the expression patterns of period (per), a circadian clock gene, of D. antiqua under temperature cycles that were square wave cycles of 12-h warm phase (W) and 12-h cool phase (C) with the temperature difference of 8 °C (WC 29:21 °C) and 1 °C (WC 25.5:24.5 °C). The phase of oscillation in per expression was found to commence 3.5h earlier under WC 25.5:24.5 °C as compared to WC 29:21 °C. This difference was in close agreement with the eclosion time difference between the two temperature cycles, suggesting that the mechanism that responds to the temperature amplitude involves the circadian clock.

Ultradian rhythm unmasked in the Pdf clock mutant of Drosophila

A diverse range of organisms shows physiological and behavioural rhythms with various periods. Extensive studies have been performed to elucidate the molecular mechanisms of circadian rhythms with an approximately 24 h period in both Drosophila and mammals, while less attention has been paid to ultradian rhythms with shorter periods. We used a video-tracking method to monitor the movement of single flies, and clear ultradian rhythms were detected in the locomotor behaviour of wild type and clock mutant flies kept under constant dark conditions. In particular, the Pigment-dispersing factor mutant (Pdf 01) demonstrated a precise and robust ultradian rhythmicity, which was not temperature compensated. Our results suggest that Drosophila has an endogenous ultradian oscillator that is masked by circadian rhythmic behaviours.

Warming Up Your Tick-Tock: Temperature-Dependent Regulation of Circadian Clocks

Circadian clocks are endogenous time-keeping mechanisms to adaptively coordinate animal behaviors and physiology with daily environmental changes. So far many circadian studies in model organisms have identified evolutionarily conserved molecular frames of circadian clock genes in the context of transcription-translation feedback loops. The molecular clockwork drives cell-autonomously cycling gene expression with ~24-hour periodicity, which is fundamental to circadian rhythms. Light and temperature are two of the most potent external time cues to reset the circadian phase of the internal clocks, yet relatively little is known about temperature-relevant clock regulation. In this review, we describe recent findings on temperature-dependent clock mechanisms in homeothermic mammals as compared with poikilothermic Drosophila at molecular, neural, and organismal levels. We propose thermodynamic transitions in RNA secondary structures might have been potent substrates for the molecular evolution of temperature-relevant post-transcriptional mechanisms. Future works should thus validate the potential involvement of specific post-transcriptional steps in temperature-dependent plasticity of circadian clocks.

Thermotaxis, circadian rhythms, and TRP channels in Drosophila

The fruit fly Drosophila melanogaster is a poikilothermic organism that must detect and respond to both fine and coarse changes in environmental temperature in order maintain optimal body temperature, synchronize behavior to daily temperature fluctuations, and to avoid potentially injurious environmental hazards. Members of the Transient Receptor Potential (TRP) family of cation channels are well known for their activation by changes in temperature and their essential roles in sensory transduction in both invertebrates and vertebrates. The Drosophila genome encodes 13 TRP channels, and several of these have key sensory transduction and modulatory functions in allowing larval and adult flies to make fine temperature discriminations to attain optimal body temperature, detect and avoid large environmental temperature fluctuations, and make rapid escape responses to acutely noxious stimuli. Drosophila use multiple, redundant signaling pathways and neural circuits to execute these behaviors in response to both increases and decreases in temperature of varying magnitudes and time scales. A plethora of powerful molecular and genetic tools and the fly's simple, well-characterized nervous system have given Drosophila neurobiologists a powerful platform to study the cellular and molecular mechanisms of TRP channel function and how these mechanisms are conserved in vertebrates, as well as how these channels function within sensorimotor circuits to generate both simple and complex thermosensory behaviors.

A novel pathway for sensory-mediated arousal involves splicing of an intron in the period clock gene

STUDY OBJECTIVES

D. melanogaster is an excellent animal model to study how the circadian (≅24-h) timing system and sleep regulate daily wake-sleep cycles. Splicing of a temperature-sensitive 3'-terminal intron (termed dmpi8) from the circadian clock gene period (per) regulates the distribution of daily activity in Drosophila. The role of dmpi8 splicing on daily behavior was further evaluated by analyzing sleep.

DESIGN

Transgenic flies of the same genetic background but expressing either a wild-type recombinant per gene or one where the efficiency of dmpi8 splicing was increased were exposed to different temperatures in daily light-dark cycles and sleep parameters measured. In addition, transgenic flies were briefly exposed to a variety of sensory-mediated stimuli to measure arousal responses.

RESULTS

Surprisingly, we show that the effect of dmpi8 splicing on daytime activity levels does not involve a circadian role for per but is linked to adjustments in sensory-dependent arousal and sleep behavior. Genetically altered flies with high dmpi8 splicing efficiency remain aroused longer following short treatments with light and non-photic cues such as mechanical stimulation.

