Multi-Oscillatory Control of Eclosion and Oviposition Rhythms inDrosophila melanogaster: Evidence from Limits of Entrainment Studies

Non-Ventral Lateral Neuron-Based, Non-PDF-Mediated Clocks Control Circadian Egg-Laying Rhythm inDrosophila melanogaster

The authors report the results of their study aimed at investigating the consequence of targeted ablation of ventral lateral neurons (LNvs—neurons regulating eclosion and locomotor activity rhythms) and genetic disruption of pigment-dispersing factor (PDF—an important output of circadian clocks) on the egg-laying rhythm of Drosophila melanogaster. The results clearly suggest that genetic ablation of LNvs and loss of function mutation of PDF abolish eclosion and locomotor activity rhythms, whereas the egg-laying rhythm continues unabated. Furthermore, the results also demonstrate that the period of egg-laying rhythm remains unchanged under different ambient temperatures and nutrition levels, suggesting that the egg-laying rhythm of D. melanogaster is temperature and nutrition compensated. Based on these results, the authors conclude that the egg-laying rhythm in D. melanogaster is regulated by non-LNv-based, non-PDF-mediated circadian clocks.

Rhythmic egg-laying behaviour in virgin females of fruit flies Drosophila melanogaster

Fruit fly Drosophila melanogaster females display rhythmic egg-laying under 12:12 h light/dark (LD) cycles which persists with near 24 h periodicity under constant darkness (DD). We have shown previously that persistence of this rhythm does not require the neurons expressing pigment dispersing factor (PDF), thought to be the canonical circadian pacemakers, and proposed that it could be controlled by peripheral clocks or regulated/triggered by the act of mating. We assayed egg-laying behaviour of wild-type Canton S (CS) females under LD, DD and constant light (LL) conditions in three different physiological states; as virgins, as females allowed to mate with males for 1 day and as females allowed to mate for the entire duration of the assay. Here, we report the presence of a circadian rhythm in egg-laying in virgin D. melanogaster females. We also found that egg-laying behaviour of 70 and 90% females from all the three male presence/absence protocols follows circadian rhythmicity under DD and LL, with periods ranging between 18 and 30 h. The egg-laying rhythm of all virgin females synchronized to LD cycles with a peak occurring soon after lights-off. The rhythm in virgins was remarkably robust with maximum number of eggs deposited immediately after lights-off in contrast to mated females which show higher egg-laying during the day. These results suggest that the egg-laying rhythm of D. melanogaster is endogenously driven and is neither regulated nor triggered by the act of mating; instead, the presence of males results in reduction in entrainment to LD cycles.

Temperature can entrain egg laying rhythm of Drosophila but may not be a stronger zeitgeber than light

In Drosophila multiple circadian oscillators and behavioral rhythms are known to exist, yet most previous studies that attempted to understand circadian entrainment have focused on the activity/rest rhythm and to some extent the adult emergence rhythm. Egg laying behavior of Drosophila females also follows circadian rhythmicity and has been seen to deviate substantially from the better characterized rhythms in a few aspects. Here we report the findings of our study aimed at evaluating how circadian egg laying rhythm in fruit flies Drosophila melanogaster entrains to time cues provided by light and temperature. Previous studies have shown that activity/rest rhythm of flies entrains readily to light/dark (LD) and temperature cycles (TC). Our present study revealed that egg laying rhythm of a greater percentage of females entrains to TC compared to LD cycles. Therefore, in the specific context of our study this result can be taken to suggest that egg laying clocks of D. melanogaster entrains to TC more readily than LD cycles. However, when TC were presented along with out-of-phase LD cycles, the rhythm displayed two peaks, one occurring close to lights-off and the other near the onset of low temperature phase, indicating that upon entrainment by TC, LD cycles may be able to exert a greater influence on the phase of the rhythm. These results suggest that temperature and light associatively entrain circadian egg laying clocks of Drosophila.

Egg-laying rhythm in Drosophila melanogaster

Extensive research has been carried out to understand how circadian clocks regulate various physiological processes in organisms. The discovery of clock genes and the molecular clockwork has helped researchers to understand the possible role of these genes in regulating various metabolic processes. In Drosophila melanogaster, many studies have shown that the basic architecture of circadian clocks is multi-oscillatory. In nature, different neuronal subgroups in the brain of D. melanogaster have been demonstrated to control different circadian behavioural rhythms or different aspects of the same circadian rhythm. Among the circadian phenomena that have been studied so far in Drosophila, the egg-laying rhythm is unique, and relatively less explored. Unlike most other circadian rhythms, the egg-laying rhythm is rhythmic under constant light conditions, and the endogenous or free-running period of the rhythm is greater than those of most other rhythms. Although the clock genes and neurons required for the persistence of adult emergence and activity/rest rhythms have been studied extensively, those underlying the circadian egg-laying rhythm still remain largely unknown. In this review, we discuss our current understanding of the circadian egg-laying rhythm in D. melanogaster, and the possible molecular and physiological mechanisms that control the rhythmic output of the egg-laying process.

