Role of the optic lobes in the regulation of the locomotor activity rhythm of Drosophila melanogaster: behavioral analysis of neural mutants

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

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

Expression of mutant CHMP2B linked to neurodegeneration in humans disrupts circadian rhythms in Drosophila

Mutations in CHMP2B, an ESCRT-III (endosomal sorting complexes required for transport) component, are associated with frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). Neurodegenerative disorders including FTD are also associated with a disruption in circadian rhythms, but the mechanism underlying this defect is not well understood. Here, we ectopically expressed the human CHMP2B variant associated with FTD (CHMP2BIntron5) in flies using the GMR-GAL4 driver (GMR>CHMP2BIntron5) and analyzed their circadian rhythms at behavioral, cellular, and biochemical level. In GMR>CHMP2BIntron5 flies, we observed disrupted eclosion rhythms, shortened free-running circadian locomotor period, and reduced levels of timeless (tim) mRNA-a circadian pacemaker gene. We also observed that the GMR-GAL4 driver, primarily known for its expression in the retina, drives expression in a subset of tim expressing neurons in the optic lobe of the brain. The patterning of these GMR- and tim-positive neurons in the optic lobe, which appears distinct from the putative clusters of circadian pacemaker neurons in the fly brain, was disrupted in GMR>CHMP2BIntron5 flies. These results demonstrate that CHMP2BIntron5 can disrupt the normal function of the circadian clock in Drosophila.

New Drosophila Long-Term Memory Genes Revealed by Assessing Computational Function Prediction Methods

A major bottleneck to our understanding of the genetic and molecular foundation of life lies in the ability to assign function to a gene and, subsequently, a protein. Traditional molecular and genetic experiments can provide the most reliable forms of identification, but are generally low-throughput, making such discovery and assignment a daunting task. The bottleneck has led to an increasing role for computational approaches. The Critical Assessment of Functional Annotation (CAFA) effort seeks to measure the performance of computational methods. In CAFA3, we performed selected screens, including an effort focused on long-term memory. We used homology and previous CAFA predictions to identify 29 key Drosophila genes, which we tested via a long-term memory screen. We identify 11 novel genes that are involved in long-term memory formation and show a high level of connectivity with previously identified learning and memory genes. Our study provides first higher-order behavioral assay and organism screen used for CAFA assessments and revealed previously uncharacterized roles of multiple genes as possible regulators of neuronal plasticity at the boundary of information acquisition and memory formation.

Reflections on contributing to "big discoveries" about the fly clock: Our fortunate paths as post-docs with 2017 Nobel laureates Jeff Hall, Michael Rosbash, and Mike Young

In the early 1980s Jeff Hall and Michael Rosbash at Brandeis University and Mike Young at Rockefeller University set out to isolate the period (per) gene, which was recovered in a revolutionary genetic screen by Ron Konopka and Seymour Benzer for mutants that altered circadian behavioral rhythms. Over the next 15 years the Hall, Rosbash and Young labs made a series of groundbreaking discoveries that defined the molecular timekeeping mechanism and formed the basis for them being awarded the 2017 Nobel Prize in Physiology or Medicine. Here the authors recount their experiences as post-docs in the Hall, Rosbash and Young labs from the mid-1980s to the mid-1990s, and provide a perspective of how basic research conducted on a simple model system during that era profoundly influenced the direction of the clocks field and established novel approaches that are now standard operating procedure for studying complex behavior.

•

2017 Nobel Prize awarded to Hall, Rosbash and Young for circadian clock mechanisms.

•

Work on fruit flies in the 1980s and 1990s were key to deciphering clock mechanisms.

•

Authors recount their experiences as postdocs in the Hall, Rosbash and Young labs.

•

The broad impacts of basic research on fruit fly clock genes.

Genotype Influences Day-to-Day Variability in Sleep in Drosophila melanogaster

Patterns of sleep often vary among individuals. But sleep and activity may also vary within an individual, fluctuating in pattern across time. One possibility is that these daily fluctuations in sleep are caused by the underlying genotype of the individual. However, differences attributable to genetic causes are difficult to distinguish from environmental factors in outbred populations such as humans. We therefore employed Drosophila as a model of intra-individual variability in sleep using previously collected sleep and activity data from the Drosophila Genetic Reference Panel, a collection of wild-derived inbred lines. Individual flies had significant daily fluctuations in their sleep patterns, and these fluctuations were heritable. Using the standard deviation of sleep parameters as a metric, we conducted a genome-wide association study. We found 663 polymorphisms in 104 genes associated with daily fluctuations in sleep. We confirmed the effects of 12 candidate genes on the standard deviation of sleep parameters. Our results suggest that daily fluctuations in sleep patterns are due in part to gene activity.

Leger RJ. The genetic basis for variation in resistance to infection in the Drosophila melanogaster genetic reference panel

Individuals vary extensively in the way they respond to disease but the genetic basis of this variation is not fully understood. We found substantial individual variation in resistance and tolerance to the fungal pathogen Metarhizium anisopliae Ma549 using the Drosophila melanogaster Genetic Reference Panel (DGRP). In addition, we found that host defense to Ma549 was correlated with defense to the bacterium Pseudomonas aeruginosa Pa14, and several previously published DGRP phenotypes including oxidative stress sensitivity, starvation stress resistance, hemolymph glucose levels, and sleep indices. We identified polymorphisms associated with differences between lines in both their mean survival times and microenvironmental plasticity, suggesting that lines differ in their ability to adapt to variable pathogen exposures. The majority of polymorphisms increasing resistance to Ma549 were sex biased, located in non-coding regions, had moderately large effect and were rare, suggesting that there is a general cost to defense. Nevertheless, host defense was not negatively correlated with overall longevity and fecundity. In contrast to Ma549, minor alleles were concentrated in the most Pa14-susceptible as well as the most Pa14-resistant lines. A pathway based analysis revealed a network of Pa14 and Ma549-resistance genes that are functionally connected through processes that encompass phagocytosis and engulfment, cell mobility, intermediary metabolism, protein phosphorylation, axon guidance, response to DNA damage, and drug metabolism. Functional testing with insertional mutagenesis lines indicates that 12/13 candidate genes tested influence susceptibility to Ma549. Many candidate genes have homologs identified in studies of human disease, suggesting that genes affecting variation in susceptibility are conserved across species.