CONCLUSIONS

We propose that the thermal regulation of dmpi8 splicing acts as a temperature-calibrated rheostat in a novel arousal mechanism, so that on warm days the inefficient splicing of the dmpi8 intron triggers an increase in quiescence by decreasing sensory-mediated arousal, thus ensuring flies minimize being active during the hot midday sun despite the presence of light in the environment, which is usually a strong arousal cue for diurnal animals.

Phosphorylation of a central clock transcription factor is required for thermal but not photic entrainment

Transcriptional/translational feedback loops drive daily cycles of expression in clock genes and clock-controlled genes, which ultimately underlie many of the overt circadian rhythms manifested by organisms. Moreover, phosphorylation of clock proteins plays crucial roles in the temporal regulation of clock protein activity, stability and subcellular localization. dCLOCK (dCLK), the master transcription factor driving cyclical gene expression and the rate-limiting component in the Drosophila circadian clock, undergoes daily changes in phosphorylation. However, the physiological role of dCLK phosphorylation is not clear. Using a Drosophila tissue culture system, we identified multiple phosphorylation sites on dCLK. Expression of a mutated version of dCLK where all the mapped phospho-sites were switched to alanine (dCLK-15A) rescues the arrythmicity of Clk^out flies, yet with an approximately 1.5 hr shorter period. The dCLK-15A protein attains substantially higher levels in flies compared to the control situation, and also appears to have enhanced transcriptional activity, consistent with the observed higher peak values and amplitudes in the mRNA rhythms of several core clock genes. Surprisingly, the clock-controlled daily activity rhythm in dCLK-15A expressing flies does not synchronize properly to daily temperature cycles, although there is no defect in aligning to light/dark cycles. Our findings suggest a novel role for clock protein phosphorylation in governing the relative strengths of entraining modalities by adjusting the dynamics of circadian gene expression.

Circadian clocks are synchronized to local time by daily cycles in light-dark and temperature. Although light is generally thought to be the most dominant entraining cue in nature, daily cycles in temperature are sufficient to synchronize clocks in a large range of organisms. In Drosophila, dCLOCK is a master circadian transcription factor that drives cyclical gene expression and is likely the rate-limiting component in the transcriptional/translational feedback loops that underlie the timekeeping mechanism. dCLOCK undergoes temporal changes in phosphorylation throughout a day, which is also observed for mammalian CLOCK. However, the role of CLOCK phosphorylation at the organismal level is still unclear. Using mass-spectrometry, we identified more than a dozen phosphorylation sites on dCLOCK. Blocking global phosphorylation of dCLOCK by mutating phospho-acceptor sites to alanine increases its abundance and transcriptional activity, leading to higher peak values and amplitudes in the mRNA rhythms of core clock genes, which likely explains the accelerated clock speed. Surprisingly, the clock-controlled daily activity rhythm fails to maintain synchrony with daily temperature cycles, although there is no observable defect in aligning to light/dark cycles. Our findings suggest a novel role for clock protein phosphorylation in governing the effective strengths of entraining modalities by adjusting clock amplitude.

Simulating natural light and temperature cycles in the laboratory reveals differential effects on activity/rest rhythm of four Drosophilids

Recent studies under semi-natural conditions have revealed various unique features of activity/rest rhythms in Drosophilids that differ from those under standard laboratory conditions. An additional afternoon peak (A-peak) has been reported for Drosophila melanogaster and another species D. malerkotliana while D. ananassae exhibited mostly unimodal diurnal activity. To tease apart the role of light and temperature in mediating these species-specific behaviours of four Drosophilid species D. melanogaster, D. malerkotliana, D. ananassae, and Zaprionus indianus we simulated gradual natural light and/or temperature cycles conditions in laboratory. The pattern observed under semi-natural conditions could be reproduced in the laboratory for all the species under a variety of simulated conditions. D. melanogaster and D. malerkotliana showed similar patterns where as D. ananassae consistently exhibited predominant morning activity under almost all regimes. Z. indianus showed clearly rhythmic activity mostly when temperature cycles were provided. We find that gradually changing light intensities reaching a sufficiently high peak value can elicit A-peak in D. melanogaster, D. malerkotliana, and D. ananassae even at mild ambient temperature. Furthermore, we show that high mid-day temperature could induce A-peak in all species even under constant light conditions suggesting that this A-peak is likely to be a stress response.

