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

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

Varying the Length of Dim Nocturnal Illumination Differentially Affects the Pacemaker Controlling the Locomotor Activity Rhythm of Drosophila Jambulina

Photic entrainment of animals in the field is basically attributed to their exposure to the dimly lit nights flanked by the dawn and dusk twilight transitions. This implicates the functional significance of the dimly lit nights as that of the twilight transitions. Recently, the authors have demonstrated that the dimly lit night at 0.0006 lux altered the attributes of the circadian rhythm of locomotor activity of Drosophila jambulina. The present study examined whether the durations of such dimly lit nights affect the entrainment and free-running rhythmicity of D. jambulina. Flies were subjected for 10 days to two types of 24-h lighting regimes in which the photophase (L) was at 10 lux for all flies but the scotophase, which varied in duration from 9 to 15?h, was either at 0 lux (D phase) for control flies or 0.0006 lux (the artificial starlight or S phase) for experimental flies. Thereafter, they were transferred to constant darkness (DD) to compare the after-effects of the dimly lit nights on the period (τ) of free-running rhythm in DD with that of the completely dark nights. Control flies were entrained by all LD cycles, but the experimental flies were entrained only by five LS cycles in which the duration of the S phases ranged from 10 to 14?h. The two LS cycles with very short (9?h) and long (15?h) S phases rendered the flies completely arrhythmic. Control flies started activity shortly before lights-on and continued well after lights-off. The experimental flies, however, commenced activity several hours prior to lights-on but ended activity abruptly at lights-off as the result of a negative masking effect of nocturnal illumination. Length of the midday rest was considerably shorter in the control than in the experimental flies in each lighting regime. The active phase in the control flies was predictably shortened; nonetheless, it was invariable in the experimental flies as the nights lengthened. Transfer from lighting regimes to DD initiated robust free-running rhythmicity in all flies including the arrhythmic ones subjected to LS cycles with 9 and 15?h of scotophases. The τ was profoundly affected by the nocturnal irradiance of the prior entraining lighting regime, as it was always shorter in the experimental than in the control flies. Thus, these results indisputably demonstrate the changes in fundamental properties of the circadian pacemaker of D. jambulina were solely attributed to the extremely dim nocturnal irradiance. This strain of D. jambulina is entrained essentially by the dimly lit natural nights, since it is never exposed to the prevailing photic cues such as the twilight transitions or bright photoperiod, owing to the dense vegetation of its habitat. (Author correspondence: drdsjoshi08@gmail.com)     

LIGHT AT NIGHT ALTERS THE PARAMETERS OF THE ECLOSION RHYTHM IN A TROPICAL FRUIT FLY,DROSOPHILA JAMBULINA

We investigated the effects of natural light at night (LAN) in the field and artificial LAN in the laboratory on the circadian rhythm of pupal eclosion in a tropical wild type strain of Drosophila jambulina captured at Galle, Sri Lanka (6.1^oN, 80.2^oE). The influence of natural LAN, varying in intensity from 0.004 lux (starlight intensity) to 0.45 lux (moonlight intensity), on the entrainment pattern of the circadian rhythm of eclosion at 25^o?±?0.5^oC was examined by subjecting the mixed-aged pupae to natural cycles of light and darkness at the breeding site of this strain in the field. The eclosion peak was ～2?h prior to sunrise, and the 24?h rhythmicity was the most robust. Effects of artificial LAN at 25^o?±?0.5^oC were determined in the laboratory by subjecting pupae to LD 12:12 cycles in which the light intensity of the photophase was 500 lux in all LD cycles, while that of the scotophase was either 0 lux (complete darkness, DD), 0.5, 5, or 50 lux. In the 0 lux LAN condition (i.e., the control experiment), the eclosion peak was ～2?h after lights-on, and the 24?h eclosion rhythm was not as strong as in the 0.5 lux LAN condition. The entrainment pattern in 0.5 lux LAN was strikingly similar to that in the field, as the 0.5 lux LAN condition is comparable to the full moonlight intensity in the tropics. LAN at 0.5 lux dramatically altered both parameters of entrainment, as the eclosion peak was advanced by ～4?h and the 24?h eclosion rhythm was better than that of the control experiment. LAN at 5 lux, however, resulted in a weak eclosion rhythm that peaked in the subjective forenoon. Interestingly, the 50 lux LAN condition rendered the eclosion events unambiguously arrhythmic. After-effects of LAN on the period (τ) of the free-running rhythm and the nature of eclosion rhythm were also determined in DD by a single LD 12:12 to DD transfer. After-effects of the LAN intensity were observed on both the τ and nature of the eclosion rhythm in all four experiments. Pupae raised in 0.5 lux LAN exhibited the shortest τ (20.6?±?0.2?h, N?=?11 for this and subsequent values) and the most robust rhythm, while pupae raised in 50 lux LAN had the longest τ (29.5?±?0.2?h) and weakest rhythm in DD. Thus, these results demonstrate the intensity of LAN, varying from 0 to 50 lux, profoundly influences the parameters of entrainment as well as free-running rhythmicity of D. jambulina. Moreover, the observed arrhythmicity in LD 12:12 cycles caused by the 50 lux LAN condition appeared to be the masking effect of relatively bright light at night, as the LD 12:12 to DD transfer restored the rhythmicity, although it was rather weak. (Author correspondence: drdsjoshi08@gmail.com)     

