Ultradian Rhythm of Activity in Japanese Quail Groups under Semi-Natural Conditions during Ontogeny: Functional Aspects and Relation to Circadian Rhythm

The function of ultradian rhythms is not yet clearly elucidated. In particular, short-term rhythms are expressed during early ontogeny, especially in broods of precocial birds. We investigated the relationship between the clarity of the ultradian rhythm of the activity/rest cycle of a group of young Japanese quail (Coturnix japonica) and the level of social synchronisation and spatial cohesion between the birds within that group. The subjects were descended from two lines selected for either very pronounced rhythmic or arrhythmic circadian activity. We found a positive relationship between the clarity of the ultradian rhythm of the activity/rest cycle when birds were young and the clarity of the circadian rhythm of feeding activity when birds were older, but still immature. The temporal organisation of the behaviour of the chicks from these two lines was observed in outdoor aviaries, when they were 4, 8, 12 and 15 days old. The mean ultradian period expressed by groups of 12 chicks was variable, with a minimum of 6 minutes. The ultradian period lengthened regularly as chicks grew older, and reached approximately 40 min on day 15. The clarity of the ultradian rhythmicity of group activity was linked to the level of inter-individual social synchronisation and of spatial cohesion; the more pronounced the ultradian rhythms of a group, the greater the temporal and spatial cohesion of the chicks within the group. Moreover, these characteristics varied with the age of the chicks. Finally, chicks in the less rhythmic groups weighed less. These results stress the adaptive value of this temporal organisation strategy under natural conditions.     

Ultradian Rhythm of Activity in Japanese Quail Groups under Semi-Natural Conditions during Ontogeny: Functional Aspects and Relation to Circadian Rhythm

The function of ultradian rhythms is not yet clearly elucidated. In particular, short-term rhythms are expressed during early ontogeny, especially in broods of precocial birds. We investigated the relationship between the clarity of the ultradian rhythm of the activity/rest cycle of a group of young Japanese quail (Coturnix japonica) and the level of social synchronisation and spatial cohesion between the birds within that group. The subjects were descended from two lines selected for either very pronounced rhythmic or arrhythmic circadian activity. We found a positive relationship between the clarity of the ultradian rhythm of the activity/rest cycle when birds were young and the clarity of the circadian rhythm of feeding activity when birds were older, but still immature. The temporal organisation of the behaviour of the chicks from these two lines was observed in outdoor aviaries, when they were 4, 8, 12 and 15 days old. The mean ultradian period expressed by groups of 12 chicks was variable, with a minimum of 6 minutes. The ultradian period lengthened regularly as chicks grew older, and reached approximately 40 min on day 15. The clarity of the ultradian rhythmicity of group activity was linked to the level of inter-individual social synchronisation and of spatial cohesion; the more pronounced the ultradian rhythms of a group, the greater the temporal and spatial cohesion of the chicks within the group. Moreover, these characteristics varied with the age of the chicks. Finally, chicks in the less rhythmic groups weighed less. These results stress the adaptive value of this temporal organisation strategy under natural conditions.     

Effect of Light Intensity on Circadian Activity in Developing Japanese Quail

The effects of constant light on the expression of the circadian rhythm of feeding activity in Japanese quail, and in particular on the clarity of the rhythm were investigated. We used 46 4-week-old birds (35 females and 11 males) issued from two lines selected for a more (line R: 25 females and 10 males) or less (line A: 10 females and 1 male) clear circadian rhythm of feeding activity. The birds, were placed successively under three light schedules: constant darkness (DD), constant dim green light (LLdim) and constant bright light (LLbright). Schedules were changed every 2 weeks. Feeding activity was recorded continuously, analysed by autocorrelation and spectral analysis, and the ratio of the correlation coefficients and the area of the spectrum peak were used as indexes to quantify the clarity of the circadian rhythm. During the experiment, some birds showed gonadal development. Therefore, we analysed separately birds showing either a high or low degree of sexual development at the end of the experiment. In DD, 35 birds showed a circadian feeding rhythm with a mean period of 22.5 ± 0.1 h, whereas 11 birds showed an arrhythmic activity. In LLdim, 27 birds were rhythmic (22 birds R and 5 birds A), and in LLbright, only 3 birds showed a rhythmic circadian activity. For the R-line birds (for females and males), the rhythm clarity decreased in LLdim compared to DD, except for the not developed females. For the A-line birds (for females), the rhythm clarity of the immature birds increased in LLdim and that of the developed birds remained stable. In LLbright, circadian activity became arrhythmic. In LLdim, the active phases of 12 birds showed two main peaks, with mean periods of 22.7 h and 25.1 h, respectively. Therefore, constant light appeared to have an inhibitory effect on the expression of the circadian rhythm. We postulate that two hierarchically coupled oscillators could control circadian feeding activity, and arrhythmia in LLbright could be the results of internal desynchronization of the pacemakers.     