Temperature sensitivity of circadian clocks is conserved across Drosophila species melanogaster, malerkotliana and ananassae

Light and temperature are the major environmental cycles that can synchronize circadian rhythms in a variety of organisms. Previously, we have shown that under light/dark cycles of various photoperiods, the Drosophila species ananassae exhibits unimodal activity pattern with a prominent morning activity peak in contrast with Drosophila melanogaster and Drosophila malerkotliana, which show bimodal activity pattern with morning and evening activity peaks. Here we report that circadian clocks controlling activity/rest rhythm of these two less-studied species D. malerkotliana and D. ananassae can be synchronized by temperature cycles and that even under temperature cycles D. ananassae exhibits only a pronounced morning (thermophase onset) activity peak. Although D. melanogaster and D. ananassae exhibit differences in the phase of activity/rest rhythm under temperature cycles, circadian clocks of both show similar sensitivity to warm temperature pulses. Circadian period of activity/rest rhythm of D. ananassae differs from the other two species at some moderate-range temperatures; however, in conditions that are more extreme, circadian clocks of D. melanogaster, D. malerkotliana and D. ananassae appear to be largely temperature compensated.

Breakdown of selection-mediated correlation between development time and clock period

Previously we have reported that selection for faster pre-adult development in fruit flies speeded-up development by ~29-h and shortened the clock period (τ) by ~0.5h, which suggests that development time and τ are correlated. Since it is known that τ is altered following exposure to light/dark (LD) cycles, we asked whether this correlation persists in the faster developing (FD) and control (BD) flies by examining the τ of the activity/rest rhythm and its difference between the two stocks following exposure to a variety of cyclic conditions. We assayed the activity/rest behavior of FD and BD flies under DD, following a week-long exposure to (a) LD cycles of 10:10h, 12:12h and 14:14h, or (b) LD12:12h with different light intensities (10, 100 and 1000lx), or (c) 12:12h warm/cold (WC) cycles of 25:18°C (WC1) and 29:25°C (WC2), or (d) WC1 or WC2, in-phase or out-of-phase with LD. The results revealed that both LD and WC altered the τ of FD and BD flies, and considerably reduced the selection-mediated difference between the two stocks. LD10:10 caused more severe after-effects on τ compared to LD12:12 and LD14:14. Among the WC cycles, WC1 which had a higher contrast caused period shortening. Irrespective of the phase relationship, imposition of LD cycles on WC cycles made no difference to the extent of after-effects; however, interestingly there was a reversal in the trend, in that, now WC2 with LD caused most drastic reduction in τ. These results suggest that cyclic environments modulate the circadian organization of Drosophila melanogaster altering the selection-mediated correlation between pre-adult development time and clock period.

The circadian system: plasticity at many levels

Over the years it has become crystal clear that a variety of processes encode time-of-day information, ranging from gene expression, protein stability, or subcellular localization of key proteins, to the fine tuning of network properties and modulation of input signals, ultimately ensuring that physiology and behavior are properly synchronized to a changing environment. The purpose of this review is to put forward examples (as opposed to generate a comprehensive revision of all the available literature) in which the circadian system displays a remarkable degree of plasticity, from cell autonomous to circuit-based levels. In the literature, the term circadian plasticity has been used to refer to different concepts. The obvious one, more literally, refers to any change that follows a circadian (circa=around, diem=day) pattern, i.e. a daily change of a given parameter. The discovery of daily remodeling of neuronal structures will be referred herein as structural circadian plasticity, and represents an additional and novel phenomenon modified daily. Finally, any plasticity that has to do with a circadian parameter would represent a type of circadian plasticity; as an example, adjustments that allow organisms to adapt their daily behavior to the annual changes in photoperiod is a form of circadian plasticity at a higher organizational level, which is an emergent property of the whole circadian system. Throughout this work we will revisit these types of changes by reviewing recent literature delving around circadian control of clock outputs, from the most immediate ones within pacemaker neurons to the circadian modulation of rest-activity cycles.

Significance of activity peaks in fruit flies, Drosophila melanogaster, under seminatural conditions

Studies on circadian entrainment have traditionally been performed under controlled laboratory conditions. Although these studies have served the purpose of providing a broad framework for our understanding of regulation of rhythmic behaviors under cyclic conditions, they do not reveal how organisms keep time in nature. Although a few recent studies have attempted to address this, it is not yet clear which environmental factors regulate rhythmic behaviors in nature and how. Here, we report the results of our studies aimed at examining (i) whether and how changes in natural light affect activity/rest rhythm and (ii) what the functional significance of this rhythmic behavior might be. We found that wild-type strains of fruit flies, Drosophila melanogaster, display morning (M), afternoon (A), and evening (E) peaks of activity under seminatural conditions (SN), whereas under constant darkness in otherwise SN, they exhibited M and E peaks, and under constant light in SN, only the E peak occurred. Unlike the A peak, which requires exposure to bright light in the afternoon, light information is dispensable for the M and E peaks. Visual examination of behaviors suggests that the M peak is associated with courtship-related locomotor activity and the A peak is due to an artifact of the experimental protocol and largely circadian clock independent.

