HOURGLASS AND OSCILLATOR EXPRESSIONS OF PHOTOPERIODIC DIAPAUSE RESPONSE IN THE CABBAGE MOTH MAMESTRA-BRASSICAE

Both oscillator and hourglass features are found in the photoperiodic response that controls the pupal winter diapause of Mamestra brassicae. The expression of oscillatory response to extended long-night cycles is temperature dependent, i.e. circadian resonance appears at 23 and 25-degrees-C but not at 20 and 28-degrees-C. At 20-degrees-C, scanning of extended scotophases by a short light pulse does not reveal any clear circadian rhythmicity. However, a circadian feature of the photoperiodic response is indicated even at 20-degrees-C by a bistability phenomenon, i.e. either one of the two dark periods in symmetrical skeleton photoperiods determines the diapause response depending on the phase angle with the preceding (entraining) light-dark cycles. At 20 and 25-degrees-C, the incidence of diapause increases as a function of the number of light - dark cycles regardless of the cycle length (T), if T is 24 h or 2 x 24 h (with a 12 h light period). A non-diel cycle (T = 36 h) is less effective, suggesting that disturbance of the circadian organization partly impairs the diapause-inducing function. The inductive effect of a long night is largely affected by temperature, and becomes saturated with eight cycles at 20-degrees-C and 14 cycles at 25-degrees-C. Presumably, an hourglass mechanism measures the dark time, and a circadian component involved in some later sequence of the photoperiodic response may or may not be expressed depending on the mode of interaction between them.