Influence of Photoperiod in Accelerating the Reentrainment in Drosophila

Efficacy of the short photoperiod (Spp) and the long photoperiod (Lpp) in accelerating the reentrainment was assessed in Drosophila biarmipes. The Spp accelerated the reentrainment after the phase advance of light-dark (LD) cycles, which was associated with the early activity onset (Ψo) and the short period of free-running rhythm (τ). The Lpp accelerated the reentrainment after the phase delay of LD cycles, which was associated with the late Ψo and the long τ. This study indicates that the photoperiodic modulation of the circadian waveform of the underlying pacemaker that controls activity rhythm influenced the rate of reentrainment in D. biarmipes. (Author correspondence: drdsjoshi08@gmail.com)     

Circadian Rhythms and time Measurement in Locomotor Activity of the Scorpion Leiurus Quinquestriatus (Buthidae)

The circadian rhythms of locomotor activity of the scorpion Leiurus quinqueslriatus were examined under different light-dark cycles and in free-running conditions. The circadian rhythm is bimodal in LD 12:12 with alternating cycles of temperature (35°-25°C) with high intensity (1300 lux) or in LD 12: 12 with constant temperature 35° C with 300 lux. In LD 12:12 (1300 lux), in long or in short light spans with constant temperature, the bimodal pattern is slightly changed with the appearance of a third minor peak of activity. In free-running conditions, the bimodal rhythm of locomotor activity persists in DD with T about 24 hr, but in LL the rhythm becomes unimodal with T about 24 hr. Cosinor and power spectrum analysis showed the presence of more than one periodic component. It seems that there is a correlation between the range of light regimens, temperature, light intensity and the coincidence of these components. These components are independently entrained by the environmental light cycle. The mechanism of entrainment of components is discussed.     

Varying the length of dim nocturnal illumination differentially affects the pacemaker controlling the locomotor activity rhythm of Drosophila jambulina