Cryptochrome antagonizes synchronization of Drosophila's circadian clock to temperature cycles

BACKGROUND

In nature, both daily light:dark cycles and temperature fluctuations are used by organisms to synchronize their endogenous time with the daily cycles of light and temperature. Proper synchronization is important for the overall fitness and wellbeing of animals and humans, and although we know a lot about light synchronization, this is not the case for temperature inputs to the circadian clock. In Drosophila, light and temperature cues can act as synchronization signals (Zeitgeber), but it is not known how they are integrated.

RESULTS

We investigated whether different groups of the Drosophila clock neurons that regulate behavioral rhythmicity contribute to temperature synchronization at different absolute temperatures. Using spatially restricted expression of the clock gene period, we show that dorsally located clock neurons mainly mediate synchronization to higher (20°C:29°C) and ventral clock neurons to lower (16°C:25°C) temperature cycles. Molecularly, the blue-light photoreceptor CRYPTOCHROME (CRY) dampens temperature-induced PERIOD (PER)-LUCIFERASE oscillations in dorsal clock neurons. Consistent with this finding, we show that in the absence of CRY very limited expression of PER in a few dorsal clock neurons is able to mediate behavioral temperature synchronization to high and low temperature cycles independent of light.

CONCLUSIONS

We show that different subsets of clock neurons operate at high and low temperatures to mediate clock synchronization to temperature cycles, suggesting that temperature entrainment is not restricted to measuring the amplitude of such cycles. CRY dampens temperature input to the clock and thereby contributes to the integration of different Zeitgebers.

Natural variation in the Drosophila melanogaster clock gene period modulates splicing of its 3'-terminal intron and mid-day siesta

Drosophila melanogaster exhibits circadian (≅24 hr) regulated morning and evening bouts of activity that are separated by a mid-day siesta. Increases in daily ambient temperature are accompanied by a progressively longer mid-day siesta and delayed evening activity. Presumably, this behavioral plasticity reflects an adaptive response that endows D. melanogaster with the ability to temporally optimize daily activity levels over a wide range of physiologically relevant temperatures. For example, the shift in activity towards the cooler nighttime hours on hot days might minimize the risks associated with exposure to mid-day heat, whereas on cold days activity is favored during the warmer daytime hours. These temperature-induced shifts in the distribution of daily activity are partly based on the thermal sensitive splicing of an intron found in the 3' untranslated region (UTR) of the circadian clock gene termed period (per). As temperature decreases, splicing of this 3'-terminal intron (termed dmpi8) is gradually increased, which is causally linked to a shorter mid-day siesta. Herein we identify several natural polymorphisms in the per 3' UTR from wild-caught populations of flies originating along the east coast of the United States. Two non-intronic closely spaced single nucleotide polymorphisms (SNPs) modulate dmpi8 splicing efficiency, with the least efficiently spliced version associated with a longer mid-day siesta, especially at lower temperatures. Although these SNPs modulate the splicing efficiency of dmpi8 they have little to no effect on its thermal responsiveness, consistent with the notion that the suboptimal 5' and 3' splice sites of the dmpi8 intron are the primary cis-acting elements mediating temperature regulation. Our results demonstrate that natural variations in the per gene can modulate the splicing efficiency of the dmpi8 intron and the daily distribution of activity, providing natural examples for the involvement of dmpi8 splicing in the thermal adaptation of behavioral programs in D. melanogaster.

Robustness of circadian timing systems evolves in the fruit fly Drosophila melanogaster as a correlated response to selection for adult emergence in a narrow window of time

Robustness is a fundamental property of biological timing systems that is likely to ensure their efficient functioning under a wide range of environmental conditions. Here we report the findings of our study aimed at examining robustness of circadian clocks in fruit fly Drosophila melanogaster populations selected to emerge as adults within a narrow window of time. Previously, we have reported that such flies display enhanced synchrony, accuracy, and precision in their adult emergence and activity/rest rhythms. Since it is expected that accurate and precise circadian clocks may confer enhanced stability in circadian time-keeping, we decided to examine robustness in circadian rhythms of flies from the selected populations by subjecting them to a variety of environmental conditions comprising of a range of photoperiods, light intensities, ambient temperatures, and constant darkness. The results revealed that adult emergence and activity/rest rhythms of flies from the selected stocks were more robust than controls, as they displayed enhanced stability under a wide variety of environmental conditions. These results suggest that selection for adult emergence within a narrow window of time results in the evolution of robustness in circadian timing systems of the fruit fly D. melanogaster.

