Internal Coincidence Research Articles

Evidence for a circadian component in the photoperiodic mechanism of the blackheaded bunting, Emberiza melanocephala

Photosensitive male blackheaded buntings were exposed to three different light-dark cycles of 22 to 30 h duration, in which a 3 h photophase was coupled with varying durations of scotophases, for a total duration of 75 days. Gonadal growth was initiated in the 30-h cycle (3L:27D), whereas the same photoperiod (3 h) in 22 h and 24 h cycles (3L:19D, 3L:21D) did not induce testicular recrudescence. The results are consistent with the Bünning hypothesis (external coincidence model), and can be interpreted to mean that the positive ultra-short-day photoperiodic response is the result of an advance in the photosensitive phase of the response system so that it becomes coincident, at least in part, with the environmental photophase. In a further experiment, buntings long-day (20L:4D)-photostimulated for 30 days were transferred to continuous darkness (DD) and short days (8L:16D) for 40 days. When examined at the end, birds in both experiments showed testicular regression. The results are explainable in terms of the external coincidence model, but could be explained also by a relatively simple internal coincidence model. Nevertheless, application of the Law of Parsimony may favour the external coincidence system in the photoperiodic response mechanism of this species.

The photo-gonadal response of a migratory bunting: evidence of an external coincidence system.

Male redheaded buntings (Emberiza bruniceps) were subjected to continuous darkness (DD) or short days (8L/16D) after photo-stimulation (15L/9D) for either 15 or 30 days. Two groups of birds (four each), after photo stimulation for 30 days, were continued on a regimen of 15L/9D. Another two groups (of four each) of photo-sensitive birds were also subjected to DD or 8L/16D; they served as controls. The effect of transfer to DD or 8L/16D on the size of testes was found not the be dependent upon the size attained at the time of transfer. The rate of testicular regression was nearly the same under both the treatments, i.e., in those birds transferred either to DD or to 8L/16D. The birds that were maintained on 15L/9D maintained enlarged testes; this excluded the possibility of the onset of photo-refractoriness during the treatment period. On the other hand, testes were not photo-stimulated in the photo-sensitive birds subjected to DD or 8L/16D. These results are consistent with the Bünning-Pittendrigh avian external coincidence model, but could be explained alternatively by a relatively simple internal coincidence model. Nevertheless, the application of the law of parsimony suggests the acceptance of the hypothesis of the external coincidence model.

The effects of ultra-short photoperiods on the seasonal clock inSarcophaga argyrostoma

The ‘external coincidence’ model for the photoperiodic clock, in its original form (e.g. Pittendrigh 1966), fails to account for the entireshape of the photoperiodic response curve in an insect such asSarcophaga argyrostoma (see Fig. 1) because it predicts as much diapause in ‘ultra-short’ daylengths (and in continuous darkness) as it does in ‘strong’ short daylengths closer to the critical photoperiod. This paper suggests, however, that a multioscillator version of external coincidence can explain the lowered incidence of pupal diapause in photoperiods of less than about 9 h, and therefore implies that external coincidence should not be rejected on this account alone. 1. Using trains of 4 h light pulses (LD 4∶20, 240 uW cm−2) with the first pulse commencing at all phases or circadian times, the incidence of diapause was shown to be negatively correlated with the number of non-steady-state or transient cycles before entrainment is achieved. Furthermore, trains of ‘weak’ pulses (1 to 8 h) gave a higher incidence of diapause when commencing at Ct 12 (the end of the subjective day) than at Ct 24 (the beginning of the subjective day), although trains of ‘stronger’ pulses (10 to 21 h) showed no such difference. 2. Increasing the irradiance of light pulses from 240 μW cm−2 to 16,000 μW cm−2 significantly increased diapause induction with 2 and 4 h pulses, but less so with pulses of 8 and 12 h duration. 3. These results are consistent with a ‘multioscillator’ photoperiodic clock inS. argyrostoma (see also Saunders 1978). Night-length measurement by such a clock is assumed to be more effective when the circadian system isat steady state with the driving light cycle. Thus, ‘weak’ light pulses starting initially far from the steady state phase relationship cause the circadian system to pass through a series of transients before entrainment, and result in a lowered diapause incidence because an insufficient number of effective (steady state) long nights are accumulated before the end of the larval sensitive period. Raising the intensity of these shorter pulses increases their ‘strength’, probably lowers the number of transients, and increases the incidence of pupal diapause. The reduced effectiveness of night-length measurement during transient cycles might be caused by an incoherence among the oscillators comprising the clock. These results, however, do nothing to distinguish ‘ external coincidence ’ from the various forms of ‘internal coincidence’, but a multioscillator version of the former remains a valid type of model forS. argyrostoma. 4. It is probable that insect species with a high sensitivity to light (i.e. a transition from Winfree's Type 1 to Type 0 resetting with short pulse durations) show a photoperiodic response curve (PPRC) in which ultra-short photoperiods (∼1 h) are strongly inductive, whereas those (likeS. argyrostoma) in which Type 1-Type 0 transitions occur with longer pulse durations show a ‘bell-shaped’ PPRC.

