Hour-glass behavior of the circadian clock controlling eclosion of the silkmoth Antheraea pernyi.

The emergence of the Pernyi silkmoth from the pupal exuviae is dictated by a brain-centered, photosensitive clock. In continuous darkness the clock displays a persistent free-running rhythm. In photoperiod regimens the interaction of the clock with the daily lightdark cycle produces a characteristic time of eclosion. But, in the majority of regimens (from 23L:1D to 4L:20D), the eclosion clock undergoes a discontinuous "hourglass" behavior. Thus, during each daily cycle, the onset of darkness initiates a free-running cycle of the clock. The next "lights-on" interrupts this cycle and the clock comes to a stop late in the photophase. The moment when the Pernyi clock stops signals the release of an eclosion-stimulating hormone and is demonstrated to be a function of the time when the free-running cycle is interrupted by lights-on. Moreover, the width (duration) of the eclosion peak in a photoperiod is shown to be dependent upon the length of the dark phase, and, consequently, upon the amount of the free-running cycle that is completed. This relationship demonstrates that the free-running cycle may be divided into two parts. The attainment of maximal accuracy (and thus the narrowest eclosion peak) is dependent upon the completion of only the first 2 hr of the free-running cycle. The completion of succeeding portions of the cycle, while having an effect upon the time of eclosion, no longer affects the accuracy of the clock. A mechanistic model of the eclosion clock is presented.