Effect of Carotenoid Depletion on the Hatching Rhythm of the Silkworm, Bombyx mori

Abstract

To investigate the functional involvement of carotenoid in the insect circadian rhythm, we observed the effect of carotenoid depletion on the hatching patterns of the silkworm under several light conditions. When eggs were transferred from continuous light (LL) to continuous darkness (DD), an overt hatching rhythm was initiated. The first hatching peak, which was observed at 13.2 h after the transfer in the carotenoid-depleted silkworm, was reduced remarkably in both control groups (carotenoid-rich and carotenoid-depleted but with lutein supplementation), though subsequent peaks occurred with similar timing. Under LD 4 : 20, LD 12 : 12 and LD 20 :4, hatching patterns depended on the dietary carotenoid and the light intensity of the photophase. At a low light intensity, carotenoid depletion suppressed hatching as a masking effect just after light-on. Under LD 4 : 20 at a low light illumination, hatchings in the carotenoid-depleted silkworm were observed during scotophase, but few larvae hatched during scotophase in the control groups. Our findings suggest that carotenoid is not indispensable for the photoreception, formation and entrainment of the circadian hatching rhythm, but that a carotenoid-dependent process that is induced by light-off and damps out in an hour-glass mode, suppresses the hatching during darkness without affecting the circadian oscillation. The possibility that this carotenoid-dependent process is involved in the photoperiodic induction in the silkworm was discussed.