Photic entrainment of animals in the field is basically attributed to their exposure to the dimly lit nights flanked by the dawn and dusk twilight transitions. This implicates the functional significance of the dimly lit nights as that of the twilight transitions. Recently, the authors have demonstrated that the dimly lit night at 0.0006 lux altered the attributes of the circadian rhythm of locomotor activity of Drosophila jambulina. The present study examined whether the durations of such dimly lit nights affect the entrainment and free-running rhythmicity of D. jambulina. Flies were subjected for 10 days to two types of 24-h lighting regimes in which the photophase (L) was at 10 lux for all flies but the scotophase, which varied in duration from 9 to 15 h, was either at 0 lux (D phase) for control flies or 0.0006 lux (the artificial starlight or S phase) for experimental flies. Thereafter, they were transferred to constant darkness (DD) to compare the after-effects of the dimly lit nights on the period (τ) of free-running rhythm in DD with that of the completely dark nights. Control flies were entrained by all LD cycles, but the experimental flies were entrained only by five LS cycles in which the duration of the S phases ranged from 10 to 14 h. The two LS cycles with very short (9 h) and long (15 h) S phases rendered the flies completely arrhythmic. Control flies started activity shortly before lights-on and continued well after lights-off. The experimental flies, however, commenced activity several hours prior to lights-on but ended activity abruptly at lights-off as the result of a negative masking effect of nocturnal illumination. Length of the midday rest was considerably shorter in the control than in the experimental flies in each lighting regime. The active phase in the control flies was predictably shortened; nonetheless, it was invariable in the experimental flies as the nights lengthened. Transfer from lighting regimes to DD initiated robust free-running rhythmicity in all flies including the arrhythmic ones subjected to LS cycles with 9 and 15 h of scotophases. The τ was profoundly affected by the nocturnal irradiance of the prior entraining lighting regime, as it was always shorter in the experimental than in the control flies. Thus, these results indisputably demonstrate the changes in fundamental properties of the circadian pacemaker of D. jambulina were solely attributed to the extremely dim nocturnal irradiance. This strain of D. jambulina is entrained essentially by the dimly lit natural nights, since it is never exposed to the prevailing photic cues such as the twilight transitions or bright photoperiod, owing to the dense vegetation of its habitat.     

Effects of photophase and altitude on oviposition rhythm of the himalayan strains of Drosophila ananassae

The effects of varying photophase and altitude of origin on the phase angle difference (Ψ) of the circadian rhythm of oviposition during entrainment to light-dark (LD) cycles and the aftereffects of such photophases on the period of the free-running rhythm (τ) in constant darkness (DD) were evaluated in two Himalayan strains of Drosophila ananassae, the high-altitude (HA) strain from Badrinath (5,123 m above sea level=ASL) and the low-altitude (LA) strain from Firozpur (179 m ASL). The Ψ (i.e., the hours from lights-on of the LD cycle to oviposition median) of both strains was determined in LD cycles in which the photophase at 100 lux varied from 6 to 18 h/24 h. The HA strain was entrained by all LD cycles except the one with 6 h photophase in which it was weakly rhythmic, but the LA strain was entrained by only three LD cycles with photophases of 10, 12, and 14 h, but photophases of 6, 8, 16, and 18 h rendered it arrhythmic. Lights-off transition of LD cycles was the phase-determining signal for both strains as oviposition medians of the HA strain occurred∼6 h prior to lights-off, while those of the LA strain occurred∼1 h after lights-off. The Ψ of the HA strain increased from∼2 h in 8 h photophase to∼11 h in 18 h photophase, while that of the LA strain increased from∼11 h in 10 h photophase to∼15 h in 14 h photophase. The aftereffects of photophase of the prior entraining LD cycles on τ in DD were determined by transferring flies from LD cycles to DD. The τ of the HA strain increased from∼19 to∼25 h when transferred to DD from LD 8:16 and LD 18:6 cycles, respectively, whereas the τ of the LA strain increased from∼26 to∼28 h when transferred to DD from LD 10:14 and LD 14:10 cycles, respectively. Thus, these results demonstrate that the photophases of entraining LD cycles and the altitude of origin affected several parameters of entrainment and the period of the free-running rhythm of these strains.     

Effects of Photophase and Altitude on Oviposition Rhythm of the Himalayan Strains of Drosophila Ananassae

