Photoperiodic Time Measurement in Insects: A Review of Clock Models

Abstract

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.

Tracking the seasons: the internal calendars of vertebrates

“…As day length changes, the changing phase relation between the two oscillators encodes seasonal time. Several variations of these models have been developed more recently to account for findings in insects (see Vaz Nunes & Saunders 1999).…”

“…While the skeleton photoperiods described above suggest that the circadian system is used for PTM, they do not rule out the use of other non-circadian timing mechanisms. Nanda-Hamner, Bunsow and bi-stability experiments provide the best evidence for circadian involvement in PTM ( Vaz Nunes & Saunders 1999). The Nanda-Hamner protocol, also referred to as resonance photocycles, has been used extensively (see also Elliott 1976).…”

“…These results are compatible with a circadian mechanism in which light in the non-24 h photocycles falls during the postulated photo-inductive phase every few cycles or alters the phase relation between morning and evening circadian oscillators. Circadian involvement in PTM has been implicated in insects ( Vaz Nunes & Saunders 1999), fishes (Baggerman 1972;Bromage et al 1990), reptiles (Underwood & Hyde 1990), birds (Hamner 1963(Hamner , 1964) and mammals (Elliott et al 1972;Almeida & Lincoln 1982;Nelson et al 1982) and is believed to be ubiquitous among birds and mammals. In insects, reliance on the circadian system for PTM is more variable, with some species using an hourglass mechanism ( Vaz Nunes & Saunders 1999).…”

Extrinsic light:dark cycles, rather than endogenous circadian cycles, affect the photoperiodic counter in the pitcher-plant mosquito, Wyeomyia smithii

“…Photoperiodic time measurement in arthropods is generally considered to be the result of two separate, though related, components: (1) a photoperiodic timer that distinguishes between long and short days (or nights), and (2) a photoperiodic counter that accumulates information from the timer and then triggers downstream processes when some threshold of information has been reached (vaz Nunes and Saunders 1999;Saunders 2002). Herein, we are concerned with the role of light in the accumulation of inductive ("long-day") cycles by the photoperiodic counter used to terminate diapause and how that role changes with latitude and altitude.…”

“…Models of the physiological mechanism of the photoperiodic counter fall into two categories: (1) models that require the repeated input of light for the counter to accumulate inductive cycles and, (2) models that require light only to set internal circadian oscillators that are then able to accumulate inductive cycles in the absence of light (vaz Nunes and Saunders 1999). The first category includes two classes of models that do or do not rely on circadian rhythmicity.…”