Paradoxical masking effects of bright photophase and high temperature in Drosophila malerkotliana

Synergic contribution of light and temperature is known to cause a paradoxical masking effect (inhibition of activity by bright light and high temperature) on various rhythms of animals. The present study reports the paradoxical masking effects of 1000-lux photophase at 25°C on the locomotor activity rhythm of Drosophila malerkotliana. Flies were subjected to light (L)-dark (D) 12:12 cycles wherein the photophase was varied from 10 to 1000 lux, whereas the scotophase was set to 0 lux in these and subsequent LD cycles. At 10, 100, and 500 lux, the flies were diurnal; however, at 1000 lux they were nocturnal. Transfer from LD 12:12 cycles to continuous darkness (DD) initiated free-running rhythmicity in all flies. Free-running rhythms of the flies switched from the 10-lux to the 500-lux groups started from the last activity-onset phase of the rhythm following 3-5 transient cycles, suggesting involvement of the circadian pacemaker. In contrast, the free-running rhythm of the flies of the 1000-lux group began abruptly from the last lights-on phase of the LD cycle, indicating noninvolvement of the pacemaker. Furthermore, all flies showed nocturnal activity in the two types of LD 12:12 cycles when the photophase was 1000 lux. The first type of LD cycles had three succeeding photophases of 100, 1000, and again 100 lux, whereas the second type of LD cycles had only one photophase of 1000 lux, but the LD 12:12 cycles were reversed to DL 12:12 cycles. Apparently, the combined effects of light and temperature caused such paradoxical masking effects. This hypothesis was tested by repeating the above experiments at 20°C. Flies in all experiments exhibited a diurnal activity pattern, even when the photophase was 1000 lux. Thus, the present study demonstrates that the paradoxical masking effect in D. malerkotliana was caused by the additive influence of light intensity and temperature. This strategy appears to have physiological significance, i.e., to shun and thus protect against the bright photophase at high temperature in the field.

The Ability to Entrain to Long Photoperiods Differs between 3 Drosophila melanogaster Wild-Type Strains and Is Modified by Twilight Simulation

The ability to adapt to different environmental conditions including seasonal changes is a key feature of the circadian clock. Here, we compared the ability of 3 Drosophila melanogaster wild-type strains to adapt rhythmic activity to long photoperiods simulated in the laboratory. Fruit flies are predominantly crepuscular with activity bouts in the morning (M) and evening (E). The M peak follows dawn and the E peak follows dusk when the photoperiod is extended. We show that this ability is restricted to a certain extension of the phase angle between M and E peaks, such that the E peak does not delay beyond a certain phase under long days. We demonstrate that this ability is significantly improved by simulated twilight and that it depends additionally on the genetic background and the ambient temperature. At 20 °C, the laboratory strain CantonS had the most flexible phase angle between M and E peaks, a Northern wild-type strain had an intermediate one, and a Southern wild-type strain had the lowest flexibility. Furthermore, we found that the 3 strains differed in clock light sensitivity, with the CantonS and the Northern strains more light sensitive than the Southern strain. These results are generally in accord with the recently discovered polymorphisms in the timeless gene ( tim) that affect clock light sensitivity.

A portable system for monitoring the behavioral activity of Drosophila

We describe a low-cost system for monitoring the behavioral activity of the fruit fly, Drosophila melanogaster. The system is readily adaptable to one or more cameras for simultaneous recordings of behavior from different angles and can be used for monitoring multiple individuals in a population at the same time. Signal processing allows discriminating between active and inactive periods during locomotion or flying, and quantification of subtler movements related to changes in position of the wings or legs. The recordings can be taken continuously over long periods of time and can thus provide information about the dynamics of a population. The system was used to monitor responses to caffeine, changes in temperature and g-force, and activity in a variable size population.

Genome-wide analysis of light- and temperature-entrained circadian transcripts in Caenorhabditis elegans

Transcriptional profiling experiments identify light- and temperature-entrained circadian transcripts in C. elegans.

Most organisms have an endogenous circadian clock that is synchronized to environmental signals such as light and temperature. Although circadian rhythms have been described in the nematode Caenorhabditis elegans at the behavioral level, these rhythms appear to be relatively non-robust. Moreover, in contrast to other animal models, no circadian transcriptional rhythms have been identified. Thus, whether this organism contains a bona fide circadian clock remains an open question. Here we use genome-wide expression profiling experiments to identify light- and temperature-entrained oscillating transcripts in C. elegans. These transcripts exhibit rhythmic expression with temperature-compensated 24-h periods. In addition, their expression is sustained under constant conditions, suggesting that they are under circadian regulation. Light and temperature cycles strongly drive gene expression and appear to entrain largely nonoverlapping gene sets. We show that mutations in a cyclic nucleotide-gated channel required for sensory transduction abolish both light- and temperature-entrained gene expression, implying that environmental cues act cell nonautonomously to entrain circadian rhythms. Together, these findings demonstrate circadian-regulated transcriptional rhythms in C. elegans and suggest that further analyses in this organism will provide new information about the evolution and function of this biological clock.

