M. K. Chandrashekaran

The effect of different light regimes on adult lifespan in Drosophila melanogaster is partly mediated through reproductive output

The effects of different light regimes on the fitness of organisms have typically been studied using mean or median adult life span as the sole index of physiological well-being. It is, however, known that life span is inversely related to reproductive output in many species. Moreover, the effects of a given environmental treatment on life span can be due to effects on either age-independent mortality or the “rate of aging,” or a combination of both. Drawing evolutionary inferences from the effects of light regime on mean or median adult life span alone is difficult and, at best, speculative. We examined the effects of constant light (LL), alternating light-dark cycles (LD 12:12 h), and constant darkness (DD) on the life span of reproducing and virgin flies in four populations of Drosophila melanogaster and also estimated lifetime fecundity in the three light regimes. The light regime effects on life span were further dissected by examining the age-independent mortality and the Gompertz rate of aging under the three light regimes. While mean adult life span of reproducing males and females and virgin females was significantly shorter in LL compared to LD 12:12 and DD, lifetime egg production was highest in LL. Life span of virgin males was not significantly affected by light regime. The rate of aging in reproducing females was higher in LL as compared to DD, whereas age-independent mortality was higher in DD. As reproductive output, especially early in life, is a far more significant contributor to fitness than is life span, our results suggest that the earlier reported deleterious effects of LL on fitness are partly an artifact of examining life span alone, without considering other components of adult fitness that trade off with life span. Our results suggest that detailed investigation of the effects of light regime on the physiological and behavioral processes that accompany reproduction is necessary to fully understand the effects of different light regimes on adult fitness in Drosophila.

Possible role of eclosion rhythm in mediating the effects of light-dark environments on pre-adult development in Drosophila melanogaster

BackgroundIn insects, circadian clocks have been implicated in affecting life history traits such as pre-adult development time and adult lifespan. Studies on the period (per) mutants of Drosophila melanogaster, and laboratory-selected lines of Bactrocera cucurbitae suggested a close link between circadian clocks and development time. There is a possibility of clock genes having pleiotropic effects on clock period and pre-adult development time. In order to avoid such pleiotropic effects we have used wild type flies of same genotype under environments of different periodicities, which phenotypically either speeded up or slowed down the eclosion clock of D. melanogaster.ResultsWe assayed pre-adult development time and pre-adult survivorship of four laboratory populations of D. melanogaster, under five different light regimes, continuous light (LL), continuous darkness (DD), and light-dark (LD) cycles of 10:10 h (T20), 12:12 h (T24), and 14:14 h (T28). Although the development time was significantly different in most light regimes, except for females under T24 &T28, pre-adult survivorship remained largely unaffected. The development time was shortest under LL, followed by T20, DD, T24 and T28 regimes, in that order. Interestingly the development time showed a positive correlation with the period of eclosion rhythm, i.e., faster oscillations were associated with faster development, and slower oscillations with slower development.ConclusionBased on these results we conclude that periodicity of imposed LD cycles, and/or of eclosion rhythm plays a key role in regulating the duration of pre-adult development in D. melanogaster in a manner that does not involve direct pleiotropic effects of clock genes on both clock period and development time.

Persistence of eclosion rhythm in Drosophila melanogaster after 600 generations in an aperiodic environment.

Abstract The ubiquity of circadian rhythms suggests that they have an intrinsic adaptive value (Ouyang et al. 1998; Ronneberg and Foster 1997). Some experiments have shown that organisms have enhanced longevity, development time or growth rates when maintained in environments whose periodicity closely matches their endogenous period (Aschoff et al. 1971; Highkin and Hanson 1954; Hillman 1956; Pittendrigh and Minis 1972; Went 1960). So far there has been no experimental evidence to show that circadian rhythms per se (i.e. periodicity itself, as opposed to phasing properties of a rhythm) confer a fitness advantage. We show that the circadian eclosion rhythm persists in a population of the fruitfly Drosophila melanogaster maintained in constant conditions of light, temperature, and humidity for over 600 generations. The results suggest that even in the absence of any environmental cycle there exists some intrinsic fitness value of circadian rhythms.

