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Notes: [Auszug] Each ovary of G. morsitans consists of two poly-trophic ovarioles, each at a different stage of development4. Only one egg matures at a time, and a sequence of ovulation occurs so that the first and third eggs are ovulated from the right ovary and the second and fourth eggs from the left ovary. ...

Notes: Summary 1. Larvae of the flesh-flySarcophaga argyrostoma were exposed to ‘skeleton’ photoperiods consisting of two unequal pulses of light (asymmetrical skeletons) or two equal pulses of light (symmetrical skeletons) per 24 h period. Asymmetrical skeletons (‘night interruption experiments’) revealed two apparently light-sensitive phases within the night, the second, occurring about 9 h after ‘dusk’ in all regimes, being thought to correspond to the photoinducible phase (Φ i) of the photoperiodic oscillator (Fig. 1). 2. Symmetrical skeletons consisting of two 1 h pulses of light closely simulated the diapause-promoting effects of complete photoperiods of the same duration up to about 10 h. Skeletons longer than 16 h were again diapause-promoting in contrast to the diapause-averting (long day) effects of corresponding complete photoperiods. The incidence of diapause was reduced in the ambiguous symmetrical skeletons close to 12 h (in the ‘Bistability Zone’) (Fig. 3). 3. The ambiguity of symmetrical skeletons in the ‘Bistability Zone’ was eliminated by transferring cultures from continuous light (LL) into regimes consisting of two 1 h pulses placed 11 h apart. When the shorter interval (11 h) was ‘seen’ first after theLL-DD transition, long-day effects (low incidence of diapause) were observed; when the longer interval (15 h) was ‘seen’ first short-day effects (high diapause) were observed (Fg. 5). These results are consistent with the hypothesis that the photoperiodic oscillator is damped out by light periods of more than 12 h and resumes its motion on transfer to darkness.

Notes: Summary 1. To discriminate between hourglass- and oscillator-based time measurement in the photoperiodic clock, responses ofSarcophaga argyrostoma andCalliphora vicina to light-dark sequences containing the same number of 12 h or 36 h nights were compared with reference to a test devised by Veerman and Vaz Nunes (1987). 2. According to their predictions, 12 h and 36 h nights should be equally inductive with an hourglass clock because all nightlengths greater than the critical value are equivalent. A clock of the oscillator type, however, would be expected to reset itself in the extended (36 h) night, to perform two acts of time measurement, and therefore to produce a higher incidence of diapause than in regimes with the same number of 12 h nights. 3. Contrary to either of these expectations, diapause incidence in sequences of 36 h nights waslower than in sequences containing 12 h nights. This was found for both species of fly. 4. These apparently perplexing results could be accounted for using the ‘damped circadian oscillator’ model for photoperiodism (Lewis and Saunders 1987). The experimental data (and the simulations) are regarded as evidence that the photoperiodic clocks ofS. argyrostoma andC. vicina are not based on hour glasses, nor on fully selfsustained oscillators. The experimental observations are consistent with the idea that the photoperiodic clocks in these species are based on a system of moderately damped circadian oscillators (Saunders and Lewis 1987a, b).

Notes: Abstract RH 5849, a non-steroidal ecdysteroid mimic, was found to cause consistent phase shifts in the circadian rhythm of locomotor activity of the blowfly, Calliphora vicina. This compound causes phase advances in the early subjective night and phase delays in the late subjective night. This effect is the opposite, but not the mirror image of the phase response curve obtained for 1 h light pulses. This suggests that ecdysteroids might act as entraining agents via the output pathway by feedback to “clock” neurons in the brain. A computer model based on 12 pacemaker neurons with circadian periods (τ values) from short to long without simulated feedback from the ecdysteroid system becomes arrhythmic; with feedback, the oscillators become synchronized to a common period. The possible role of ecdysteroids as endogenous synchronizing agents in the insect circadian system is discussed.