Daily (circadian) rhythms in behavior and physiology allow organisms to adapt to periodic cues such as light and temperature associated with the rotation of the earth. Subsets of molecular components of the internal clock that drive these rhythms, as well as effector genes for behavioral outputs, also exhibit rhythmic expression in many organisms. While circadian rhythms in behavior have previously been described in the nematode Caenorhabditis elegans, no transcriptional rhythms or clock genes have been identified, leaving open the question of the nature of the clock in this organism. Here, we identify light- and temperature-entrained cycling genes in C. elegans via genome-wide transcriptional profiling. Transcripts showing circadian regulation (including expression with a 24-h period maintained upon removal of the entraining stimulus) and temperature compensation were identified. Light and temperature appear to entrain independent sets of genes. We also identify large sets of light- or temperature-driven genes. Mutations in a channel gene previously implicated in sensory transduction in a small set of sensory neurons abolish entrainment of gene expression by environmental signals. This work demonstrates the presence of circadian transcriptional rhythms in C. elegans, and provides the foundation for future investigations into the underlying mechanisms.

Synergic Entrainment of Drosophila’s Circadian Clock by Light and Temperature

Daily light and temperature cycles are considered the most important zeitgebers for circadian clocks in many organisms. The influence of each single zeitgeber on the clock has been well studied, but little is known about any synergistic effects of both zeitgebers on the clock. In nature, light and temperature show characteristic daily oscillations with the temperature rising during the light phase and reaching its maximum in the late afternoon. Here, we studied behavioral and molecular rhythms in Drosophila melanogaster under simulated natural low light-dark (LD) and temperature (T) cycles that typically occur during the September equinox. Wild-type flies were either subjected to simulated LD or T cycles alone or to a combination of both. Behavioral rhythms and molecular rhythms in the different clock neurons were assessed under the 3 different conditions. Although behavioral rhythms entrained to all conditions, the rhythms were most robust under the combination of LD and T cycles. The clock neurons responded differently to LD and T cycles. Some were not entrained by T cycles alone; others were only slightly entrained by LD cycles alone. The amplitude of the molecular cycling was not different between LD alone and T cycles alone; but LD alone could set the pacemaker neurons to similar phases, whereas T cycles alone could not. The combination of the 2 zeitgebers entrained all clock neurons not only with similar phase but also enhanced the amplitude of Timeless cycling in the majority of cells. Our results show that the 2 zeitgebers synergistically entrain behavioral and molecular rhythms of Drosophila melanogaster.

Temperature entrainment of Drosophila's circadian clock involves the gene nocte and signaling from peripheral sensory tissues to the brain

Circadian clocks are synchronized by the natural day/night and temperature cycles. Our previous work demonstrated that synchronization by temperature is a tissue autonomous process, similar to synchronization by light. We show here that this is indeed the case, with the important exception of the brain. Using luciferase imaging we demonstrate that brain clock neurons depend on signals from peripheral tissues in order to be synchronized by temperature. Reducing the function of the gene nocte in chordotonal organs changes their structure and function and dramatically interferes with temperature synchronization of behavioral activity. Other mutants known to affect the function of these sensory organs also interfere with temperature synchronization, demonstrating the importance of nocte in this process and identifying the chordotonal organs as relevant sensory structures. Our work reveals surprising and important mechanistic differences between light- and temperature-synchronization and advances our understanding of how clock resetting is accomplished in nature.

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.

Effects of Temperature, Photoperiod, and Light Intensity on the Eclosion Rhythm of the High-Altitude Himalayan Strain ofDrosophila ananassae

