The prothoracic glands of Rhodnius prolixus contain a photosensitive circadian clock that regulates rhythmic steroidogenesis

The cellular responses comprising insect development are elicited by steroid moulting hormones (ecdysteroids) synthesized by the prothoracic glands (PGs). We recently showed that ecdysteroid synthesis by the PGs of Rhodnius prolixus is rhyhmic and under circadian control in vivo. The PGs receive daily rhythmic cues in vivo from prothoracicotropic hormone (PTTH), whose release is also under circadian control. However, the rhythm of ecdysteroid synthesis in not a passive 'slave' driven by rhythmic PTTH input. We report here that PGs in vitro are directly photosensitive and contain their own circadian clock. Arrhythmic PGs (from animals reared in continuous light) were maintained in vitro for 48 h. A 'lights-off cue given at any time in vitro promptly elicits a free-running rhythm of ecdysteroid synthesis with peaks in subjective nights. Therefore, the PGs contain a photosensitive circadian clock that controls ecdysteroid synthesis.