Notes: Summary 1. The pupal eclosion rhythm imSarcophaga argyrostoma was compared with the photoperiodic induction of diapause in the same species. The eclosion rhythm is controlled by a circadian system which free-runs in continuous darkness (DD), after exposure to light/dark cycles (LD) or to continuous light (LL), with an endogenous period, τ, close to 24 h. InLD cycles photoperiod controls the rate of larval development, and which eclosion gates are used by the emerging flies. 2. Sequential transfer of cultures fromLL to DD, or exposure to a 7 h delay in the light cycle at different ages, showed that the response (in terms of initiation and phase control of the eclosion rhythm) was maximal with young larvae, less with older larvae, and only marginal with the early intra-puparial stages. Older intrapuparial stages (developing adults) appear to show a renewed responsiveness to light pulses. 3. Phase response curves for the eclosion rhythm were “weak” or Type 1 with 1 h pulses; those for 8 h and 12 h pulses were “strong” or Type 0. After light periods in excess of about 12 h the eclosion peaks occurred at characteristic time (11.3+modulo τ h) after the light-off or “dusk” signal, provided that theLD cycles were experimentally discontinued before the end of larval development. 4. Cultures transferred fromLL into a dark (D) and a final light (L) period which together added up to a value (D+L) close to 24 or 48 h gave rise to highly rhythmic eclosion patterns. Those in whichD + L values were close to 36 or 60 h, however, were far less coherent. This difference is attributed to the multi-oscillator construction of the circadian system, and its internal temporal organisation. 5. A phase response curve for the photoperiodic oscillator was determined for 15 min resetting pulses by the use of 3-point “skeleton” photoperiods. Like the 1 h response curve for pupal eclosion, this was of the “weak” type with small “amplitude” phase changes. 6. The similarities between the eclosion rhythm and the photoperiodic oscillator inS. argyrostoma (same responsive period, same phase response curve, same phase relationships to driving light cycles) are discussed. The two systems are considered to be sufficiently alike for the overt system (eclosion) to be used as a measure of phase in the analysis of the photoperiodic clock, as recommended by Pittendrigh and Minis (1964).

Notes: Summary 1. The circadian rhythm of pupal eclosion inSarcophaga argyrostoma was used as a “measure” of the phase of the covert photoperiodic oscillation in an experimental and formal analysis of photoperiodic induction, particularly in terms of entrainment and phase coherence within the multioscillator circadian system. 2. Phase response curves for the eclosion pacemaker were obtained in single pulse resetting experiments for a series of pulse-lengths (1 to 20 h). These data were also analysed for “coherence” of the eclosion peaks in terms of Winfree's (1970) arrhythmicity or R-values. 3. One and 3 h pulses of white light (240 μW cm−2) gave rise to “weak” or Type 1 resetting curves, whereas pulses of 5 h or more gave rise to “strong” or Type 0 curves. Pulses close to 4 h in duration starting at points close to 4 h after the transition fromLL to DD gave rise to highly arrhythmic eclosion patterns, suggesting that this combination of pulse duration and phase moved the system on to its “singularity.” 4. Arrhythmicity was also observed in cultures in which the number of hours of darkness between theLL/DD transition and the beginning of the resetting pulse (D hours), and the duration of the pulse (L hours) added up to a value (D+L) close to 12, 36, 60, 84 or 108 h (modulo τ+1/2τ). Highly rhythmic cultures, however, were obtained when D+L added up to values close to 24, 48, 72, 96 or 120 h (modulo τ). 5. The phase response curves for single 12 h pulses of white light were followed through the first 5 days of larval life. In the first two cycles following the transition fromLL to DD these pulses gave rise to strong (Type 0) curves; in subsequent cycles these curves “decayed” from Type 0 to Type 1. This change is thought to be associated with a developmental change, perhaps in the photoreceptor or its coupling to the “clock” or merely to a change in behaviour. 6. The data obtained from phase response curves were used in a computer program to calculate steady-state entrainment to a variety of “complete” and “skeleton” photoperiods. These computed data were also compared with those experimentally determined for cultures exposed to identical regimes, and to diapause induction data obtained from earlier experiments. 7. In “complete” photoperiods (T=24 h) the median of eclosion (φr) was shown to phase-lead dawn in cycles containing less than 14 h of light, but to phase-lag dawn in longer photoperiods. The point at whichφ r crossed the “dawn threshold” closely matched the value of the critical photoperiod for diapause induction. In symmetrical “skeletons” (T= 24 h) and asymmetrical “skeletons” (T=24 and 72 h) computed phases ofφ r, observed phases ofφ r, and diapause induction data were all in close agreement. 8. The results are interpreted in terms of Pittendrigh's (1966) “external coincidence” model for the photoperiodic clock, the model which appears to offer the most plausible explanation for photoperiodic induction in this species. The model is modified, however, to incorporate the degree of internal organisation or disorganisation within the multioscillator circadian system. In particular, this accounts for the fall in diapause incidence in ultra-short daylengths, and for the results of “resonance” experiments. 9. The “external coincidence” model, as adopted forS. argyrostoma, is compared with the formal properties of the photoperiodic clock in other insect species which have been adequately investigated. In particular, the strong similarities betweenS. argyrostoma and the aphid,Megoura viciae (Lees, 1973), are stressed.

