Marlies Vaz Nunes

PHOTOPERIODIC TIME MEASUREMENT IN INSECTS : A REVIEW OF CLOCK MODELS

Based on analyses of responses of insects and mites to a wide range of diel and nondiel experimental light-dark schedules, a variety of models have been developed for the photoperiodic clocks in these species by nearly as many investigators. According to some of these models, the photoperiodic clock is based on a mechanism separate from the circadian system, that is, a so-called “hourglass.” According to other models, the clock is based on one or more circadian oscillators that may be coupled to each other and that may or may not show a certain degree of damping. In this context, a rapidly damping oscillator could be regarded as an hourglass. The present article gives an overview of the many different clock models and their philosophies, and it makes comparisons among them to provide a better understanding about how these models are related, if at all, and why the double circadian oscillator model is the most favored model at present.

Aphid photoperiodic clocks

Abstract An overview is given of photoperiodic research performed with English and Scottish clones of the vetch aphid, Megoura viciae , and the black bean aphid, Aphis fabae , during the last decade, with emphasis on the photoperiodic photoreceptor and the clock–counter mechanisms. Photoperiodic photoreceptor . The photoperiodic photoreceptors of the vetch aphid, M. viciae , are located in the brain. Immunocytochemical techniques have recently indicated that an anterior dorsal region of the protocerebrum possesses antigenic sites that are consistently labelled with a number of antibodies raised to invertebrate and vertebrate opsins and phototransduction proteins. Counter . In two clones of both M. viciae and A. fabae , long-night accumulation appears to be temperature compensated, whereas short-night accumulation is temperature sensitive. Clock . (1) The clocks of the English and Scottish clones of M. viciae can be modelled by a slowly damping long-night (LN) system and a rapidly damping short-night (SN) system. The LN system of the Scottish clone damps more slowly and has a shorter period than that of the English clone. The critical night length (CNL) of the English clone is highly temperature compensated, that of the Scottish clone is less so. (2) The clock in presumptive gynoparae of an English clone of A. fabae can be modelled by rapidly damping LN and SN systems; the SN systems damping rate being temperature dependent, but that of the LN system being temperature compensated. (3) The clocks in gynopara and male producers of the English clone of A. fabae are mimicked by rapidly damping LN systems and self-sustained SN systems, while in a Scottish clone they are assumed to consist of slowly damping LN systems and self-sustained SN systems. (4) The CNLs for gynopara and male production are temperature compensated in both the English and the Scottish clones of A. fabae .

The effect of temperature on the photoperiodic ‘counters’ for female morph and sex determination in two clones of the black bean aphid, Aphis fabae

Experiments were performed on two clones of the black bean aphid, Aphis fabae Scopoli – one from Aberdeen, Scotland (57°N), the other from Cambridge, England (52°N) − to determine the number of long‐ or short‐night cycles required for 50% induction of winged versus wingless females on the one hand and males versus females on the other (i.e. required day number, RDN), at three temperatures, 12.5, 15 and 17.5°C. In the case of female morph determination, the RDN for long‐night cycles was temperature compensated, whereas that for short‐night cycles was highly temperature dependent. For sex determination, the RDN for long‐night cycles was again temperature compensated, whereas, due to the mechanism of sex determination, male production was close to 100% in our protocol, even with a maximal number of short‐night cycles, and the RDN could therefore not be assessed. Model‐generated response curves, using the recently developed ‘double circadian oscillator model’ for photoperiodic time measurement in insects and mites, closely resembled the observations. It could also be shown that differences observed between response curves of female morph and sex determination in the Scottish clone were due, according to the model, to differences in their photoperiodic ‘counters’, rather than to differences in their clocks.

A model for the photoperiodic counter in the aphid Megoura viciae

The photoperiodic counter in the aphid Megoura viciae was analysed by means of a previously developed general model of the photoperiodic counter mechanism in insects and mites. It was shown that the counter in M. viciae accumulated long scotophases as well as short scotophases. It could further be shown that the inductive strength, or “value”, of long and short scotophases depended to a certain extent on photophase duration: long, 12-h scotophases accompanied by long photophases (> 18 h) decreased in value, whereas the inductive strength of short, 8-h scotophases decreased when accompanied by short photophases (< 16 h). Sensitivity to photoperiod changed slightly during the 10-day postnatal sensitive period at 15°C. Although sensitivity to short-night cycles (16 h light-8 h dark) showed a maximum at around day 7 and thereafter decreased sharply, sensitivity to long-night cycles (12 h light-12 h dark) did not appear to change. In M. viciae the “required day number”, which is the number of long- or short-night cycles required to elicit a 50% response, could only be determined at 12 and 15°C, as at higher temperatures (17 and 20°C) the long-night response (ovipara-production) was attenuated. For the low temperatures it was found that the accumulation of short-night cycles was far more temperature sensitive than that of long-night cycles. The observed attenuation of the long-night response at the higher temperatures could be explained in terms of the model, by assuming that the “minimally required induction sum” (Smin) is extremely sensitive to temperature, with a temperature coefficient much higher than the temperature coefficients of the photoperiodic values of both long-night cycles and short-night cycles.