Formal properties of the circadian system underlying photoperiodic time-measurement in Japanese quail

The photosensitive phase for the photoperiodic response of Japanese quail was delineated with 3 h main photoperiods and 0.25 h night breaks in cycles (T) of 24 and 27 h. UnderT24 there was one peak of induction of testicular growth and luteinizing hormone release, while underT27 there were two, the first of which had a phase angle which was about 1.5 h more positive than that of the peak inT24. In contrast, the phase angle of locomotor activity underT27 was 3–4 h more positive than underT24. During entrainment to 1 h photoperiods in cycles between 19.1 and 25.7 h in length the rate of testicular growth (k) remained close to zero even though the subjective night of the activity rhythm was illuminated in some treatments. The ratek also remained close to zero when quail were exposed to 3 h photoperiods in cycles between 21 and 36 h in length, and underT30 the critical daylength for photoperiodic induction was only 1.5 h shorter than that underT24. The results suggest that asT is altered the changes in the phase angle of the photoinducible phase are smaller than those of the rhythm of locomotor activity, indicating the involvement of oscillators with different entrainment properties. This hypothesis is neither supported nor excluded by consideration of the internal coincidence model.

External coincidence and the photoinducible phase in theSarcophaga photoperiodic clock

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 (φi) 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).

Internal and external coincidence and the apparent diversity of photoperiodic clocks in the insects

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.

Dawn signal as a rhythmical timer for the seasonal adaptive variation of CAM: a model

Abstract Photoperiodism, known to control the level of CAM in different types of CAM plants (e.g. Kalanchoe blossfeldiana, Bryophyllum daigremontianum), would act as a reliable timer for seasonal coherent adaptation. Two different endogenous rhythms (malic enzyme activity, PEP carboxylase capacity) appear to be separately coupled to dawn and dusk, respectively, thus achieving time‐compartmentation of CAM; this feature suggests involvement of an ‘internal coincidence’ type of clock mechanism.Persistence in continuous darkness of the rhythm of malic enzyme activity (measured by label incorporation into pyruvate or by CO2 output) establishes its endogenous character and shows that light is not necessary for malate decarboxylation. The role of the dawn signal would be to entrain the CAM system, i.e. to phase the endogenous rhythm of malic enzyme activity correctly to local time. The possibility of an effect of the phase of the PEP carboxylase rhythm on the phase of the decarboxylation rhythm is ruled out by the presence of the intermediary malate storage step. Phase responses to red and to far‐red show that phytochrome is involved in rephasing the rhythm of malic enzyme activity.The relative position of dusk affords a ‘measurement’ of the season by the CAM system entrained by dawn. According to the dusk‐dawn interval, the level of PEP carboxylase capacity (amount of active enzyme) is modified, resulting in changes of CAM level (high activity under short days).