The effects of varying photophase and altitude of origin on the phase angle difference (Ψ) of the circadian rhythm of oviposition during entrainment to light‐dark (LD) cycles and the aftereffects of such photophases on the period of the free‐running rhythm (τ) in constant darkness (DD) were evaluated in two Himalayan strains of Drosophila ananassae, the high‐altitude (HA) strain from Badrinath (5,123 m above sea level=ASL) and the low‐altitude (LA) strain from Firozpur (179 m ASL). The Ψ (i.e., the hours from lights‐on of the LD cycle to oviposition median) of both strains was determined in LD cycles in which the photophase at 100 lux varied from 6 to 18 h/24 h. The HA strain was entrained by all LD cycles except the one with 6 h photophase in which it was weakly rhythmic, but the LA strain was entrained by only three LD cycles with photophases of 10, 12, and 14 h, but photophases of 6, 8, 16, and 18 h rendered it arrhythmic. Lights‐off transition of LD cycles was the phase‐determining signal for both strains as oviposition medians of the HA strain occurred～6 h prior to lights‐off, while those of the LA strain occurred～1 h after lights‐off. The Ψ of the HA strain increased from～2 h in 8 h photophase to～11 h in 18 h photophase, while that of the LA strain increased from～11 h in 10 h photophase to～15 h in 14 h photophase. The aftereffects of photophase of the prior entraining LD cycles on τ in DD were determined by transferring flies from LD cycles to DD. The τ of the HA strain increased from～19 to～25 h when transferred to DD from LD 8:16 and LD 18:6 cycles, respectively, whereas the τ of the LA strain increased from～26 to～28 h when transferred to DD from LD 10:14 and LD 14:10 cycles, respectively. Thus, these results demonstrate that the photophases of entraining LD cycles and the altitude of origin affected several parameters of entrainment and the period of the free‐running rhythm of these strains.     

History-dependent changes in entrainment of the activity rhythm in the Syrian hamster (Mesocricetus auratus)

The authors have studied the activity rhythm of Syrian hamsters exposed to square LD cycles with a 22-h period (T22) with the aim of testing the effects of the previous history on the rhythmic pattern. To do so, sequential changes of different lighting environments were established, followed by the same LD condition. Also, the protocol included T22 cycles with varying lighting contrasts to test the extent to which a computational model predicts experimental outcomes. At the beginning of the experiment, exposure to T22 with 300 lux and dim red light occurring respectively at photophase and scotophase (LD300/dim red) mainly generated relative coordination. Subsequent transfer to cycles with approximately 0.1-lux dim light during the scotophase (LD300/0.1) promoted entrainment to T22. However, a further reduction in light intensity to 10 lux during the photophase (LD10/0.1) generated weak and unstable T22 rhythms. When, after that, animals were transferred again to the initial LD300/dim red cycles, the amplitude of the rhythm still remained very low, and the phases were very unstable. Exposure to constant darkness partially restored the activity rhythm, and when, afterwards, the animals were submitted again to LD300/dim red cycles, a robust T22 rhythm appeared. The results demonstrate history-dependent changes in the hamster circadian system because the locomotor activity pattern under the same T22 cycle can show relative coordination or unstable or robust entrainment depending on the prior lighting condition. This suggests that the circadian system responds to environmental stimuli depending on its previous history. Moreover, computer simulations allow the authors to predict entrainment under LD300/0.1 cycles and indicate that most of the patterns observed in the animals due to the light in the scotophase can be explained by different degrees of coupling among the oscillators of the circadian system.     

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

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

Temperature dependent eclosion rhythmicity in the high altitude Himalayan strains of Drosophila ananassae

The circadian pacemaker controlling the eclosion rhythm of the high altitude Himalayan strains of Drosophila ananassae captured at Badrinath (5123 m) required ambient temperature at 21°C for the entrainment and free-running processes. At this temperature, their eclosion rhythms entrained to 12h light, 12h dark (LD 12:12) cycles and free-ran when transferred from constant light (LL) to constant darkness (DD) or upon transfer to constant temperature at 21°C following entrainment to temperature cycles in DD. These strains, however, were arrhythmic at 13 or 17°C under identical experimental conditions. Eclosion medians always occurred in the thermophase of temperature cycles whether they were imposed in LL or DD; or whether the thermophase coincided with the photophase or scotophase of the concurrent LD 12:12 cycles. The temperature dependent rhythmicity in the Himalayan strains of D. ananassae is a rare phenotypic plasticity that might have been acquired through natural selection by accentuating the coupling sensing mechanism of the pacemaker to temperature, while simultaneously suppressing the effects of light on the pacemaker.     