Effect of Light Intensity on Circadian Activity in Developing Japanese Quail

The effects of constant light on the expression of the circadian rhythm of feeding activity in Japanese quail, and in particular on the clarity of the rhythm were investigated. We used 46 4-week-old birds (35 females and 11 males) issued from two lines selected for a more (line R: 25 females and 10 males) or less (line A: 10 females and 1 male) clear circadian rhythm of feeding activity. The birds, were placed successively under three light schedules: constant darkness (DD), constant dim green light (LLdim) and constant bright light (LLbright). Schedules were changed every 2 weeks. Feeding activity was recorded continuously, analysed by autocorrelation and spectral analysis, and the ratio of the correlation coefficients and the area of the spectrum peak were used as indexes to quantify the clarity of the circadian rhythm. During the experiment, some birds showed gonadal development. Therefore, we analysed separately birds showing either a high or low degree of sexual development at the end of the experiment. In DD, 35 birds showed a circadian feeding rhythm with a mean period of 22.5 ± 0.1 h, whereas 11 birds showed an arrhythmic activity. In LLdim, 27 birds were rhythmic (22 birds R and 5 birds A), and in LLbright, only 3 birds showed a rhythmic circadian activity. For the R-line birds (for females and males), the rhythm clarity decreased in LLdim compared to DD, except for the not developed females. For the A-line birds (for females), the rhythm clarity of the immature birds increased in LLdim and that of the developed birds remained stable. In LLbright, circadian activity became arrhythmic. In LLdim, the active phases of 12 birds showed two main peaks, with mean periods of 22.7 h and 25.1 h, respectively. Therefore, constant light appeared to have an inhibitory effect on the expression of the circadian rhythm. We postulate that two hierarchically coupled oscillators could control circadian feeding activity, and arrhythmia in LLbright could be the results of internal desynchronization of the pacemakers.     

Circadian rhythm of activity during the annual phases in the European quail, Coturnix coturnix

Migratory birds, such as the European quail, present an annual cycle with the following phases: moult, fattening, migration and reproduction. This study aimed at determining how variations in the circadian rhythm of feeding during the annual cycle took endogenous rhythmic characteristics into account. The birds (n = 8) were maintained under constant dim light from the age of 1 to 9 months. Feeding activity was recorded using infra-red detectors. The birds expressed all the phases, except migration. Activity was arrhythmic when they were moulting. A circadian rhythm of feeding activity appeared during the fattening phase. In males, the circadian period lengthened and the clarity of the rhythm increased during sexual development. These results appear to confirm the effects of physiological state on the temporal organisation of activity. Variations of the circadian rhythm could influence the ability to synchronize with exogenous cycles such as the alternation of day and night.     

Cyclic melatonin synchronizes the circadian rhythm of feeding activity in Japanese quail,Coturnix c. japonica

In Japanese quail, we can observe the circadian rhythm of feeding activity in constant conditions, especially in birds from selected lines. In order to try to test the importance of melatonin as hormonal output for the circadian system, we gave a 24-h period cycle of exogenous melatonin to some of these birds when they were free running. We used castrated males firstly in order to cancel the known effect of steroids on circadian organisation. Secondly, as castrated birds generally expressed a very short periodicity, it allowed us to check induced synchronisation more easily. We maintained ten castrated males in constant dim light. We divided the experiment into five successive phases. The birds received a 24-h period cycle of melatonin (M phase) or of control solution with only the alcoholic solvent (C phase) as a drink. Before and after each one of these two phases, we gave water continually to drink (W1, W2 and W3 phases). Thus, the successive phases were W1-M-W2-C-W3. We measured intake of liquids and plasma melatonin concentrations to check melatonin ingestion. We automatically recorded individual feeding activity by infrared detectors, and analysed this by spectral analysis. At the beginning of the experiment, eight birds showed a rhythmic feeding activity, with a mean period of 22.9 +/- 0.2 h, and the two others an arrhythmic circadian activity. During the 24-h period cycle of exogenous melatonin, for the rhythmic birds, the circadian period became approximately 24 h (23.9 +/- 0.2 h), the inactive phase corresponding to the period of melatonin availability. During the W2 and C phases, the circadian period was similar to that expressed during the W1 phase. Moreover, when birds only drink water, we found a significant positive relationship between the clarity of the circadian rhythm and the ratio, between the melatonin level of the inactive phase and that of the active phase. These facts support the hypothesis of the role of this hormone in the regulation of the circadian system, at least for feeding activity, in quail.     