From neurogenetic studies in the fly brain to a concept in circadian biology

This paper is dedicated to Karl-Friedrich Fischbach, who has always shared with me the interest in the function of the fly brain, especially that of its optic lobes. He has accompanied me during my first steps in scientific research. The paper tells the story how our first common attempts to localize the circadian clock in the fly brain finally helped in phrasing the two-oscillator principle of circadian clocks that seems to be valid far beyond the fly circadian system. I hope that Karl-Friedrich will take this story as praise for his generosity in supporting younger scientists outside his own lab, even without the reward of a common paper.

Drosophila TRPA1 functions in temperature control of circadian rhythm in pacemaker neurons

Most animals from flies to humans count on circadian clocks to synchronize their physiology and behaviors. Daily light cycles are well known environmental cues for setting circadian rhythms. Warmer and cooler temperatures that mimic day and night are also effective in entraining circadian activity in most animals. Even vertebrate organisms can be induced to show circadian responses through exposure to temperature cycles. In poikilothermic animals such as Drosophila, temperature differences of only 2-3°C are sufficient to synchronize locomotor rhythms. However, the molecular sensors that participate in temperature regulation of circadian activity in fruit flies or other animals are enigmatic. It is also unclear whether such detectors are limited to the periphery or may be in the central brain. Here, we showed that Drosophila TRPA1 (transient receptor potential cation channel A1) was necessary for normal activity patterns during temperature cycles. The trpA1 gene was expressed in a subset of pacemaker neurons in the central brain. In response to temperature entrainment, loss of trpA1 impaired activity, and altered expression of the circadian clock protein period (Per) in a subset of pacemaker neurons. These findings underscore a role for a thermoTRP in temperature regulation that extends beyond avoidance of noxious or suboptimal temperatures.

Selective entrainment of the Drosophila circadian clock to daily gradients in environmental temperature

BACKGROUND

Circadian clocks are internal daily time keeping mechanisms that allow organisms to anticipate daily changes in their environment and to organize their behavior and physiology in a coherent schedule. Although circadian clocks use temperature compensation mechanisms to maintain the same pace over a range of temperatures, they are also capable of synchronizing to daily temperature cycles. This study identifies key properties of this process.

RESULTS

Gradually ramping daily temperature cycles are shown here to synchronize behavioral and molecular daily rhythms in Drosophila with a remarkable efficiency. Entrainment to daily temperature gradients of amplitudes as low as 4 degrees C persisted even in the context of environmental profiles that also included continuous gradual increases or decreases in absolute temperature. To determine which elements of daily temperature gradients acted as the key determinants of circadian activity phase, comparative analyses of daily temperature gradients with different wave forms were performed. The phases of ascending and descending temperature acted together as key determinants of entrained circadian phase. In addition, circadian phase was found to be modulated by the relative temperature of release into free running conditions. Release at or close to the trough temperature of entrainment consistently resulted in phase advances. Re-entrainment to daily temperature gradients after large phase shifts occurred relatively slowly and required several cycles, allowing flies to selectively respond to periodic rather than anecdotal signals. The temperature-entrained phase relationship between clock gene expression rhythms and locomotor activity rhythms strongly resembled that previously observed for light entrainment. Moreover, daily temperature gradient and light/dark entrainment reinforced each other if the phases of ascending and descending temperature were in their natural alignment with the light and dark phases, respectively.

CONCLUSION

The present study systematically examined the entrainment of clock-controlled behavior to daily environmental temperature gradients. As a result, a number of key properties of circadian temperature entrainment were identified. Collectively, these properties represent a circadian temperature entrainment mechanism that is optimized in its ability to detect the time-of-day information encoded in natural environmental temperature profiles. The molecular events synchronized to the daily phases of ascending and descending temperature are expected to play an important role in the mechanism of circadian entrainment to daily temperature cycles.

Unique Self-Sustaining Circadian Oscillators Within the Brain ofDrosophila melanogaster

In Drosophila circadian rhythms persist in constant darkness (DD). The small ventral Lateral Neurons (s-LNv) mainly control the behavioral circadian rhythm in consortium with the large ventral Lateral Neurons (l-LNv) and dorsal Lateral Neurons (LNd). It is believed that the molecular oscillations of clock genes are the source of this persistent behavior. Indeed the s-LNv, LNd, Dorsal Neurons (DN)-DN2 and DN3 displayed self-sustained molecular oscillations in DD both at RNA and protein levels, except the DN2 oscillates in anti-phase. In contrast, the l-LNv and DN1 displayed self-sustained oscillations at the RNA level, but protein oscillations quickly dampened. Having self-sustained and dampened molecular oscillators together in the DN groups suggested that they play different roles. However, all DN groups seemed to contribute together to the light-dark (LD) behavioral rhythm. The LD entrainment of LN oscillators is achieved through Rhodopsin (RH) and Cryptochrome (CRY). CRY's expression in all DN groups implicates also its role in LD entrainment of DN, like in DN1. However, mutations in cry and glass that did not inflict LD synchronization of the DN2, DN3 oscillator implicate the existence of a novel photoreceptor at least in DN3.

Pigment-Dispersing Factor (PDF) Has Different Effects onDrosophila's Circadian Clocks in the Accessory Medulla and in the Dorsal Brain

The neuropeptide pigment-dispersing factor (PDF) is a key transmitter in the circadian clock of Drosophila melanogaster. Here we studied the rhythmic behavior of neural mutants with modified arborizations of the large PDF neurons. In sine oculis1( so1) mutants we found a higher density of PDF fibers in the fly's pacemaker center, the accessory medulla. These flies exhibited a significantly longer period (24.6 h) than control flies. When PDF levels were elevated to very high levels in the dorsal brain as true for somdamutants and small optic lobes;so1double mutants ( sol1;so1), a short-period component split off the long period in behavioral rhythmicity. The short period became shorter the higher the amount of PDF in this brain region and reached a value of ~21 h. The period alterations were clearly dependent on PDF, because so1;Pdf 01 and somda;Pdf 01 double mutants showed a single free-running component with a period similar to Pdf 01 mutants (~22.5 h) and significantly longer than the short period of somdamutants. These observations indicate that PDF feeds back on the clock neurons and changes their period. Obviously, PDF lengthens the period of some clock neurons and shortens that of others.