Thermoperiodic Responses in Insects and Mites Simulated with the Double Circadian Oscillator Clock

The “double circadian oscillator model” for the photoperiodic clock has been used to simulate thermoperiodic responses in insects and mites. Two assumptions have been made: (1) the clock measures cryophase in a similar way to scotophase, and (2) temperature cycles are able to entrain the clock in a similar way to LD cycles. Simulations showed that Assumption 1 causes the “critical cryophase” to be of about equal duration as the critical night length. Assumption 2 is not always needed if diapause incidences in DD are high at low temperatures but low or zero at high temperatures. The latter assumption is needed, however, if high diapause occurs in thermoperiodic cycles in DD, whereas nondiapause occurs in DD with both high and low constant temperatures. The model accounts for the observation that the amplitude of the temperature cycle is important in some insects, whereas the temperature of the cryophase is crucial in others.

Differential photoperiodic responses in genetically identical winged and wingless pea aphids, Acyrthosiphon pisum, and the effect of day length on wing development.

Abstract. Winged and wingless individuals of a pink clone of the pea aphid, Acyrthosiphon pisum (Harris), showed differences in the response curves for photoperiodic induction of both males and sexual females (oviparae). The critical night length (CNL) for ovipara induction in winged aphids was 0.75 h shorter than in wingless aphids, whereas the CNL for male induction in winged aphids was 1.0h longer than in wingless aphids. This means that in winged aphids the CNL for male induction in winged aphids was 0.5 h longer than that for ovipara induction, while in wingless aphids the CNL for male induction was 1.0–1.5h shorter than that for ovipara induction, and also the shapes of the curves differed.

A pacemaker-slave model for the photoperiodic clock in the vetch aphid, Megoura viciae

Abstract Recent investigations into the photoperiodic clock of Megoura viciae indicated that night-length measurement in this aphid is accomplished by a circadian-based oscillator mechanism, despite its typical “hourglass” responses in Nanda-Hamner and Bunsow experiments. To reconcile these apparently contradicting observations, it is suggested here that although long-night determination is executed in a repetitive manner, this is not true for short-night determination. To test this hypothesis, the previously developed “coupled oscillator model” has been modified such that once a scotophase has exceeded the critical night length (as in the Nanda-Hamner experiments) it will always be determined as “long”, no matter whether a subsequent “photo-inducible phase” coincides with darkness or with light. In this respect the modified model differs from all known circadian-clock models. Model-generated responses are compared with a variety of experimental results previously observed with Megoura and the validity of the model as a description of the photoperiodic clock-counter mechanism in Megoura is discussed.

‘Bistability’ experiments and the photoperiodic clock in the spider mite Tetranychus urticae

In insects and mites, photoperiodic induction of diapause comprises at least two processes: night-length measurement by means of a photoperiodic ‘clock’ and the subsequent accumulation of photoperiodic information contained in a sequence of light-dark cycles (photoperiodic ‘counter’). It has been shown that in many species the circadian system is involved in photoperiodic induction, but whether it plays a role in nightlength measurement itself (i.e. as the clock) or is more indirectly involved in the induction process, is still largely a matter of dispute. Many experimental protocols have been designed to try to solve this problem, and one of those is the so-called ‘bistability’ protocol. This protocol consists of so-called ‘symmetrical skeleton’ photoperiods (i.e. consisting of two short light pulses per 24 h (Pittendrigh, 1966)), with both light pulses close to 12 h apart, for example L1:D10:L1:D12. Work on circadian rhythms (e.g., the eclosion rhythm in Drosophila pseudoobscura (Pittendrigh,1966)) demonstrated that symmetrical skeleton photoperiods simulated ‘complete’ photoperiods, such that the shorter of the two dark phases was always ‘interpreted’ as light. For example, L1:D8:L1:D14 and L1:D14:L1:D8 both always simulated L10:D14. However, when the light pulses were placed close to 12 h apart, both the shorter and the longer dark phase could be interpreted as light. This region with two possible ‘interpretations’ was called the ‘zone of bistability’ (Pittendrigh, 1966). Which interpretation was adopted, depended on (1) the circadian time at which the first light pulse started and (2) the length of the first dark phase. The bistability protocol was applied to the spider mite, Tetranychus urticae. Another experimental protocol had already shown, that the mite’s clock is based on a non-circadian, or ‘hourglass’, mechanism (Veerman & Vaz Nunes, 1987). It was expected, therefore, that bistability would not occur in photoperiodic induction in this species.

The photoperiodic counter in the black bean aphid, Aphis fabae

Abstract The photoperiodic counter mechanism in the black bean aphid, Aphis fabae, was analysed by exposing newly-born, presumptive gynoparae to a variety of photoperiodic regimes at 15°C, and by the development of a model. It was shown that the counter accumulated long as well as short scotophases and that the “inductive strength”, or “value”, of long and short scotophases depended on photophase duration. Whilst the value of a long, 12-h, scotophase increased when accopanied by long photophases (>32 h), the value of a short, 8-h, scotophase decreased when accompanied by short photophases (

Photoperiodic induction of winged females in the black bean aphid. Aphis fabae

Abstract. The Photoperiodic of winged females (alatae) in the black bean aphid, Aphis fabae Scop. (Homopetera: Aphididae), is investigated in detail with emphasis on the interaction of the maternal and embryonic/young larval photoperiodic clocks. Previous work had shown that in uncrowded conditions the induction of gynoparae (winged females that produce sexual females) requires both prenatal and postnatal exposure to long‐night (12 h) Photoperidic cycles: present results show that sole postantal exposure to long nights of any lenght does not induce wing formation in early‐born aphids.