Evidence for ‘dawn’ and ‘dusk’ oscillators in the Nasonia photoperiodic clock

Females of Nasonia vitripennis were maintained in light cycles from 12 to 72 hr in length, with 4 to 28 hr photoperiods, and their offspring examined for larval diapause. This ‘resonance’ technique revealed periodic maxima of diapause induction, about 24 hr apart. The ‘ascending slopes’ of these maxima appeared to obtain their principal time cue from dusk and the ‘descending slopes’ from dawn. This suggests that two independent—dawn and dusk—oscillators are involved in the Nasonia photoperiodic clock. The results are interpreted in terms of ‘internal coincidence’. N. vitripennis was shown to be able to distinguish between 12 and 18 hr of red light (>600 nm) in the photoperiodic sense. A ‘positive’ resonance experiment using such a red light was also performed. This shows that the spectral sensitivity of the pigment coupling the circadian system to the environmental light cycle extends into the red end of the spectrum.

Thermoperiodic Control of Diapause in an Insect: Theory of Internal Coincidence

Females of the parasitic wasp Nasonia vitripennis raised from the egg stage in the total absence of light but subjected to daily temperature cycles (13 degrees to 23 degrees C), are able to distinguish a "short-day" thermoperiod ( 13 hours at 23 degrees C per day) from a "long-day" thermoperiod ( 13 hours at 23 degrees C per day) and produce diapausing or developing progeny accordingly.

Timing system for Ge(Li) detectors

Abstract A timing system for coincidence and nuclear lifetime measurements with Ge(Li) detectors is described. The timing discriminator, based on the ARC-principle, is designed using integrated circuits. Uncompensated pulses from low energy events near the threshold of the timing discriminator are eliminated by a special internal coincidence circuit. Output pulses caused by noise, are excluded by duration discrimination. The detector signals are amplified by a preamplifier and a fast amplifier with 2.5 and 3.5 ns risetimes, respectively. The time resolution has been obtained 15.6, 2.7 and 1.1 ns fwhm for energies 50, 511 and 1330 keV, respectively, by using a planar detector (1.9 cm 3 ) with a collection time of 40 ns. The differential peak shift in the range from 10 to 1330 keV was 1.2 ns.

Timing System for Ge(Li)-Detectors

A timing system for coincidence and nuclear lifetime measurements with Ge(Li)-detectors is described. The timing discriminator, based on the ARC-principle, is designed using integrated circuits. Uncompensated pulses from low energy events near the threshold of the timing discriminator are eliminated by a special internal coincidence circuit. Output pulses caused by noise, are excluded by duration discrimination. The detector signals are amplified by a preamplifier and a fast amplifier with 2.5 and 3.5 nsec risetimes,respectively. The time resolution has been obtained 15.6 , 2.7 and 1.1 nsec FWHM for energies 50, 511 and 1330 keV, respectively, by using of a planar detector (1.9 cm3) with a charge collection time of 40 nsec. The differential peak shift in a range from 10 to 1330 keV was 1.2 nsec.

Testicular Responses of House Sparrows and White-Crowned Sparrows to Short Daily Photoperiods with Low Intensities of Light

Testicular Responses of House Sparrows and White-Crowned Sparrows to Short Daily Photoperiods with Low Intensities of Light

Determination of the Fierz Interference Term From the K-Capture/ +Ratio for the Pure Gamow-Teller Decay of58Co

A direct determination has been made of the Fierz interference term b for the pure Gamow-Teller decay of 58Co. The K/β+ ratio has been determined by an internal source scintillation counter coincidence method. The value obtained was 4.92 ± 0.09. This may be compared with the recent predicted value of 4.87. Allowing for the variation in the theoretical value arising from β+ end point uncertainty this yields a value for b of (+0.6 ± 2.3)%. Two interpretations of the result are possible on the basis of two-component neutrino theory. If, as is customarily assumed, CA = CA', CT = -CT', then b should vanish identically; and the small experimental value for b confirms the theory. If alternatively, CA = CA', CT = +CT' then the experimental result requires that CT, CT' be very small.

Testicular Response of the House Sparrow, Passer Domesticus, to Short Photoperiods and Low Intensities

Testicular Response of the House Sparrow, Passer Domesticus, to Short Photoperiods and Low Intensities