Circadian- and light-dependent regulation of resting membrane potential and spontaneous action potential firing of Drosophila circadian pacemaker neurons

The ventral lateral neurons (LNvs) of adult Drosophila brain express oscillating clock proteins and regulate circadian behavior. Whole cell current-clamp recordings of large LNvs in freshly dissected Drosophila whole brain preparations reveal two spontaneous activity patterns that correlate with two underlying patterns of oscillating membrane potential: tonic and burst firing of sodium-dependent action potentials. Resting membrane potential and spontaneous action potential firing are rapidly and reversibly regulated by acute changes in light intensity. The LNv electrophysiological light response is attenuated, but not abolished, in cry(b) mutant flies hypomorphic for the cell-autonomous light-sensing protein CRYPTOCHROME. The electrical activity of the large LNv is circadian regulated, as shown by significantly higher resting membrane potential and frequency of spontaneous action potential firing rate and burst firing pattern during circadian subjective day relative to subjective night. The circadian regulation of membrane potential, spontaneous action potential firing frequency, and pattern of Drosophila large LNvs closely resemble mammalian circadian neuron electrical characteristics, suggesting a general evolutionary conservation of both physiological and molecular oscillator mechanisms in pacemaker neurons.

Pigment dispersing factor-dependent and -independent circadian locomotor behavioral rhythms

Circadian pacemaker circuits consist of ensembles of neurons, each expressing molecular oscillations, but how circuit-wide coordination of multiple oscillators regulates rhythmic physiological and behavioral outputs remains an open question. To investigate the relationship between the pattern of oscillator phase throughout the circadian pacemaker circuit and locomotor activity rhythms in Drosophila, we perturbed the electrical activity and pigment dispersing factor (PDF) levels of the lateral ventral neurons (LNv) and assayed their combinatorial effect on molecular oscillations in different parts of the circuit and on locomotor activity behavior. Altered electrical activity of PDF-expressing LNv causes initial behavioral arrhythmicity followed by gradual long-term emergence of two concurrent short- and long-period circadian behavioral activity bouts in approximately 60% of flies. Initial desynchrony of circuit-wide molecular oscillations is followed by the emergence of a novel pattern of period (PER) synchrony whereby two subgroups of dorsal neurons (DN1 and DN2) exhibit PER oscillation peaks coinciding with two activity bouts, whereas other neuronal subgroups exhibit a single PER peak coinciding with one of the two activity bouts. The emergence of this novel pattern of circuit-wide oscillator synchrony is not accompanied by concurrent change in the electrical activity of the LNv. In PDF-null flies, altered electrical activity of LNv drives a short-period circadian activity bout only, indicating that PDF-independent factors underlie the short-period circadian activity component and that the long-period circadian component is PDF-dependent. Thus, polyrhythmic behavioral patterns in electrically manipulated flies are regulated by circuit-wide coordination of molecular oscillations and electrical activity of LNv via PDF-dependent and -independent factors.

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.

Dominant-negative CK2alpha induces potent effects on circadian rhythmicity

Circadian clocks organize the precise timing of cellular and behavioral events. In Drosophila, circadian clocks consist of negative feedback loops in which the clock component PERIOD (PER) represses its own transcription. PER phosphorylation is a critical step in timing the onset and termination of this feedback. The protein kinase CK2 has been linked to circadian timing, but the importance of this contribution is unclear; it is not certain where and when CK2 acts to regulate circadian rhythms. To determine its temporal and spatial functions, a dominant negative mutant of the catalytic alpha subunit, CK2α^Tik, was targeted to circadian neurons. Behaviorally, CK2α^Tik induces severe period lengthening (∼33 h), greater than nearly all known circadian mutant alleles, and abolishes detectable free-running behavioral rhythmicity at high levels of expression. CK2α^Tik, when targeted to a subset of pacemaker neurons, generates period splitting, resulting in flies exhibiting both long and near 24-h periods. These behavioral effects are evident even when CK2α^Tik expression is induced only during adulthood, implicating an acute role for CK2α function in circadian rhythms. CK2α^Tik expression results in reduced PER phosphorylation, delayed nuclear entry, and dampened cycling with elevated trough levels of PER. Heightened trough levels of per transcript accompany increased protein levels, suggesting that CK2α^Tik disturbs negative feedback of PER on its own transcription. Taken together, these in vivo data implicate a central role of CK2α function in timing PER negative feedback in adult circadian neurons.

The molecular mechanism that governs organization of physiology and behavior into 24-h rhythms is a conserved transcriptional feedback process that is strikingly similar across distinct phyla. Notably, cyclic phosphorylation of negative feedback regulators is critical to time molecular rhythms. Indeed, mutation of a putative phosphoacceptor site in the human PERIOD2 gene, a key negative regulator, is associated with Advanced Sleep Phase Syndrome. This study reveals a critical role for the protein kinase CK2 for setting the period of behavioral and molecular oscillations in Drosophila. Circadian phenotypes due to CK2 disruption are due to a direct requirement in adult circadian pacemakers. These findings further demonstrate that CK2 modification of the negative feedback regulator PERIOD alters its cyclical phosphorylation, protein abundance, nuclear translocation, and transcriptional repression activity. These studies place CK2 as a central kinase in circadian timing.

