Tomoyosi Nisimura

Bimodal life cycle of the burying beetle Nicrophorus quadripunctatus in relation to its summer reproductive diapause

Abstract 1. Under natural conditions in Kyoto, Japan, the reproductive activities of Nicrophorus quadripunctatus Kraatz (Coleoptera: Silphidae) decreased in summer and the species showed a bimodal life cycle.

Circannual rhythm in the varied carpet beetle, Anthrenus verbasci

Although circannual rhythms controlling different physiological processes and various aspects of behavior have been reported in numerous organisms, our understanding of the underlying biological mechanisms is still quite limited. We examined the mechanisms controlling the circannual pupation rhythm of the varied carpet beetle, Anthrenus verbasci. This rhythm is self-sustainable, exhibits temperature compensation of the periodicity, and is entrainable to environmental changes. In addition, the circannual phase response curves to a photoperiod pulse display Type 0 or Type 1 resetting, depending on the duration of the pulse. Thus, we infer that this rhythm is derived from a self-sustaining biological oscillator with a period of about a year, that is, a circannual clock, analogous to the circadian clock. Further, a circadian clock appears to mediate photoperiodic time measurement for phase resetting of the circannual clock. Based on these results and previous research performed in other organisms, we discuss the general characteristics of the physiological mechanisms underpinning circannual rhythmicity.

A circadian system is involved in photoperiodic entrainment of the circannual rhythm of Anthrenus verbasci.

In the circannual pupation rhythm of the varied carpet beetle, Anthrenus verbasci, entrainment to annual cycles is achieved by phase resetting of the circannual oscillator in response to photoperiodic changes. In order to examine whether a circadian system is involved in expression of the periodic pattern and phase resetting of the circannual rhythm as photoperiodic responses, we exposed larvae to light-dark cycles with a short photophase followed by a variable scotophase (the Nanda-Hamner protocol). When the cycle length (T) was a multiple of 24h, i.e., 24, 48, or 72 h, short-day effects were clearer than when T was far from a multiple of 24h, i.e., 36 or 60 h. Exposure to light-dark cycles of T=36 h had effects similar to exposure to long-day cycles of T=24h. The magnitude of phase shifts depended on the duration and the phase of exposure to the cycles of T=36 or 60 h. It was therefore concluded that a circadian system is involved in photoperiodic time measurement for phase resetting of the circannual oscillator of A. verbasci.

Circannual pupation rhythm in the varied carpet beetle Anthrenus verbasci under different nutrient conditions

Anthrenus verbasci pupates in spring and the timing of pupation is controlled by a circannual rhythm. Although A. verbasci is considered to be a univoltine species in Japan, it is assumed that larval development in its natural habitats, including bird nests, varies with nutrient availability, and that the life cycle often takes two or more years to complete. In the present study, larval development and pupation times were compared under constant and outdoor conditions in larvae provided a diet of either high‐nutrient bonito powder or low‐nutrient pigeon feathers. Although a circannual pupation rhythm was observed irrespective of the diet used, larval development was slower on feathers than on bonito powder. The pupation times on feathers varied over three years or more under both constant and outdoor conditions. Under outdoor conditions, larvae grown on feathers needed three years to approach the weight gained within a year by larvae grown on bonito powder. It is considered that life cycle length in A. verbasci is often two years or more in nutritionally unstable natural habitats, and that this species has probably evolved a circannual rhythm as a seasonal adaptation to nutrient‐poor environments.

Effect of temperature on circadian rhythm controlling the crepuscular activity of the burying beetle Nicrophorus quadripunctatus Kraatz (Coleoptera: Silphidae)

Locomotor activity rhythm was examined at various temperatures under a 16 h light : 8 h dark photoperiod (LD 16:8) or LD 12:12 using adults of the burying beetle Nicrophorus quadripunctatus. At 20°C, the locomotor activity of the beetles showed a bimodal daily pattern with two peaks around lights on and lights off under both photoperiods. This bimodal activity rhythm persisted under constant darkness; therefore, the activity of adult N. quadripunctatus is controlled by a circadian clock. Adults showed a bimodal activity pattern for temperatures ranging from 15 to 25°C. The evening peak of the daily activity was earlier at lower temperatures. These findings suggest that in the field, N. quadripunctatus shows crepuscular activity, and is active earlier in the afternoon in cooler seasons. In this species, therefore, temperature appears to play an important role in the determination of daily activity patterns.

Circannual Rhythms in Insects

Although many insects adapt to seasonal changes by photoperiodism, a small proportion of insect species use a circannual rhythm for seasonal adaptations. The circannual pupation rhythm of the varied carpet beetle Anthrenus verbasci shows a periodicity of approximately 40 weeks under constant conditions, and the change in photoperiod acts as a zeitgeber. The circannual rhythm of A. verbasci, of which the larval duration varies from one to several years, probably plays an important role for synchronizing the pupation and breeding times with spring each year. There are only slight differences in the critical daylength for circannual entrainment among geographically distinct populations in Japan, and they pupate in the same period under natural conditions in Osaka. Therefore, A. verbasci can adapt to seasonal changes in different regions without changing the parameters of the circannual rhythm. Long-term endogenous rhythms have also been reported in oviposition and pupation of some ant species. These insects are considered to refer not only to external cues but also to the phase of an endogenous clock for maintaining appropriate seasonality.