The photoperiodic clock and counter mechanism in two species of flies: evidence for damped circadian oscillators in time measurement

Abstract

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).