Organization of cell and tissue circadian pacemakers: a comparison among species

In most animal species, a circadian timing system has evolved as a strategy to cope with 24-hour rhythms in the environment. Circadian pacemakers are essential elements of the timing system and have been identified in anatomically discrete locations in animals ranging from insects to mammals. Rhythm generation occurs in single pacemaker neurons and is based on the interacting negative and positive molecular feedback loops. Rhythmicity in behavior and physiology is regulated by neuronal networks in which synchronization or coupling is required to produce coherent output signals. Coupling occurs among individual clock cells within an oscillating tissue, among functionally distinct subregions within the pacemaker, and between central pacemakers and the periphery. Recent evidence indicates that peripheral tissues can influence central pacemakers and contain autonomous circadian oscillators that contribute to the regulation of overt rhythmicity. The data discussed in this review describe coupling and synchronization mechanisms at the cell and tissue levels. By comparing the pacemaker systems of several multicellular animal species (Drosophila, cockroaches, crickets, snails, zebrafish and mammals), we will explore general organizational principles by which the circadian system regulates a 24-hour rhythmicity.

Rhythm defects caused by newly engineered null mutations in Drosophila's cryptochrome gene

Much of the knowledge about cryptochrome function in Drosophila stems from analyzing the cryb mutant. Several features of this variant's light responsiveness imply either that CRYb retains circadian-photoreceptive capacities or that additional CRY-independent light-input routes subserve these processes. Potentially to resolve these issues, we generated cry knock-out mutants (cry0's) by gene replacement. They behaved in an anomalously rhythmic manner in constant light (LL). However, cry0 flies frequently exhibited two separate circadian components in LL, not observed in most previous cryb analyses. Temperature-dependent circadian phenotypes exhibited by cry(0) flies suggest that CRY is involved in core pacemaking. Further locomotor experiments combined cry0 with an externally blinding mutation (norpAP24), which caused the most severe decrements of circadian photoreception observed so far. cryb cultures were shown previously to exhibit either aperiodic or rhythmic eclosion in separate studies. We found cry0 to eclose in a solidly periodic manner in light:dark cycles or constant darkness. Furthermore, both cry0 and cryb eclosed rhythmically in LL. These findings indicate that the novel cry0 type causes more profound defects than does the cryb mutation, implying that CRYb retains residual activity. Because some norpAP24 cry0 individuals can resynchronize to novel photic regimes, an as-yet undetermined light-input route exists in Drosophila.

Possible evidence for morning and evening oscillators in Drosophila melanogaster populations selected for early and late adult emergence

In this paper, we report the results of our study aimed at a systematic analysis of the circadian phenotypes of fruit flies Drosophila melanogaster selected for early and late adult emergence, in light of the "morning and evening oscillator" (M and E) model for circadian clocks. We monitored adult emergence and activity/rest rhythms in these flies under light/dark (LD) cycles with short (8:16 h), normal (12:12 h) and long (16:8 h) photoperiods, as well as under constant darkness (DD). Across all the three LD cycles, the early populations displayed a morning phenotype with peak of emergence and activity occurring earlier than the controls and greater anticipation to "lights-on" and weak anticipation to "lights-off", while the late populations showed an evening phenotype with peak of emergence and activity occurring later than the controls and greater anticipation to lights-off and weak anticipation to lights-on. The gate of adult emergence and duration of activity in the early populations was narrower than the controls, while those of the late populations were wider than the controls. In addition, the circadian periodicities of adult emergence and activity/rest rhythms of the early flies were significantly shorter than the controls, while those of the late flies were significantly longer than the controls. In summary, the circadian phenotypes indicate that the early populations have evolved a dominant M oscillator, while the late populations have evolved a dominant E oscillator, thus providing an empirical support for the M and E model in Drosophila.

Development and morphology of the clock-gene-expressing lateral neurons of Drosophila melanogaster

The clock-gene-expressing lateral neurons are essential for the locomotor activity rhythm of Drosophila melanogaster. Traditionally, these neurons are divided into three groups: the dorsal lateral neurons (LN(d)), the large ventral lateral neurons (l-LN(v)), and the small ventral lateral neurons (s-LN(v)), whereby the latter group consists of four neurons that express the neuropeptide pigment-dispersing factor (PDF) and a fifth PDF-negative neuron. So far, only the l-LN(v) and the PDF-positive s-LN(v) have been shown to project into the accessory medulla, a small neuropil that contains the circadian pacemaker center in several insects. We show here that the other lateral neurons also arborize in the accessory medulla, predominantly forming postsynaptic sites. Both the l-LN(v) and LN(d) are anatomically well suited to connect the accessory medullae. Whereas the l-LN(v) may receive ipsilateral photic input from the Hofbauer-Buchner eyelet, the LN(d) invade mainly the contralateral accessory medulla and thus may receive photic input from the contralateral side. Both the LN(d) and the l-LN(v) differentiate during midmetamorphosis. They do so in close proximity to one another and the fifth PDF-negative s-LN(v), suggesting that these cell groups may derive from common precursors.

Electrophysiological and anatomical characterization of PDF-positive clock neurons in the intact adult Drosophila brain

Daily biological rhythms in both prokaryotes and eukaryotes are controlled by circadian clocks. In Drosophila, there is a good basic understanding of both the molecular and anatomical components of the clock. In this study we directly measure, for the first time, electrophysiological properties and anatomy of individual filled large lateral PDF-positive clock neurons, a cell group believed to be involved in synchronization of the clock in constant conditions. We find that the large PDF-positive neurons are morphologically homogeneous and that their resting membrane potential is modulated both by the clock and by light inputs. Expression of a leak channel, dORK-deltaC, which has been shown to disrupt circadian locomotor rhythms, hyperpolarizes these neurons, and blocks firing. These data imply that the firing properties of large PDF neurons are both regulated by and critical for clock function.

Functional analysis of circadian pacemaker neurons in Drosophila melanogaster

The molecular mechanisms of circadian rhythms are well known, but how multiple clocks within one organism generate a structured rhythmic output remains a mystery. Many animals show bimodal activity rhythms with morning (M) and evening (E) activity bouts. One long-standing model assumes that two mutually coupled oscillators underlie these bouts and show different sensitivities to light. Three groups of lateral neurons (LN) and three groups of dorsal neurons govern behavioral rhythmicity of Drosophila. Recent data suggest that two groups of the LN (the ventral subset of the small LN cells and the dorsal subset of LN cells) are plausible candidates for the M and E oscillator, respectively. We provide evidence that these neuronal groups respond differently to light and can be completely desynchronized from one another by constant light, leading to two activity components that free-run with different periods. As expected, a long-period component started from the E activity bout. However, a short-period component originated not exclusively from the morning peak but more prominently from the evening peak. This reveals an interesting deviation from the original Pittendrigh and Daan (1976) model and suggests that a subgroup of the ventral subset of the small LN acts as "main" oscillator controlling M and E activity bouts in Drosophila.