Effects of temperature, photoperiod, and light intensity on the eclosion rhythm of the high-altitude Himalayan strain of Drosophila ananassae

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

Effect of aging on the photic entrainment in the frugivorous bat, Rousettus leschenaulti

Aging disrupted the photic entrainment in old (∼15 years) frugivorous Rousettus leschenaulti bats as natural light - dark (LD) cycles in the field or artificial LD cycles of 12 h of light at 2000 lux and 12 h of complete darkness failed to entrain them (Vanlalnghaka & Joshi 2005; Vanlalnghaka et al. 2005). The results were attributed to the age-related decline in photic sensitivity and/or the period of zeitgeber (T) deviating too much from the free-running period (τ) of bats. In the present study, the old bats were subjected to LD cycles in which the intensity of the photophase was raised to 6000 lux and Ts of 23.2 h and 24.9 h were exactly that of τ in the scotophase of LD cycles and in constant darkness, respectively. These LD cycles also failed to entrain the old bats which unambiguously demonstrates that aging in R. leschenaulti disrupted the integrity of the photic entrainment mechanism.     

Circadian rhythm of locomotor activity in the Japanese honeybee, Apis cerana japonica

Abstract.  To reveal circadian characteristics and entrainment mechanisms in the Japanese honeybee Apis cerana japonica , the locomotor-activity rhythm of foragers is investigated under programmed light and temperature conditions. After entrainment to an LD 12 : 12 h photoperiodic regime, free-running rhythms are released in constant dark (DD) or light (LL) conditions with different free-running periods. Under the LD 12 : 12 h regime, activity offset occurs approximately 0.4 h after lights-off transition, assigned to circadian time (Ct) 12.4 h. The phase of activity onset, peak and offset, and activity duration depends on the photoperiodic regimes. The circadian rhythm can be entrained to a 24-h period by exposure to submultiple cycles of LD 6 : 6 h, as if the locomotive rhythm is entrained to LD 18 : 6 h. Phase shifts of delay and advance are observed when perturbing single light pulses are presented during free-running under DD conditions. Temperature compensation of the free-running period is demonstrated under DD and LL conditions. Steady-state entrainment of the locomotor rhythm is achieved with square-wave temperature cycles of 10 °C amplitude, but a 5 °C amplitude fails to entrain.     

Two oscillators might control the locomotor activity rhythm of the high-altitude Himalayan strain of Drosophila helvetica

The properties of the pacemaker controlling the adult locomotor activity rhythm of the high-altitude Himalayan (haH) strain (Hemkund Sahib, 4121 m above sea level) of Drosophila helvetica are strikingly different from those of the low-altitude Himalayan (laH) strain (Birahi, 1132 m above sea level) of the same species. The haH strain has a unimodal activity pattern with a delayed peak occurring about 4.5 h after lights-on of the entraining light-dark (LD) cycle, while the laH strain has a bimodal activity pattern with the morning and evening peaks. It is rather unusual for a wild type strain of any Drosophila species to have a unimodal activity pattern during entrainment as observed in the haH strain. The single activity peak of the haH strain is regarded as a consequence of delayed morning peak merging with the evening one. Three experiments were performed to test this hypothesis. The first experiment examined whether the single activity peak could be dissociated into two components by LD cycles in which photoperiods varied from 10 to 16 h per 24 h. The haH strain again exhibited a unimodal activity pattern with a delayed peak in 10, 12, and 14 h photoperiods but a bimodal activity pattern in 16 h photoperiod. The laH strain had bimodality in 10 and 12 h photoperiods, unimodality in a 14 h photoperiod, but complete arrhythmicity in a 16 h photoperiod. In the second experiment, the haH flies were transferred from LD 16:8 to LL at 5 lux to confirm whether the bimodality of this strain in LD 16:8 cycles was not the result of masking by the long photoperiod of 16 h. Bimodality of the haH strain persisted in LL too; moreover, the morning component free-ran with period (tau) <24 h, while the evening component free-ran with tau>24 h. The third experiment examined the LL-induced splitting of activity peak of the haH strain. Flies were transferred from LD 12:12 cycles to LL at 0, 1, 5, and 15 lux. The haH strain was rhythmic in LL at 0 and 1 lux with a unimodal activity pattern. It was also rhythmic in LL at 5 lux, but the single activity peak was split into two discrete components; the morning component free-ran with tau<24 h, while the evening component free-ran with tau>24 h. This strain, however, was completely arrhythmic in LL at 15 lux. The laH strain was uniformly arrhythmic in LL at all levels of light intensity. These results suggest that the single but late activity component of the haH strain during entrainment appears to be the consequence of merging the delayed morning peak with the evening one as an adaptation to the environmental conditions at the altitude of origin of this strain, where these flies begin activity in the forenoon owing to non-permissible low temperature in the morning.     