Reproductive and post-embryonic daily rhythm patterns of the malaria vector Anopheles (Kerteszia) cruzii: aspects of the life cycle

Females of Anopheles (Kerteszia) cruzii, a sporadic malaria vector in some areas of the Atlantic Forest in south and southeastern Brazil, were captured and studied under controlled conditions. In the laboratory, daily observations were conducted in natural light-dark cycles at 25.1+/-0.6 degrees C and relative humidity 57-81%. Post-embryonic development, which comprises four larval instars and the pupa, was continuously observed, and its cycles, as well as temporal components of reproduction, were registered. A preliminary study on female longevity was also performed. Oviposition, ecdysis from the third and fourth instars larvae, and pupation were visually monitored over three consecutive days and the emergence of adults over four consecutive days. Results were analyzed by circular statistics, and the null hypothesis of the absence of rhythm was assessed by Rayleigh's test at the 5% significance level. From a total of 141 females captured, 113 (80.14%) survived and 79 (69.91%) were successfully fed on blood, offered at one of two time intervals, 09:00-10:30 h (morning) or 18:30-20:00 h (evening). A total of 36 females laid 1063 eggs in 65 oviposition episodes, and 18 females presented fragmented oviposition. The average duration from egg-laying until adult emergence was 30.71+/-3.57 days, the larval stage being the longest in the post-embryonic development. Egg-laying showed a daily rhythm, with a peak at 23:24+/-3:47 h, 2 to 5 h after sunset. The time of the blood meal did not shift the phase of the egg-laying rhythm. The last larval ecdysis, pupation, and adult emergence did not follow a 24 h rhythmic pattern. A description of temporal patterns of post-embryonic development, particularly in the case of vectors, can be an important tool in research to determine methods of control.

Selection for early and late adult emergence alters the rate of pre-adult development in Drosophila melanogaster

Background

Circadian clocks have been implicated in the regulation of pre-adult development of fruit flies Drosophila melanogaster. It is believed that faster clocks speed up development and slower clocks slow it down. We established three sets of D. melanogaster populations (early, control and late). The early and late populations were raised by selecting for flies that emerged either in the morning or in the evening under 12:12 hr light/dark (LD) cycles. After 75 generations of selection, the time course and waveform of the adult emergence and activity rhythms of the early and the late populations diverged from each other as well as from the controls. In this paper, we report the consequence of this selection on the rate of pre-adult development.

Results

We assayed the pre-adult development time of the selected and control populations under 12:12 hr LD cycles and constant darkness (DD). Under LD cycles, the early populations develop faster than the controls, while the late populations develop slower than the controls. Although flies take longer to develop under DD than in LD, the relative differences between the mean development times of the selected and control populations remain unaltered in DD. In a separate experiment designed to investigate the effect of time of egg collection and experimental conditions on the duration of pre-adult stage, we assayed the development time of the selected and control populations by collecting eggs at different times of the day (morning and evening) and by assaying their pre-adult development time under constant light (LL), LD, and DD conditions. Irrespective of the time of egg collection and assay light regime, the late flies continue to develop slower than the early flies.

Conclusion

The results of our study clearly indicate that selection on the timing of adult emergence alters the rate of pre-adult development in D. melanogaster. The timing of egg collection as well as assay light regime does not have any measurable effect on the relative differences between the developmental rates of the early and the late flies. Taken together these results appear to suggest that pleiotropic effects of clock genes mediate correlated changes in the timing of adult emergence and the rate of pre-adult development in D. melanogaster.

Circadian regulation of egg-laying behavior in fruit flies Drosophila melanogaster

Significant progress has been made in our understanding of the neurogenetics of circadian clocks in fruit flies Drosophila melanogaster. Several pacemaker neurons and clock genes have now been identified and their roles in the cellular and molecular clockwork established. Some recent findings suggest that the basic architecture of the clock is multi-oscillatory; the clock mechanisms in the ventral lateral neurons (LN(v)s) of the fly brain govern locomotor activity and adult emergence rhythms, while the peripheral oscillators located in antennal cells regulate olfactory rhythm. Among circadian phenomena exhibited by Drosophila, the egg-laying rhythm is unique in many ways: (i) this rhythm persists under constant light (LL), while locomotor activity and adult emergence become arrhythmic, (ii) its circadian periodicity is much longer than 24h, and (iii) while egg-laying is rhythmic under constant darkness, the expression of two core clock genes period (per) and timeless (tim), is non-oscillatory in the ovaries. In this paper, we review our current knowledge of the circadian regulation of egg-laying behavior in Drosophila, and provide some possible explanations for its self-sustained nature. We conclude by discussing the existing limitations in our understanding of the regulatory mechanisms and propose few approaches to address them.

Neural circuits underlying circadian behavior in Drosophila melanogaster

Circadian clocks include control systems for organizing daily behavior. Such a system consists of a time-keeping mechanism (the clock or pacemaker), input pathways for entraining the clock, and output pathways for producing overt rhythms in behavior and physiology. In Drosophila melanogaster, as in mammals, neural circuits play vital roles in all three functional subdivisions of the circadian system. Regarding the pacemaker, multiple clock neurons, each with cell-autonomous pacemaker capability, are coupled to each other in a network. The outputs of different sets of clock neurons in this network combine to produce the normal bimodal pattern of locomotor activity observed in Drosophila. Regarding input, multiple sensory modalities (including light, temperature, and pheromones) use their own circuitry to entrain the clock. Regarding output, distinct circuits are likely involved for controlling the timing of eclosion and for generating the locomotor activity rhythms. This review summarizes work on all of these circadian circuits, and discusses the broader utility of studying the fly's circadian system.