The circadian system of Drosophila melanogaster and its light input pathways

The fruit fly Drosophila melanogaster has been a grateful object for circadian rhythm researchers over several decades. Behavioral, genetic, and molecular studies in the little fly have aided in understanding the bases of circadian time keeping and rhythmic behaviors not only in Drosophila, but also in other organisms, including mammals. This review summarizes our present knowledge about the fruit fly's circadian system at the molecular and neurobiological level, with special emphasis on its entrainment by environmental light-dark cycles. The results obtained for Drosophila are discussed with respect to parallel findings in mammals.

Electrical hyperexcitation of lateral ventral pacemaker neurons desynchronizes downstream circadian oscillators in the fly circadian circuit and induces multiple behavioral periods

Coupling of autonomous cellular oscillators is an essential aspect of circadian clock function but little is known about its circuit requirements. Functional ablation of the pigment-dispersing factor-expressing lateral ventral subset (LNV) of Drosophila clock neurons abolishes circadian rhythms of locomotor activity. The hypothesis that LNVs synchronize oscillations in downstream clock neurons was tested by rendering the LNVs hyperexcitable via transgenic expression of a low activation threshold voltage-gated sodium channel. When the LNVs are made hyperexcitable, free-running behavioral rhythms decompose into multiple independent superimposed oscillations and the clock protein oscillations in the dorsal neuron 1 and 2 subgroups of clock neurons are phase-shifted. Thus, regulated electrical activity of the LNVs synchronize multiple oscillators in the fly circadian pacemaker circuit.

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.

Circadian rhythms of locomotor activity in Drosophila

Drosophila is by far the most advanced model to understand the complex biochemical interactions upon which circadian clocks rely. Most of the genes that have been characterized so far were isolated through genetic screens using the locomotor activity rhythms of the adults as a circadian output. In addition, new techniques are available to deregulate gene expression in specific cells, allowing to analyze the growing number of developmental genes that also play a role as clock genes. However, one of the major challenges in circadian biology remains to properly interpret complex behavioral data and use them to fuel molecular models. This review tries to describe the problems that clockwatchers have to face when using Drosophila activity rhythms to understand the multiple facets of circadian function.

Genetics and molecular biology of rhythms in Drosophila and other insects

Application of generic variants (Sections II-IV, VI, and IX) and molecular manipulations of rhythm-related genes (Sections V-X) have been used extensively to investigate features of insect chronobiology that might not have been experimentally accessible otherwise. Most such tests of mutants and molecular-genetic xperiments have been performed in Drosophila melanogaster. Results from applying visual-system variants have revealed that environmental inputs to the circadian clock in adult flies are mediated by external photoreceptive structures (Section II) and also by direct light reception chat occurs in certain brain neurons (Section IX). The relevant light-absorbing molecuLes are rhodopsins and "blue-receptive" cryptochrome (Sections II and IX). Variations in temperature are another clock input (Section IV), as has been analyzed in part by use of molecular techniques and transgenes involving factors functioning near the heart of the circadian clock (Section VIII). At that location within the fly's chronobiological system, approximately a half-dozen-perhaps up to as many as 10-clock genes encode functions that act and interact to form the circadian pacemaker (Sections III and V). This entity functions in part by transcriptional control of certain clock genes' expressions, which result in the production of key proteins that feed back negatively to regulate their own mRNA production. This occurs in part by interactions of such proteins with others that function as transcriptional activators (Section V). The implied feedback loop operates such that there are daily variations in the abundances of products put out by about one-half of the core clock genes. Thus, the normal expression of these genes defines circadian rhythms of their own, paralleling the effects of mutations at the corresponding genetic loci (Section III), which are to disrupt or apparently eliminate clock functioning. The fluctuations in the abundance of gene products are controlled transciptionally and posttranscriptionally. These clock mechanisms are being analyzed in ways that are increasingly complex and occasionally obscure; not all panels of this picture are comprehensive or clear, including problems revolving round the biological meaning or a given features of all this molecular cycling (Section V). Among the complexities and puzzles that have recently arisen, phenomena that stand out are posttranslational modifications of certain proteins that are circadianly regulated and regulating; these biochemical events form an ancillary component of the clock mechanism, as revealed in part by genetic identification of Factors (Section III) that turned out to encode protein kinases whose substrates include other pacemaking polypeptides (Section V). Outputs from insect circadian clocks have been long defined on formalistic and in some cases concrete criteria, related to revealed rhythms such as periodic eclosion and daily fluctuations of locomotion (Sections II and III). Based on the reasoning that if clock genes can regulate circadian cyclings of their own products, they can do the same for genes that function along output pathways; thus clock-regulated genes have been identified in part by virtue of their products' oscillations (Section X). Those studied most intensively have their expression influenced by circadian-pacemaker mutations. The clock-regulated genes discovered on molecular criteria have in some instances been analyzed further in their mutant forms and found to affect certain features of overt whole-organismal rhythmicity (Sections IV and X). Insect chronogenetics touches in part on naturally occurring gene variations that affect biological rhythmicity or (in some cases) have otherwise informed investigators about certain features of the organism's rhythm system (Section VII). Such animals include at least a dozen insect species other than D. melanogaster in which rhythm variants have been encountered (although usually not looked for systematically). The chronobiological "system" in the fruit fly might better be graced with a plural appellation because there is a myriad of temporally related phenomena that have come under the sway of one kind of putative rhythm variant or the other (Section IV). These phenotypes, which range well beyond the bedrock eclosion and locomotor circadian rhythms, unfortunately lead to the creation of a laundry list of underanalyzed or occult phenomena that may or may not be inherently real, whether or not they might be meaningfully defective under the influence of a given chronogenetic variant. However, such mutants seem to lend themselves to the interrogation of a wide variety of time-based attributes-those that fall within the experimental confines of conventionally appreciated circadian rhythms (Sections II, III, VI, and X); and others that consist of 24-hr or nondaily cycles defined by many kinds of biological, physiological, or biochemical parameters (Section IV).