Dark switch in the entrainment of circadian activity rhythms in night monkeys. Aotus trivirgatus humboldt

1. The locomotor activity of the night monkey (Aotus trivirgatus) has been shown to be related to light intensity by an optimum function; here entrainment by LD cycles is examined to see whether the mechanism of synchronization of circadian periodicity in Aotus is based on this function. 2. Eleven night monkeys of various ages, previously in either a free-running phase or in LD 12:12 (10(2):10(-1) lux), were recorded in LD 12:12 with the optimal intensity (10(-1) lux) in the light part of the cycle and a suboptimal intensity (10(-3) lux) in the dark part. 3. In all cases the monkeys synchronized in such a way that their activity phase fell in the dark part of the LD cycle. 4. The implication is that Aotus is a true dark-active species, that the illumination-dependent activity maximum at 10(-1) lux does not affect the synchronization mechanism, and that the differential (direction of change) rather than proportional (absolute level) actions of light provide the decisive cue for synchronization of the circadian activity rhythm.     

Entrainment to light of circadian activity rhythms in tench (Tinca tinca)

The present article analyzes locomotor activity rhythms in Tinca tinca. To that end, three different experiments were conducted on 24 animals (20 g body weight) kept in pairs in 60-liter aquaria fitted with infrared sensors connected to a computer to continuously record fish movements. The first experiment was designed to study the endogenous circadian clock under free-running conditions [ultradian 40:40 min LD pulses and constant dark (DD)] and after shifting the LD cycle. Our results demonstrate that tench has a strictly nocturnal activity pattern, an endogenous rhythm being evident in 45.8% of the fish analyzed. The second experiment was conducted to test the influence of different photoperiods (LD 6:18, 12:12, 18:6, and 22:2) on locomotor activity, the results showing that even under an extremely long photoperiod, tench activity is restricted to dark hours. The third experiment examined the effect of light intensity on locomotor activity rhythms. When fish were exposed to decreasing light intensities (from 300:0 lux to 30:0, 3:0, and 0.3:0 lux) while maintaining a constant photoperiod (LD 12:12), the highest percentage of locomotor activity was in all cases associated with the hours of complete darkness (0 lux). In short, our results clearly show that (a) tench is a species with a strictly nocturnal behavior, and (b) daily activity rhythms gradually entrain after shifting the LD cycle and persist under free-running conditions, pointing to their circadian nature. However, light strongly influences activity rhythms, since (c) the length of the active phase is directly controlled by the photophase, and (d) strictly nocturnal behavior persists even under very dim light conditions (0.3 lux). The above findings deepen our knowledge of tench behavior, which may help to optimize the aquacultural management of this species, for example, by adjusting feeding strategies to their nocturnal behavior.     

Same Temporal Niche,Opposite Rhythmicity: Two Closely Related Bioluminescent Insects With Opposite Bioluminesce Propensity Rhythms