Notes: Summary 1. Photoperiodic induction of diapause inNasonia vitripennis andSarcophaga argyrostoma was shown to be different in a number of respects. InN. vitripennis induction proceeds to “completion” in continuous darkness after an initial five short-day (long-night) cycles, and can be accomplished in the total absence of light by the use of daily temperature cycles, or thermoperiods.S. argyrostoma, on the other hand, requires repeated exposures to short-day (long-night) cycles for “full” induction, and thermoperiods in the absence of light are apparently ineffective. Furthermore, inS. argyrostoma periods of CO2 and N2 anaesthesia applied during the light and the dark portions of the daily cycle underlined the central importance of night-length measurement, whereas periods of chilling inN. vitripennis had previously shown light and dark to be of equal importance. 2. These results are considered to be consistent with two current models for the photoperiodic clock: internal coincidence (N. vitripennis) and external coincidence (S. argyrostoma), although none of the experimental results offersunequivocal evidence for this conclusion. 3. The apparent diversity of insect photoperiodic clocks is examined, particularly in terms of Pittendrigh's “extended circadian surfaces”. It is considered likely that the apparent diversity is a product of evolutionary divergence from a common ancestral mechanism with its basis in circadian rhythmicity.

Notes: Summary 1. Data from single light pulse resetting experiments on the circadian rhythm of pupal eclosion inSarcophaga argyrostoma were used to design and predict the outcome of one- and two-pulse cycles in terms of the induction of pupal diapause. Experiments were interpreted in terms of the “External Coincidence” model of Pittendrigh (1966). 2. Two-pulse asymmetrical “skeleton” photoperiods (night-interruption experiments), in which a short supplementary pulse scans the “night” of a diapause-inductive cycle, show two points (A and B) of short-night effect (the non-diapause or “summer” pathway). The External Coincidence model suggests that the photoinducible phase (φ i) lies at point B. 3. Single short pulses of light (1 h) were used in cycles of different length (so-called T-experiments) in such a way that the light pulses fell on particular circadian phases in each cycle. “Short-night” or non-diapause effects were only produced when the light pulse fell, in each cycle, on that phase (Circadian time, Ct, 21.5) equivalent to point B. 4. In T-experiments comprising l h pulses of light in a cycle of LD 1∶20.5 (T 21.5), with the first pulse in the train starting at different circadian phases, the incidence of diapause was shown to be a function of the number of transient cycles before the circadian pacemaker achieved steady-state entrainment to the light cycle. 5. In two-pulse asymmetric “skeletons” in which the scanning pulse was arranged to fall on point A, it was shown that the diapause-averting effects of the scanning pulse could be “reversed” by a terminal dark period greater than the critical night length. In asymmetrical skeletons in which the pulse fell on point B, however, the diapause-averting effects were irreversible. These experiments demonstrate that the photoinducible phase lies late in the subjective night at a circadian phase (Ct 21.5) marked by point B in night interruption experiments. 6. Although the results do not exclude the possibility of an “Internal Coincidence” type of photoperiodic clock (Pittendrigh, 1972) they remain consistent with the External Coincidence model as adopted in earlier papers forS. argyrostoma. The results also underscore the essential similarities between the photoperiodic clocks inSarcophaga and the aphidMegoura viciae (Lees, 1973).