The regulation of circadian clocks by light in fruitflies and mice

A circadian clock has no survival value unless biological time is adjusted (entrained) to local time and, for most organisms, the profound changes in the light environment provide the local time signal (zeitgeber). Over 24 h, the amount of light, its spectral composition and its direction change in a systematic way. In theory, all of these features could be used for entrainment, but each would be subject to considerable variation or 'noise'. Despite this high degree of environmental noise, entrained organisms show remarkable precision in their daily activities. Thus, the photosensory task of entrainment is likely to be very complex, but fundamentally similar for all organisms. To test this hypothesis we compare the photoreceptors that mediate entrainment in both flies and mice, and assess their degree of convergence. Although superficially different, both organisms use specialized (employing novel photopigments) and complex (using multiple photopigments) photoreceptor mechanisms. We conclude that this multiplicity of photic inputs, in highly divergent organisms, must relate to the complex sensory task of using light as a zeitgeber.

Localization of the clock controlling circadian rhythms in the first neuropile of the optic lobe in the housefly

The visual system of a fly expresses several circadian rhythms that have been detected in the photoreceptors of the compound eye and in the first neuropile, the lamina, of the underlying optic lobe. In the lamina, axons of two classes of interneuron, L1 and L2, exhibit cyclical size changes, swelling by day and shrinking by night. These rhythmic size changes may be generated by circadian oscillators located inside and/or outside the optic lobe. To localize such oscillators, we have examined changes in the axonal cross-sectional areas of L1 and L2 within the lamina of the housefly (Musca domestica) under conditions of 12 h of light and 12 h of darkness (LD12:12), constant darkness (DD) or continuous light (LL) 24 h after the medulla was severed from the rest of the brain. After the lesion, the axon size changes of L1 and L2 were maintained only in LD conditions, but were weaker than in control flies. In DD and LL conditions, they were eliminated. This indicates that circadian rhythms in the lamina of a fly are generated central to the lamina and medulla neuropiles of the optic lobe. Cyclical changes of light and darkness in LD conditions are still able, however, to induce a weak daily rhythm in the axon sizes of L1 and L2.

Photic entrainment of the circadian clock: from Drosophila to mammals

Entrainment is as fundamental to an organism's circadian timing as are the molecular mechanisms involved in the functioning of the intracellular clock oscillator. In nature, one of the principle, although not the only, circadian entraining stimulus (Zeitgeber) is provided by the daily light--dark cycles. In animals, the visual processing apparatus alone is inadequate to accomplish the task of transducing circadian photic signals to the clockwork machinery. In fact, it is ever more appreciated by circadian biologists that organisms as divergent as plants and mammals have evolved a wonderfully complex array of partly redundant specializations which can guarantee the precise alignment of biological and environmental time. Research in circadian biology is cruising at such a rate that attempts to review the state of the art can only hope, at best, to provide a snapshot of the speeding cruiser from its wake. This paper will hopefully provide a reasonably sharp portrayal of what is at hand.

Blockade of the central generator of locomotor rhythm by noncompetitive NMDA receptor antagonists in Drosophila larvae

The noncompetitive antagonists of the vertebrate N-methyl-D-aspartate (NMDA) receptor dizocilpine (MK 801) and phencyclidine (PCP), delivered in food, were found to induce a marked and reversible inhibition of locomotor activity in Drosophila melanogaster larvae. To determine the site of action of these antagonists, we used an in vitro preparation of the Drosophila third-instar larva, preserving the central nervous system and segmental nerves with their connections to muscle fibers of the body wall. Intracellular recordings were made from ventral muscle fibers 6 and 7 in the abdominal segments. In most larvae, long-lasting (>1 h) spontaneous rhythmic motor activities were recorded in the absence of pharmacological activation. After sectioning of the connections between the brain and abdominal ganglia, the rhythm disappeared, but it could be partially restored by perfusing the muscarinic agonist oxotremorine, indicating that the activity was generated in the ventral nerve cord. MK 801 and PCP rapidly and efficiently inhibited the locomotor rhythm in a dose-dependent manner, the rhythm being totally blocked in 2 min with doses over 0.1 mg/mL. In contrast, more hydrophilic competitive NMDA antagonists had no effect on the motor rhythm in this preparation. MK 801 did not affect neuromuscular glutamatergic transmission at similar doses, as demonstrated by monitoring the responses elicited by electrical stimulation of the motor nerve or pressure applied glutamate. The presence of oxotremorine did not prevent the blocking effect of MK 801. These results show that MK 801 and PCP specifically inhibit centrally generated rhythmic activity in Drosophila, and suggest a possible role for NMDA-like receptors in locomotor rhythm control in the insect CNS.

Phase response curves of a molecular model oscillator: implications for mutual coupling of paired oscillators

Increasing evidence indicates that the accessory medulla is the circadian pacemaker controlling locomotor activity rhythms in insects. A prominent group of neurons of this neuropil shows immunoreactivity to the peptide pigment-dispersing hormone (PDH). In Drosophila melanogaster, the PDH-immunoreactive (PDH-ir) lateral neurons, which also express the clock genes period and timeless, are assumed to be circadian pacemaker cells themselves. In other insects, such as Leucophaea maderae, a subset of apparently homologue PDH-ir cells is a candidate for the circadian coupling pathway of the bilaterally symmetric clocks. Although knowledge about molecular mechanisms of the circadian clockwork is increasing rapidly, very little is known about mechanisms of circadian coupling. The authors used a computer model, based on the molecular feedback loop of the clock genes in D. melanogaster, to test the hypothesis that release of PDH is involved in the coupling between bilaterally paired oscillators. They can show that a combination of all-delay- and all-advance-type interactions between two model oscillators matches best the experimental findings on mutual pacemaker coupling in L. maderae. The model predicts that PDH affects the phosphorylation rate of clock genes and that in addition to PDH, another neuroactive substance is involved in the coupling pathway, via an all-advance type of interaction. The model suggests that PDH and light pulses, represented by two distinct classes of phase response curves, have different targets in the oscillatory feedback loop and are, therefore, likely to act in separate input pathways to the clock.