Arachnocampa species, commonly called glowworms, are flies whose larvae use light to attract prey. Here we compare rhythmicity in two of the nine described species: the Tasmanian species, Arachnocampa tasmaniensis, which inhabits caves and wet forest, and the eastern Australian mainland species, A. flava, primarily found in subtropical rainforest. Both species show the same nocturnal glowing pattern in external (epigean) environments and the same inhibition of bioluminescence by light and both species show circadian regulation of bioluminescence. We find that the underlying circadian bioluminescence propensity rhythm (BPR) of the two species peaks at opposite phases of the day:night cycle. Larvae of A. flava, placed in constant darkness in the laboratory, bioluminesce during the subjective scotophase, typical of nocturnal animals, whereas A. tasmaniensis shows the opposite tendency, bioluminescing most intensely during the subjective photophase. In A. tasmaniensis, which are exposed to natural day:night cycles, light exposure during the day overrides the high bioluminescence propensity through negative masking and leads to a release of bioluminescence after dusk when the BPR is on the wane. A consequence is that A. tasmaniensis is able to start glowing at any phase of the light:dark cycle as soon as masking by light is released, whereas A. flava is locked into nocturnal bioluminescence. We suggest that the paradoxical BPR of A. tasmaniensis is an adaptation for living in the cave environment. Observations of bioluminescence in colonies of A. tasmaniensis located in the transition from a cave mouth to the dark zone show that glowing is inhibited by light exposure but a peak bioluminescence follows immediately after “dusk” at their location. The substantial difference in the circadian regulation of bioluminescence between the two species probably reflects adaptation to the cave (hypogean) habitat in A. tasmaniensis and the forest (epigean) habitat in A. flava. (Author correspondence: d.merritt@uq.edu.au)     

PACEMAKER PHASE CONTROL VERSUS MASKING BY LIGHT: SETTING THE CIRCADIAN CHRONOTYPE IN DUAL OCTODON DEGUS

There are two main processes involved in the expression of circadian rhythmicity: entrainment and masking. Whereas the first operates via the central pacemaker to anticipate predictable environmental conditions, masking (mainly induced by light) functions as a direct modulator of the circadian output signal induced by nonpredictable events. The Chilean rodent Octodon degus presents both diurnal and nocturnal chronotypes when given free access to an exercise wheel. Two steady-entrainment phases and graded masking by light seem to generate the wide variability of chronotypes in this species. The aim of this study was to characterize the differential masking by light according to the individual chronotypes, their stability over time, and the influence of wheel running availability and ambient temperature upon the degus' nocturnality. To this end, diurnal and nocturnal degus were subjected to ultradian cycles (1:1-h light-dark [LD]), with and without wheel running availability, and under both normal and high diurnal ambient temperature cycles. The present results show that diurnal and nocturnal degus present a stable masking by light, each according to its respective chronotype. Thus, whereas diurnal animals increased their activity with light, in nocturnal degus light induced a sharp drop in wheel running activity. These two types of masking responses appeared not only when the animals were synchronized to the 12:12-h LD cycle, but also under ultradian cycles. Different masking effects persisted when wheel running was made unavailable and when the animals shifted their circadian activity patterns in response to ultradian cycles or to diurnal exposure to high temperatures. In conclusion, our results show that the positive and negative masking effects of light on diurnal and nocturnal degus, respectively, seem to occur independently of relative phase control by the central pacemaker or the negative masking induced by high environmental temperatures. (Author correspondence: angerol@um.es)     

Entrainment of eclosion rhythm in Drosophila melanogaster populations reared for more than 700 generations in constant light environment

In this paper, we report the results of our extensive study on eclosion rhythm of four independent populations of Drosophila melanogaster that were reared in constant light (LL) environment of the laboratory for more than 700 generations. The eclosion rhythm of these flies was assayed under LL, constant darkness (DD) and three periodic light-dark (LD) cycles (T20, T24, and T28). The percentage of vials from each population that exhibited circadian rhythm of eclosion in DD and in LL (intensity of approximately 100 lux) was about 90% and 18%, respectively. The mean free-running period (τ) of eclosion rhythm in DD was 22.85 ± 0.87 h (mean ± SD). Eclosion rhythm of these flies entrained to all the three periodic LD cycles, and the phase relationship (ψ) of the peak of eclosion with respect to “lights-on” of the LD cycle was significantly different in the three periodic light regimes (T20, T24, and T28). The results thus clearly demonstrate that these flies have preserved the ability to exhibit circadian rhythm of eclosion and the ability to entrain to a wide range of periodic LD cycles even after being in an aperiodic environment for several hundred generations. This suggests that circadian clocks may have intrinsic adaptive value accrued perhaps from coordinating internal metabolic cycles in constant conditions, and that the entrainment mechanisms of circadian clocks are possibly an integral part of the clockwork.