Ontogeny of the circadian system controlling release of sperm from the insect testis

In the gypsy moth, the release of sperm bundles from the testis into the vas deferens is rhythmic and is controlled by a circadian pacemaker located in the reproductive system. However, in males kept since pupation in constant darkness (DD) and temperature, the release of sperm was arrhythmic. The release of sperm became rhythmic when males were transferred from a light-dark cycle (LD 16:8) to DD 6-7 days after pupation. To further investigate the development of the circadian system during the pupal stage, we exposed DD pupae to a single 8-hr pulse of light or 8-hr pulse of a 4 degrees C temperature increase on different days after pupation. The pattern of sperm release was determined 5-6 days after the pulse. Males that were exposed to light or temperature pulses 5 days after pupation subsequently showed nonrhythmic sperm release. However, about half of the pupae that received the pulse on day 6 and most of the pupae that received it on day 7 subsequently showed synchronized sperm release. These results suggested that the clock underlying rhythmic release of sperm becomes operational at approximately 6 days after pupation--that is, 2 days prior to initiation of rhythmic sperm release from the testis.