Eclosion rhythm of the high-altitude Himalayan strain of Drosophila ananassae from Badrinath (altitude 5123 m) was temperature-dependent and at 21 degrees C, it was entrained by cycles of 12h light: 12h darkness (LD 12:12) and free-ran in constant darkness, however, it was arrhythmic at 13 degrees C or 17 degrees C under identical experimental conditions (Khare, P. V., Barnabas, R. J., Kanojiya, M., Kulkarni, A. D., Joshi, D. S. (2002). Temperature dependent eclosion rhythmicity in the high altitude Himalayan strains of Drosophila ananassae. Chronobiol. Int. 19:1041-1052). The present studies were designed to see whether or not these strains could be entrained at 13 degrees C, 17 degrees C, and 21 degrees C by two types of LD cycles in which the photoperiod at 100 lux intensity varied from 6h to 18h, and the light intensity of LD 14:10 cycles varied from 0.001 lux to 1000 lux. All LD cycles entrained this strain at 21 degrees C but not at 13 degrees C or 17 degrees C. These results demonstrate that the entrainment of eclosion rhythm depends on the ambient temperature and not on the photoperiod or light intensity of LD cycles. Thus the temperature has taken precedence over the light in the entrainment process of eclosion rhythm of the high altitude Himalayan strain of D. ananassae. This may be the result of natural selection in response to the environmental temperature at Badrinath that resembles that of the sub-Arctic region but the photoperiod or light intensity are of the subtropical region.

Effects of Light and Temperature on the Circadian System Controlling Sperm Release in MothSpodoptera littoralis

Reproductive physiology of male moths is regulated by a peripheral circadian system, which controls the timing of sperm release from the testis into the upper vas deferens (UVD) and timing of sperm transfer from the UVD to the seminal vesicles. We investigated various effects of light and temperature on sperm release and transfer rhythms in the moth Spodoptera littoralis. We report that both rhythms persist for up to 1 week in constant darkness without significant dampening and are also temperature compensated in the range from 20 degrees C to 30 degrees C. However, the duration of sperm retention in the UVD is temperature-dependent; consequently, temperature exerts a masking effect on the rhythm of sperm transfer. Experimental manipulations of light and temperature regime demonstrated that light dominates over temperature in entraining the timing of sperm release and transfer. Nevertheless, temperature plays a critical role in the absence of light Zeitgeber. Sperm release and transfer are arrhythmic in constant light (LL); however, both rhythms are restored by temperature cycles.

Natural variation in the splice site strength of a clock gene and species-specific thermal adaptation

We show that multiple suboptimal splice sites underlie the thermal-sensitive splicing of the period (per) 3'-terminal intron (dmpi8) from D. melanogaster, enabling this species to prolong its midday "siesta," a mechanism that likely diminishes the deleterious effects of heat during the longer summer days in temperate climates. In D. yakuba and D. santomea, which have a more ancestral distribution indigenous to Afro-equatorial regions wherein day length and temperature exhibit little fluctuation throughout the year, the splicing efficiencies of their per 3'-terminal introns do not exhibit thermal calibration, consistent with the little effect of temperature on the daily distribution of activity in these species. We propose that the weak splice sites on dmpi8 underlie a mechanism that facilitated the acclimation of the widely colonized D. melanogaster (and possibly D. simulans) to temperate climates and that natural selection operating at the level of splicing signals plays an important role in the thermal adaptation of life forms.

The Two-Spotted Cricket Gryllus bimaculatus: An Emerging Model for Developmental and Regeneration Studies

INTRODUCTIONThe two-spotted cricket Gryllus bimaculatus De Geer (Orthoptera: Gryllidae), which is one of the most abundant cricket species, inhabits the tropical and subtropical regions of Asia, Africa, and Europe. G. bimaculatus can be easily bred in the laboratory and has been widely used to study insect physiology and neurobiology. Recently, this species has become established as a model animal for studies on molecular mechanisms of development and regeneration because its mode of development is more typical of arthropods than that of Drosophila melanogaster, and the cricket is probably ancestral for this phylum. Moreover, the cricket is a hemimetabolous insect, in which nymphs possess functional legs with a remarkable capacity for regeneration after damage. Because RNA interference (RNAi) works effectively in this species, the elucidation of mechanisms of development and regeneration has been expedited through loss-of-function analyses of genes. Furthermore, because RNAi-based techniques for analyzing gene functions can be combined with assay systems in other research areas (such as behavioral analyses), G. bimaculatus is expected to become a model organism in various fields of biology. Thus, it may be possible to establish the cricket as a simple model system for exploring more complex organisms such as humans.

RNA interference of the clock gene period disrupts circadian rhythms in the cricket Gryllus bimaculatus

Periodic expression of so-called clock genes is an essential part of the circadian clock. In Drosophila melanogaster the cyclic expression of per and tim through an autoregulatory feedback loop is believed to play a central role in circadian rhythm generation. However, it is still elusive whether this hypothesis is applicable to other insect species. Here it is shown that per gene plays a key role in the rhythm generation in the cricket Gryllus bimaculatus. Measurement of per mRNA levels in the optic lobe revealed the rhythmic expression of per in light cycles with a peak in the late day to early night, persisting in constant darkness. A single injection of per double-stranded RNA (dsRNA) into the abdomen of the final instar nymphs effectively knocked down the mRNA levels as adult to about 50% of control animals. Most of the per dsRNA-injected crickets completely lost the circadian locomotor activity rhythm in constant darkness up to 50 days after the injection, whereas those injected with DsRed2 dsRNA as a negative control clearly maintained it. The electrical activity of optic lobe efferents also became arrhythmic in the per dsRNA-injected crickets. These results not only suggest that per plays an important role in the circadian rhythm generation also in the cricket but also show that RNA interference is a powerful tool to dissect the molecular machinery of the cricket circadian clock.

