Circadian clock controlling egg hatching in the cricket (Gryllus bimaculatus).

ABSTRACT

Adult crickets (Gryllus bimaculatus) were maintained under a 12-h light12-h dark cycle (LD 1212). After oviposition, their eggs were incubated under different lighting regimens at 23 degrees C, and temporal profiles of egg hatching were examined. When the eggs were incubated in LD 1212 or in DL 1212 with a phase difference of 12h from LD 1212, throughout embryogenesis, 88% to 97% of hatching occurred within 3 h of the dark-light transition on days 17 and 18 of embryogenesis; the phases of the egg-hatching rhythms in the LD 1212 and DL 1212 groups differed by about 12 h. In eggs incubated in constant darkness (DD) throughout embryogenesis, a circadian (about 24 h) rhythm of hatching was found, and the phase of the rhythm was similar to that seen in eggs incubated in LD 1212, but not DL 1212, throughout embryogenesis. When eggs that had been incubated in DD after oviposition were transferred to DL 1212 in the middle or later stages of embryogenesis and were returned to DD after three cycles of DL 1212, the rhythm of hatching synchronized (entrained) to DL 1212. However, when eggs in the earlier stages of embryogenesis were transferred from DD to DL 1212 and returned to DD after three cycles, 52% to 94% of hatching did not entrain to DL 1212. To determine whether photoperiodic conditions to which the parents had been exposed influenced the timing of egg hatching, adult crickets were maintained in DL 1212, and their eggs were incubated in LD 1212, DL 1212, or DD throughout embryogenesis. The egg-hatching rhythm was also found in the eggs incubated under these three lighting regimens. In DD, the phase of the rhythm was similar to that seen in eggs incubated in DL 1212, not LD 1212, throughout embryogenesis. The results indicate that in the cricket, the timing of egg hatching is under circadian control and that the circadian rhythm of hatching entrains to 24-h lightdark cycles, but only if the lightdark cycles are imposed midway through embryogenesis. Therefore, by midembryogenesis, a circadian clock has been formed in the cricket, and this is entrainable to lightdark cycles. In addition, the photoperiodic conditions to which the parents (probably the mothers) have been exposed influence the timing of hatching, suggesting that maternal factors may regulate the timing of egg hatching.