The suprachiasmatic nucleus and the circadian time-keeping system revisited

Many physiological and behavioral processes show circadian rhythms which are generated by an internal time-keeping system, the biological clock. In rodents, evidence from a variety of studies has shown the suprachiasmatic nucleus (SCN) to be the site of the master pacemaker controlling circadian rhythms. The clock of the SCN oscillates with a near 24-h period but is entrained to solar day/night rhythm by light. Much progress has been made recently in understanding the mechanisms of the circadian system of the SCN, its inputs for entrainment and its outputs for transfer of the rhythm to the rest of the brain. The present review summarizes these new developments concerning the properties of the SCN and the mechanisms of circadian time-keeping. First, we will summarize data concerning the anatomical and physiological organization of the SCN, including the roles of SCN neuropeptide/neurotransmitter systems, and our current knowledge of SCN input and output pathways. Second, we will discuss SCN transplantation studies and how they have contributed to knowledge of the intrinsic properties of the SCN, communication between the SCN and its targets, and age-related changes in the circadian system. Third, recent findings concerning the genes and molecules involved in the intrinsic pacemaker mechanisms of insect and mammalian clocks will be reviewed. Finally, we will discuss exciting new possibilities concerning the use of viral vector-mediated gene transfer as an approach to investigate mechanisms of circadian time-keeping.

Differential control of morning and evening components in the activity rhythm of Drosophila melanogaster--sex-specific differences suggest a different quality of activity

The rhythms of locomotor activity of male and virgin or mated female flies were compared in the Drosophila melanogaster wild-type strains CantonS, Berlin, and OregonR. Under light-dark conditions, most flies showed a bimodal activity pattern with a morning peak around lights-on and an evening peak before lights-off. For all strains, a distinct sexual dimorphism was observed in the phase of the morning peak. Males had a significantly earlier morning peak than females and consequently a larger phase angle between morning and evening peak (psi(m, e)). Under constant dark conditions, the morning component merged with the evening component to a unimodal activity band in about half of the flies. In those flies who maintained bimodality, the sex-specific difference in psi(m, e) disappeared. Other sex-specific differences were now apparent: Males showed a shorter free-running period than females, and in two of the three strains, females were more active than males. Morning and evening components seem to contribute to the free-running period. Spontaneous or externally provoked change in psi(m, e) were correlated with period changes. In some flies, the morning and the evening components showed splitting, indicating that they are the output of two different oscillators. The sexual dimorphism in the phase of the morning peak under LD-conditions suggests that the function of activity during morning and evening peak might be different, for example, during the morning peak, males are active to find females. Overall, the results underline the multioscillatory nature of Drosophila's circadian system.

Extraretinal photoreceptors at the compound eye's posterior margin in Drosophila melanogaster

Many invertebrates have supplementary extraocular photoreceptors that often are implicated in circadian rhythms. An extraretinal group of candidate photoreceptors in the fruit fly, Drosophila melanogaster, has been revealed previously at the posterior margin of the compound eye by using a photoreceptor-specific monoclonal antibody (Hofbauer and Buchner [1989] Naturwissen 76:335-336), but it never has been characterized. Here, we report the fine structure of this cell cluster reported by Hofbauer and Buchner, which is called "eyelet," as well as the further candidacy of their visual pigment and neurotransmitter. Eyelet forms a specialized, pigmented organ with cells that have numerous microvilli arranged into coherent rhabdomeres. The presence of rhabdomeric microvilli is a defining feature of a photoreceptor, reported here for the first time in eyelet. The rhabdomeres exhibit Rh6 opsin-like immunoreactivity, which provides evidence that the photoreceptors are functional: they fail to immunostain with antibodies against NINAE (Rh1), Rh4, or Rh5. The photoreceptors have been shown previously to exhibit histamine-like immunoreactivity, but they also stain with a monoclonal antiserum raised against Drosophila choline acetyltransferase (ChAT), suggesting that the photoreceptors not only may contain histamine but also can synthesize acetylcholine. A ChAT-immunoreactive axon bundle originating from eyelet terminates in the cortex of the anterior medulla. This bundle also is seen with reduced silver stains. Electron microscopic examination revealed four axon profiles of similar size in this bundle, indicating that eyelet contains at least four photoreceptors. The pathway of eyelet's axon bundle coincides with the precocious pathway of Bolwig's nerve that arises from the larval organ of sight. The origin and possible function of eyelet are discussed.

Role of posttranscriptional regulation in circadian clocks: lessons from Drosophila

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

An extraretinally expressed insect cryptochrome with similarity to the blue light photoreceptors of mammals and plants

Photic entrainment of insect circadian rhythms can occur through either extraretinal (brain) or retinal photoreceptors, which mediate sensitivity to blue light or longer wavelengths, respectively. Although visual transduction processes are well understood in the insect retina, almost nothing is known about the extraretinal blue light photoreceptor of insects. We now have identified and characterized a candidate blue light photoreceptor gene in Drosophila (DCry) that is homologous to the cryptochrome (Cry) genes of mammals and plants. The DCry gene is located in region 91F of the third chromosome, an interval that does not contain other genes required for circadian rhythmicity. The protein encoded by DCry is approximately 50% identical to the CRY1 and CRY2 proteins recently discovered in mammalian species. As expected for an extraretinal photoreceptor mediating circadian entrainment, DCry mRNA is expressed within the adult brain and can be detected within body tissues. Indeed, tissue in situ hybridization demonstrates prominent expression in cells of the lateral brain, which are close to or coincident with the Drosophila clock neurons. Interestingly, DCry mRNA abundance oscillates in a circadian manner in Drosophila head RNA extracts, and the temporal phasing of the rhythm is similar to that documented for the mouse Cry1 mRNA, which is expressed in clock tissues. Finally, we show that changes in DCry gene dosage are associated predictably with alterations of the blue light resetting response for the circadian rhythm of adult locomotor activity.