Epigenetic Maternal Effects on Endogenous Rhythms in Precocial Birds

Development involves interactions between genetic and environmental influences. Vertebrate mothers are generally the first individuals to encounter and interact with young animals. Thus, their role is primordial during ontogeny. The present study evaluated non‐genomic effects of mothers on the development of rhythms of precocial Japanese quail (Coturnix c. japonica). First, we investigated the influence of mothering on the ontogeny of endogenous rhythms of young. We compared circadian and ultradian rhythms of feeding activity of quail reared with or without adoptive mothers. More brooded than non‐brooded quail presented a circadian and/or an ultradian rhythm. Thus, the presence of the mother during the normal brooding period favors, in the long term, expression of rhythms in the young. Second, we investigated the influence of rhythmic phenotype of the mother on the development of endogenous rhythms of young by comparing quail brooded by circadian‐rhythmic adoptive mothers (R) to quail brooded by circadian‐arrhythmic adoptive mothers (A). More R‐brooded than A‐brooded quail expressed circadian rhythmicity, and circadian rhythm clarities were greater in R‐brooded than A‐brooded quail. Ultradian rhythmicity did not differ between R‐ and A‐brooded quail, nor between R and A adoptive mothers. Thus, the rhythmic phenotypes of quail mothers influence the rhythmic phenotypes of their young. Our results demonstrate that mothers of precocial birds influence epigenetically the ontogeny of endogenous rhythms of the young they raise.     

Circadian Rhythm of Activity in Japanese Quail in Constant Darkness: Variability of Clarity and Possibility of Selection

In conditions of constant darkness, interindividual variability in the clarity of circadian rhythmicity was observed in sexually immature young quail, with birds classified as more or less rhythmic or arrhythmic. The relative clarity of this circadian rhythm was observed on the actograms by measurement of the autocorrelation coefficient ratio over 12 cycles. Autocorrelation coefficients were calculated from sequential series of total activity over 12-minute periods. Crosses of selected phenotypes with different clarities of rhythmicity were conducted in order to study the possibility of selection of this characteristic. From a random population (N = 42, twice), pairs of the most rhythmic birds (3 families), and pairs of arrhythmic birds (4 families) were reared. Autocorrelation coefficient ratios of Fl birds from rhythmic families (N = 54) were greater than those of Fl birds (N = 48) from arrhythmic families (t-test, p <. 0001). These ratios in offspring were significantly correlated with that of the mean parent of each clutch of siblings (N= 102, r =.35, p =. 0003). This result was maintained in a second generation (F2) of birds, for which significant differences in expressed rhythmicity were observed. That is, autocorrelation coefficient ratios of F2 birds from two rhythmic families (TV = 30) were greater than those of F2 birds from arrhydimic families (N = 20) (t-test, p =. 039). Comparison of F2 outbred and inbred birds from rhythmic pairs showed greater values of autocorrelation coefficient ratios in the case of inbred birds (N = 16) than for outbred birds (N = 30; t-test, p =. 036). There was no difference between outbred (N = 20) and inbred birds (N = 15) from arrhythmic pairs. Therefore, selection of a rhythmic strain seems possible, whereas crosses between two arrhythmic birds may also give rise to rhythmic birds. Comparisons between rhythmic birds of different families did not show differences in the free-running period of the circadian rhythm, which is true also for rhythmic birds bred from two arrhythmic parents. Therefore, our selection procedure did not seem to be based on the characteristics of the pacemaker itself, but rather on a downstream event. Although the parents were not selected on the basis of quantity of activity per cycle or on the duration of the active phase, significant differences among the offspring of different families were shown. (Chronobiology International, 75(3), 219–230, 1998)     

Attenuated blood corticosterone rhythm in rats with jejunal resection

Circadian rhythmic changes in blood corticosterone concentration were studied in rats after resection of the jejunum or the ileum. The rats with ideal resection showed a normal corticosterone rhythm, with a peak at the beginning of the dark period when they were fed ad libitum, and the phase of the rhythm shifted when the feeding time was restricted to a specific time of day during the light period. The rats with jejunal resection also showed a similar corticosterone rhythm, but its amplitude was lower compared to that of the rats with ideal resection. There were no differences in body weight and the circadian rhythm of blood urea concentration between two groups of rats. We conclude that the jejunum is an important site where the sense of food is received as an entraining signal for the corticosterone rhythm.     

