The behaviour and coupling of the photoreceptor and hour-glass photoperiod timer at low temperature in the aphid Megoura viciae

In Megoura viciae photoperiodic time is measured during the scotophase by an hour-glass mechanism which is temperature compensated within the range 6–20°C. At lower temperatures the clock “loses time”, running at about half the normal rate at 0°C. The present experiments, based on the analysis of critical night lengths, showed that for short periods of scotophase chilling (4h or less) the same diminished rate is maintained irrespective of the position of the low temperature period within the scotophase. This occurs despite the known changes in spectral sensitivity of the photoreceptor. With longer periods of chilling (>6h) a normal critical response was not observed unless several hours of darkness intervened between the end of the chilling period and the temperature-modified critical night length. This is interpreted as a “recovery” process involving the photoreceptor but not the hour-glass. Additional data indicated that the sequence of changes in the photoreceptor, including the early resetting phase, is closely coupled to the progression of the timer even when photosensitivity is temporarily impaired by low temperature.