Spectral sensitivity of the photoreceptors responsible for phase shifting the circadian rhythm of activity in the bat, Hipposideros speoris

Summary1.The spectral sensitivity of the photoreceptors responsible for phase shifting the circadian rhythm of flight activity in the bat,Hipposideros speoris was investigated. For this purpose we studied the phase shifts evoked with 15 min and 2.77 h pulses of monochromatic light at various phases of the rhythm freerunning in DD.2.A PRC for the circadian rhythm of flight activity inH. speoris was constructed with white light pulses (1,000 lx for 15 min) against DD background (Fig. 1). In the first set of experiments 15 min monochromatic light pulses of varying intensities were administered to two phases of the rhythm: the phase of the rhythm at which maximal phase advances occur CT 4, and the phase of the rhythm at which maximal phase delays occur CT 18. The intensities of the 15 min monochromatic light pulses required to produce 50% of the phase shifts evoked with white light pulses (1,000 lx for 15 min) at these two phases were determined. The spectral sensitivity curve for advance phase shifts has a maximum at the wavelength 520 nm and the spectral sensitivity curve for delay phase shifts has a maximum at the wavelength 430 nm (Fig. 5).3.In the second set of experiments 2.77 h monochromatic light pulses of equal energy of 100 μW/cm2 were used. We studied the wavelength dependent phase shifts at four phases of the rhythm: CT 2, CT 4, CT 12 and CT 18. The pulses of 430 and 520 nm evoked unequivocal delay and advance phase shifts, respectively, at all four phases (Fig. 7). These results suggest that at this photopic level of pulse energy, there might be a clear antagonism between the two photoreceptor classes, one having a maximum at the wavelength 430 nm and the other having a maximum at the wavelength 520 nm.4.We suggest that there may exist two different classes of photoreceptors in the retinas ofH. speoris. The S photoreceptors (short wavelength sensitive) having a maximum at the wavelength 430 nm and the M photoreceptors (middle wavelength sensitive) having a maximum at the wavelength 520 nm that mediate delay and advance phase shifts, respectively.

Social synchronization of the activity rhythm in a cave-dwelling insectivorous bat

can be the determining factor specific to the species tested and differences in these structural orders may specify the adult tissue pattern in other species too. A comparable situation exists in the aggregation stage of slime mold [7], where the cells are spread out in two dimensions and the original center of the aggregate becomes the anterior tip of the slug, retaining its dominant role later. The segregation of specialized cells at two poles of the aggregate and directed development based on this pattern also has phylogenetic significance. The origination of sponges has been ascribed to the symbiosis of flagellate and ameboid cells [8]. One can visualize how in an aggregate with such an assembly of cells the functional order would be initiated by designing the pattern with specific localization of different cell types.

A case for multiple oscillators controlling different circadian rhythms in Drosophila melanogaster

A population of the fruit fly Drosophila melanogaster was raised in periodic light/dark (LD) cycles of 12:12 h for about 35 generations. Eclosion, locomotor activity, and oviposition were found to be rhythmic in these flies, when assayed in constant laboratory conditions where the light intensity, temperature, humidity and other factors which could possibly act as time cue for these flies, were kept constant. These rhythms also entrained to a LD cycle of 12:12 h in the laboratory with each of them adopting a different temporal niche. The free-running periods (tau) of the eclosion, locomotor activity and oviposition rhythms were significantly different from each other. The peak of eclosion and the onset of locomotor activity occurred during the light phase of the LD cycle, whereas the peak of oviposition was found to occur during the dark phase of the LD cycle. Based on these results, we conclude that different circadian oscillators control the eclosion, locomotor activity and oviposition rhythms in the fruit fly D. melanogaster.