Circadian rhythms of oviposition and feeding activity in Japanese quail: effects of cyclic administration of melatonin

The aim of these experiments was to test the effect of a cyclic administration of melatonin, by mimicking the daily rhythm of hormone levels, on the circadian organization of two distinct functions in quail: oviposition and feeding activity. Laying and feeding rhythms under photoperiodic conditions and constant darkness (DD) were investigated. Under DD, where the two rhythms were free running, a daily rhythm of melatonin was administered. In LD 14h:10h, two different individual profiles of laying were established, with stable females laying at the same time each day and delayed females laying progressively later each day. For feeding activity, all birds were clearly synchronized to the photoperiodic cycle. In DD, the laying birds showed a free-running rhythm of oviposition with a period longer than 24 h for both profiles but the delayed profile females had a longer period than stable profile females. In comparison, the free-running period of feeding rhythm of the same birds was shorter than 24 h. A cyclic administration of melatonin had no effect on laying rhythm, which continued to free-run in DD, whereas feeding activity was synchronized as soon as the first cycle of melatonin was administered. From these results, it seems that two different circadian systems drive each of the two types of behavior separately. Melatonin could be the main synchronizer for the temporal control of feeding behavior, but it does not play a part in the control of oviposition in Japanese quail.     

Biorhythms and pineal gland

Endocrine biorhythms are classified according to the period time, as one of the most characteristic properties of biorhythms. Each endocrine organ has parallel more than one biorhythms with different period time (e. g. circadian and circannual rhythms). The time of acrophase of the biorhythms at the different endocrine organs is fairly variant. This review summarizes the rhythmic function of the THS-thyroid, gonadotrophic-gonadal and ACTH-adrenocortical systems. Pineal gland plays an integrative role in the regulation of rhythmic function of the endocrine system. The melatonin secretion of this gland also reveals conspicuous circadian and circannual rhythms both in mammals and in birds. Mammalian pineal is functional only if its peripheral sympathetic innervation from the superior cervical ganglion is intact. In contrast, melatonin secretion and its circadian rhythm is also maintained in birds under isolated conditions (explanted into an in vitro superfusion system). The 24 hours period time of melatonin circadian rhythm can not be changed by light impulses. The phases of the circadian rhythm, however, can be turned by changing the time of environmental light-dark phases. The wavelength of the artificial light used for reversal of circadian rhythm is an important factor. The development of the entrainment and synchronization of the circadian melatonin rhythm in birds is independent of the rhythmic day-night changes in environmental lighting condition. The differences in the main elements of the biological clock between mammals and birds are discussed.     

Diel profile of behaviour in the smooth newt, Triturus vulgaris (L.): an analysis of environmental cues and endogenous timing

Diel activity in a population of smooth newts during the aquatic period, was compared with rhythmic activity in newts under controlled light-dark and temperature cycles in the laboratory. The light-dark cycle was the primary synchronizer of activity and entrained a bimodal activity rhythm. This rhythm was endogenous and persisted for several cycles under constant conditions. Under natural conditions this rhythm was modified by illumination and temperature to produce a crepuscular activity pattern with a dominant second peak in activity. Diel activity under natural conditions was partitioned into sexual behaviour and feeding. Whereas sexual behaviour was entirely crepuscular, feeding behaviour was nocturnal. Feeding was probably unrelated to the underlying circadian rhythm and was engendered by the diel migration of zooplankton in the ponds. Ascents to the pond surface for air were most frequent during periods of sexual behaviour.     

Melatonin and the circadian rhythms of feeding and perch-hopping in the European starling,Sturnus vulgaris

Summary The role of melatonin plasma titers in the control of free-running circadian rhythms was investigated in European starlings,Sturnus vulgaris, held in continuous dim light. Simultaneous recordings of plasma melatonin, perch-hopping and feeding activity revealed synchronous circadian variations in all three functions with high melatonin titers during resting and low titers during activity periods. Implanting birds with melatonin silastic capsules resulted in a 100-fold increase in plasma concentrations and an abolition of the endogenous melatonin rhythm. Feeding rhythms persisted during this treatment, although the activity time () was lengthened and the circadian period () shortened. Similar changes were found in perchhopping activity, but in some birds, this activity was more or less suppressed. These data demonstrate that although melatonin plasma titers freerun synchronously with other circadian parameters, fluctuations in peripheral melatonin are not essential for the expression of circadian activity.     