Synchronization of the Drosophila circadian clock by temperature cycles

The natural light/dark and temperature cycles are considered to be the most prominent factors that synchronize circadian clocks with the environment. Understanding the principles of temperature entrainment significantly lags behind our current knowledge of light entrainment in any organism subject to circadian research. Nevertheless, several effects of temperature on circadian clocks are well understood, and similarities as well as differences to the light-entrainment pathways start to emerge. This chapter provides an overview of the temperature effects on the Drosophila circadian clock with special emphasis on synchronization by temperature cycles. As in other organisms, such temperature cycles can serve as powerful time cues to synchronize the clock. Mutants that specifically interfere with aspects of temperature entrainment have been isolated and will likely help to reveal the underlying mechanisms. These mechanisms involve transcriptional and posttranscriptional regulation of clock genes. For synchronization of fly behavior by temperature cycles, the generation of a whole organism or systemic signal seems to be required, even though individual fly tissues can be synchronized under isolated culture conditions. If true, the requirement for such a signal would reveal a fundamental difference to the light-entrainment mechanism.

Thermosensitive splicing of a clock gene and seasonal adaptation

Similar to many diurnal animals, the daily distribution of activity in Drosophila exhibits a bimodal pattern with clock-controlled morning and evening peaks separated by a midday "siesta." In prior work, we showed that the thermosensitive splicing of a 3'-terminal intron in the RNA product from the Drosophila period (per) gene (dper) is critical for temperature-induced adjustments in the timing of evening activity. Cold temperatures enhance the splicing efficiency of this intron (termed dmpi8, Drosophila melanogaster per intron 8), an event that stimulates the daily accumulation of dper RNA and protein, leading to earlier evening activity. Conversely, warm temperatures attenuate dmpi8 splicing efficiency contributing to delayed evening activity, likely ensuring that flies avoid activity during the hot midday sun when they are at increased risk of desiccation. Here, we discuss the underlying molecular mechanisms governing the thermosensitive splicing of dmpi8 and how it contributes to seasonal changes in the daily activity patterns of Drosophila. On a broader perspective, RNA-RNA interactions likely have fundamental roles in the thermal adaptation of life forms to the daily and seasonal changes in temperature.

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.

Cis-combination of the classic per(S) and per(L) mutations results in arrhythmic Drosophila with ectopic accumulation of hyperphosphorylated PERIOD protein

The 1st circadian "clock" gene identified was the X-linked period (per) gene in Drosophila melanogaster. In the pioneering initial report, Konopka and Benzer (1971) characterized 3 alleles of per that shortened (per (S); approximately 19 h), lengthened (per (L); approximately 29 h), or abolished (per (0)) circadian behavioral rhythms. They also showed that transheterozygotes carrying the per (S) and per (L) mutations exhibit robust behavioral rhythms with nearly normal periods of approximately 23 h, highlighting the semidominant nature of many clock mutants. In this study, per (0) flies bearing a doubly mutated per transgene that carries both the per (S) and per (L) alleles (per (0); per (S/L)) were analyzed for behavioral and molecular rhythms. Unlike singly mutated versions, the per (0);per ( S/L) transgenic flies are arrhythmic in constant dark conditions and exhibit little, if any, entrainment to daily light-dark cycles. In a wildtype per (+) background, expression of per ( S/L) abolishes behavioral rhythms, indicating that it functions in a transdominant negative fashion. Biochemical analysis of head extracts revealed that only hyperphosphorylated isoforms of the PERS/L protein are detected throughout a daily cycle, and the levels remain constant. Intriguingly, little if any PERS/L is observed in key pacemaker neurons that control daily activity rhythms, consistent with the notion that hyperphosphorylated isoforms of PER are unstable. Nonetheless, PERS/L is detected in ectopic cells in the brain, in which it exhibits an unusual localization, mainly staining the periphery of the nucleus. These results suggest that posttranslational mechanisms play a key role in limiting the accumulation of PER to specific cells. On a broader scope, our results indicate that the semidominant effects of period-altering alleles observed in trans are not necessarily preserved in the cis-configuration and that novel phenotypes can emerge.