Organization of the circadian system in insects

The circadian systems of different insect groups are summarized and compared. Emphasis is placed on the anatomical identification and characterization of circadian pacemakers, as well as on their entrainment, coupling, and output pathways. Cockroaches, crickets, beetles, and flies possess bilaterally organized pacemakers in the optic lobes that appear to be located in the accessory medulla, a small neuropil between the medulla and the lobula. Neurons that are immunoreactive for the peptide pigment-dispersing hormone (PDH) arborize in the accessory medulla and appear to be important components of the optic lobe pacemakers. The neuronal architecture of the accessory medulla with associated PDH-immunoreactive neurons is best characterized in cockroaches, while the molecular machinery of rhythm generation is best understood in fruit flies. One essential component of the circadian clock is the period protein (PER), which colocalizes with PDH in about half of the fruit fly's presumptive pacemaker neurons. PER is also found in the presumptive pacemaker neurons of beetles and moths, but appears to have different functions in these insects. In moths, the pacemakers are situated in the central brain and are closely associated with neuroendocrine functions. In the other insects, neurons associated with neuroendocrine functions also appear to be closely coupled to the optic lobe pacemakers. Some crickets and flies seem to possess central brain pacemakers in addition to their optic lobe pacemakers. With respect to neuronal organization, the circadian systems of insects show striking similarities to the vertebrate circadian system.

Neural substrates of Drosophila rhythms revealed by mutants and molecular manipulations

In the fruit-fly Drosophila, rhythmic expression of the clock gene period is detected in cells throughout the body. Whereas these cells could be pacemakers for circadian rhythms of unknown physiological processes, the brain pacemakers are known to be responsible for circadian behavior. Recent progress in genetic and molecular studies of clock genes in Drosophila has permitted the identification of brain pacemakers at the cellular level and their output pathways to rhythmic behavior. Similar studies in other insect species have suggested considerable diversity in the anatomical and neurochemical properties of pacemaker cells, as well as in the mechanisms of clock-gene regulation.

Response of the timeless protein to light correlates with behavioral entrainment and suggests a nonvisual pathway for circadian photoreception

The period (per) and timeless (tim) genes are required for circadian behavioral rhythms in Drosophila. The current model for how these rhythms entrain to light is based upon the light induced decrease in timeless protein (TIM) levels. We show here that the TIM response to light correlates with the effect of light on the behavioral rhythm. To identify components of the entrainment pathway, we also assayed the TIM response in flies with mutant visual systems. Flies that lacked eyes displayed a normal response in lateral neurons. The TIM response to a light pulse was attenuated in flies that were mutant for the transient receptor potential (trp) and trp-like (trpl) genes, which are required for calcium conductance in the visual transduction cascade. The reduced TIM response was accompanied by a reduced phase shift in the behavioral rhythm, but neither response was completely eliminated, and the trpl;trp flies entrain to light-dark cycles, suggesting that these genes perturb some aspect of circadian entrainment when mutated but are not essential for it. The TIM response was also unaffected in ninaE flies that lack the rhodopsin protein (rh1). These results support the hypothesis that circadian entrainment does not rely on the visual system and likely involves a dedicated pathway for photoreception.

Multiple circadian-regulated elements contribute to cycling period gene expression in Drosophila

A new regulatory element necessary for the correct temporal expression of the period (per) gene was identified by monitoring real-time per expression in living individual flies carrying two different period-luciferase transgenes. luciferase RNA driven from only the per promoter was not sufficient to replicate the normal pattern of per RNA cycling; however, a per-luc fusion RNA driven from a transgene containing additional per sequences cycled identically to endogenous per. The results indicate the existence of at least two circadian-regulated elements--one within the promoter and one within the transcribed portion of the per gene. Phase and amplitude analysis of both per-luc transgenes revealed that normal per expression requires the regulation of these elements at distinct phases and suggests a mechanism by which biological clocks sustain high-amplitude feedback oscillations.

Quantitative analysis of Drosophila period gene transcription in living animals

To determine the in vivo regulatory pattern of the clock gene period (per), the authors recently developed transgenic Drosophila carrying a luciferase cDNA fused to the promoter region of per. They have now carried out noninvasive, high time-resolution experiments allowing high-throughput monitoring of circadian bioluminescence rhythms in individual living adults for several days. This immediately solved several problems (resulting directly from individual asynchrony within a population) that have accompanied previous biochemical experiments in which groups of animals were sacrificed at each time point. Furthermore, the authors have developed numerical analysis methods for automatically determining rhythmicity associated with bioluminescence records from single flies. This has revealed some features of per gene transcription that were previously unappreciated and provides a general strategy for the analysis of rhythmic time series in the study of molecular rhythms.

Daily rhythms in cells of the fly's optic lobe: taking time out from the circadian clock

Considerable progress has recently been reported in locating the cellular basis and molecular mechanisms of the circadian clock in the fruitfly, Drosophila melanogaster. To advance beyond the clock, towards the outputs that lie between the clock itself and the circadian rhythms in behaviour that it regulates, will present new challenges. This is because most behaviours are generated by complex neuronal circuits, which are themselves difficult to unravel. Recently described anatomical changes in the optic lobe of the related housefly, Musca domestica, exhibit a circadian rhythm that is, by contrast, relatively easy to assay. This rhythm is apparently controlled by at least two sets of diffuse modulatory neurones. One of these, immunoreactive to the peptide pigment-dispersing hormone, also expresses in Drosophila the product of the period (per) gene, the most widely studied of the so-called clock genes that are essential for the correct expression of circadian rhythmicity. The second, called LBO5HT, is immunoreactive to 5-HT, a widely invoked transmitter system in insect circadian rhythms. The identification of these elements, and a widening cascade of events which their actions apparently trigger, opens up new opportunities to examine old problems in the regulation of circadian rhythms in the nervous system.