Exogenous melatonin reduces the resynchronization time after phase shifts of a nonphotic zeitgeber in the house sparrow (Passer domesticus)

Continuous melatonin administration via silastic implants accelerates the resynchronization of the circadian locomotor activity rhythm in house sparrows (Passer domesticus) after exposure to phase shifts of a weak light-dark cycle. Constant melatonin might induce this effect either by increasing the sensitivity of the visual system to a light zeitgeber or by reducing the degree of self-sustainment of the circadian pacemaker. To distinguish between these two possible mechanisms, two groups of house sparrows, one carrying melatonin implants and the other empty implants, were kept in constant dim light and subjected to advance and delay shifts of a 12-h feeding phase. The resynchronization times of their circadian feeding rhythm following the phase shifts were significantly shorter when the birds carried melatonin implants than when they carried empty implants. In a second experiment, melatonin-implanted and control birds were released into food ad libitum conditions 2 days after either a delay or an advance phase shift. The number of hours by which the activity rhythms had been shifted on the second day in food ad libitum conditions was assessed. Melatonin-implanted house sparrows had significantly larger phase shifts in their circadian feeding rhythm than control birds. This is in accordance with the first experiment since a larger phase shift at a given time reflects accelerated resynchronization. Additionally, the second experiment also excludes any possible masking effects of the nonphotic zeitgeber. In conclusion, constant melatonin accelerates resynchronization even after phase shifts of a nonphotic zeitgeber, indicating that constant high levels of melatonin can reduce the degree of self-sustainment of the circadian pacemaker independent of any effects on the photoreceptive system.     

Differential effects of pinealectomy on circadian rhythms of feeding and perch hopping in the European starling

To study the effects of pinealectomy on the circadian rhythms of locomotor activity and feeding. European starlings (Sturnus vulgaris) were held in constant light (0.2 lux and 200 lux) and under constant temperature conditions. Locomotor activity was measured by means of perches with microswitches mounted underneath, and feeding with an infrared photocell system at the feeder. Pinealectomy consistently led to disturbances in perch-hopping rhythms and often to a complete loss of rhythmicity as revealed by periodogram analysis. In some birds, perch-hopping rhythms recovered following a period of initial arrhythmicity. When a perch-hopping rhythm was present, its period was usually shorter than it had been before pinealectomy. In contrast to its effects on perch hopping, pinealectomy had no effect on the persistence of feeding rhythmicity, although its period, like that of the hopping rhythm, decreased after this operation. These results support the hypothesis derived from previous studies that the circadian organization of feeding is different from that of perch hopping. Different circadian pacemakers may be involved, but other models may possibly explain the data just as well.     

Food Intake and Circadian Rhythms of Activity of Red-Winged Blackbirds (Agelaius phoeniceus). A Time-Course Study on the Effects of Alpha-Chloralose and Secobarbital

Anesthetic drugs like alpha-chloralose and secobarbital are used to capture wild birds. The doses are usually chosen on the basis of low mortality rates and high capture success. However, little information is available on the time necessary for the birds to fully recover from the drugs' effects after they have regained consciousness. In this work, we used circadian rhythms of activities to study the long-term effects of a mixture of alpha-chloralose and secobarbital on adult male Red-winged Blackbirds. The birds were housed individually in sound-proof boxes for 30 days, during which they received alpha-chloralose and secobarbital mixed with corn, either on day 3 or on day 15. On the day of treatment, the birds experienced a significant decrease in activity level, including feeding activities, which lasted more than 22 days for the birds treated on day 3, compared to about 8 days for the birds treated on day 15. Moreover, typical circadian distribution of activity was lost and many birds became aperiodic. Two weeks were needed for the birds treated on day 3 to recover normal rhythms, compared to less than 9 days for those treated on day 15. These results indicate that circadian parameters can be used to detect after-effects of intoxication long after the birds have regained consciousness.     