RELATIONSHIP BETWEEN LIGHT INTENSITY AND PHASE RESETTING IN A MAMMALIAN CIRCADIAN SYSTEM

The light-induced phase-resetting response of the locomotor activity rhythm in the field mouse Mus booduga was studied at two phases of the circadian cycle known to respond to light stimuli of 15 min duration and 1000 lux intensity with maximum advance (at circadian time 20 [CT20]) and maximum delay phase-shifts (at CT15). The phase-shifts evoked by natural daylight stimuli of various illuminations ranging between 0.001 lux and 10,000 lux and lasting 15 min were estimated. The results clearly demonstrate that the relationship between the phase-shifts and the intensities of light stimuli is nonlinear. Furthermore, a single light stimulus of 0.001 lux, or 0.1 lux intensity for a duration of 15 min, administered at CT20, evoked unequivocal responses; phase delays were observed instead of phase advances. The critical intensities needed for light stimuli of 15 min duration to induce saturating response were calculated and were found to be about 100 lux for CT20 and about 500 lux for CT15. These results suggest that a greater intensity of light is required at the phase CT15 to induce a saturating phase shift than is required at a later phase of the circadian cycle (CT20).

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.

Developmental plasticity of the locomotor activity rhythm of Drosophila melanogaster.

We used four replicate outbred populations of Drosophila melanogaster to investigate whether the light regimes experienced during the pre-adult (larval and pupal) and early adult stages influence the free-running period (tau(DD)) of the circadian locomotor activity rhythm of adult flies. In a series of two experiments four different populations of flies were raised from egg to eclosion in constant light (LL), in light/dark (LD) 12:12h cycle, and in constant darkness (DD). In the first experiment the adult male and female flies were directly transferred into DD and their locomotor activity was monitored, while in the second experiment the locomotor activity of the emerging adult flies was first assayed in LD 12:12h for 15 days and then in DD for another 15 days. The tau(DD) of the locomotor activity rhythm of flies that were raised in all the three light regimes, LL, LD 12:12h and in DD was significantly different from each other. The tau(DD) of the locomotor activity rhythm of the flies, which were raised in DD during their pre-adult stages, was significantly shorter than that of flies that were raised as pre-adults in LL regime, which in turn was significantly shorter than that of flies raised in LD 12:12h regime. This pattern was consistent across both the experiments. The results of our experiments serve to emphasise the fact that in order to draw meaningful inferences about circadian rhythm parameters in insects, adequate attention should be paid to control and specify the environment in which pre-adult rearing takes place. The pattern of pre-adult and early adult light regime effects that we see differs from that previously observed in studies of mutant strains of D. melanogaster, and therefore, also points to the potential importance of inter-strain differences in the response of circadian organisation to external influences.

Possible Evidence for Shift Work Schedules in the Media Workers of the Ant Species Camponotus compressus

The locomotor activity rhythm of the media workers of the ant species Camponotus compressus was monitored under constant conditions of the laboratory to understand the role of circadian clocks in social organization. The locomotor activity rhythm of most ants entrained to a 24 h light/dark (12:12 h; LD) cycle and free-ran under constant darkness (DD) with circadian periodicities. Under entrained conditions about 75% of media workers displayed nocturnal activity patterns, and the rest showed diurnal activity patterns. In free-running conditions these ants displayed three types of activity patterns (turn-around). The free-running period (τ) of the locomotor activity rhythm of some ants (10 out of 21) showed period lengthening, and those of a few (6 out of 21) showed period shortening, whereas the locomotor activity rhythm of the rest of the ants (5 out of 21) underwent large phase shifts. Interestingly, the pre-turn-around τ of those ants that showed nocturnal activity patterns during earlier LD entrainment was shorter than 24 h, which became greater than 24 h after 6–9 days of free-run in DD. On the other hand, the pre-turn-around τ of those ants, which exhibited diurnal patterns during earlier LD entrainment, was greater than 24 h, which became shorter than 24 h after 6–9 days of free-run in DD. The patterns of activity under LD cycles and the turn-around of activity patterns in DD regime suggest that these ants are shift workers in their respective colonies, and they probably use their circadian clocks for this purpose. Circadian plasticity thus appears to be a general strategy of the media workers of the ant species C. compressus to cope with the challenges arising due to their roles in the colony constantly exposed to a fluctuating environment.