Different circadian pacemakers control feeding and locomotor activity rhythms in European starlings

Summary In higher organisms, many physiological and behavioral functions exhibit daily variations, generated by endogenous circadian oscillators. It is not yet clear whether all the various rhythms that occur within an individual depend on one and the same pacemaker or whether different pacemakers are involved. To examine this question, the feeding and perch-hopping rhythms were measured in European starlings (Sturnus vulgaris) under light-dark cycles and continuous dim light. In dim light, the internal phase relationship between the feeding and perch-hopping rhythms changed systematically as a function of the circadian period, and the two rhythms could even dissociate and show different circadian periods in individuals with extremely long or extremely short circadian periods. Moreover, in some birds kept on lowamplitude light-dark cycles, the rhythm of feeding was synchronized 180° out of phase with the rhythm of locomotor activity. These results strongly suggest that in the European starling the feeding and locomotor activity rhythms are controlled by separate circadian pacemakers.     

Regulation of the Photosynthesis Rhythm in Euglena gracilis: I. Carbonic Anhydrase and Glyceraldehyde-3-Phosphate Dehydrogenase Do Not Regulate the Photosynthesis Rhythm

A circadian rhythm of O₂ evolution has been found in Euglena gracilis, Klebs strain Z. The rhythm persists for at least 5 days in constant dim light and temperature, but damps out in constant bright light. The phase of this rhythm can be shifted by a pulse of bright light and the period length is not changed over a 10 C span of growth temperature.

The O₂ evolution rhythm is found in both logarithmic and stationary phase cultures, but CO₂ uptake is clearly rhythmic only in stationary phase cultures.

The activity of glyceraldehyde-3-phosphate dehydrogenase was not rhythmic as previously reported (Walther and Edmunds [1973] Plant Physiol. 51: 250-258). Carbonic anhydrase activity was rhythmic when the cultures were maintained under a light-dark cycle with the highest enzyme activity coinciding with the fastest rate of O₂ evolution. However, the rhythm in carbonic anhydrase activity disappeared under constant conditions. Changes in the activities of these two enzymes are therefore not responsible for the rhythmic changes in photosynthetic capacity.

     

Food Intake and Circadian Rhythms of Activity of Red-Winged Blackbirds (Agelaius phoeniceus). A Time-Course Study on the Effects of Alpha-Chloralose and Secobarbital

Anesthetic drugs like alpha-chloralose and secobarbital are used to capture wild birds. The doses are usually chosen on the basis of low mortality rates and high capture success. However, little information is available on the time necessary for the birds to fully recover from the drugs' effects after they have regained consciousness. In this work, we used circadian rhythms of activities to study the long-term effects of a mixture of alpha-chloralose and secobarbital on adult male Red-winged Blackbirds. The birds were housed individually in sound-proof boxes for 30 days, during which they received alpha-chloralose and secobarbital mixed with corn, either on day 3 or on day 15. On the day of treatment, the birds experienced a significant decrease in activity level, including feeding activities, which lasted more than 22 days for the birds treated on day 3, compared to about 8 days for the birds treated on day 15. Moreover, typical circadian distribution of activity was lost and many birds became aperiodic. Two weeks were needed for the birds treated on day 3 to recover normal rhythms, compared to less than 9 days for those treated on day 15. These results indicate that circadian parameters can be used to detect after-effects of intoxication long after the birds have regained consciousness.     

A novel circadianly expressed Drosophila melanogaster gene dependent on the period gene for its rhythmic expression.

The Drosophila melanogaster period (per) gene is required for expression of endogenous circadian rhythms of locomotion and eclosion. per mRNA is expressed with a circadian rhythm that is dependent on Per protein; this feedback loop has been proposed to be essential to the central circadian pacemaker. This model would suggest the Per protein also controls the circadian expression of other genetic loci to generate circadian behavior and physiology. In this paper we describe Dreg-5, a gene whose mRNA is expressed in fly heads with a circadian rhythm nearly identical to that of the per gene. Dreg-5 mRNA continues to cycle in phase with that of per mRNA in conditions of total darkness and also when the daily feeding time is altered. Like per mRNA, Dreg-5 mRNA is not expressed rhythmically in per null mutant flies. Dreg-5 encodes a novel 298 residue protein and Dreg-5 protein isoforms also oscillate in abundance with a circadian rhythm. The phase of Dreg-5 protein oscillation, however, is different from that of Per protein expression, suggesting that Dreg-5 and per have common translational but different post-translational control mechanisms. These results demonstrate that the per gene is capable of modulating the rhythmic expression of other genes; this activity may form the basis of the output of circadian rhythmicity in Drosophila.