Role of neurosecretory cells in the photoperiodic induction of pupal diapause of the tobacco hornworm Manduca sexta

In the tobacco hornworm, Manduca sexta, pupal diapause can be induced by exposure of fifth-instar larvae to a short-day photoperiod. We studied the effect of surgical ablation of tissues containing the neurosecretory cells of the brain of fifth-instar larvae on the photoperiodic induction of pupal diapause. At the end of the experiments, we immunostained the neurosecretory cells to determine the success of the ablations. Under long-day conditions (LD 16:8 at 22°C), all intact larvae, most of the sham-operated larvae, and control-operated larvae developed into nondiapausing pupae. Under short-day conditions (LD 10:14 at 22°C), most intact, sham-operated, and control-operated larvae developed into diapausing pupae. Removal of type-II cells did not interfere with the photoperiodic response. Under long-day conditions, elimination of type-Ia₁ cells did not affect the incidence of nondiapausing pupae. When type-Ia₁ cells were removed under short-day conditions, however, the incidence of nondiapausing pupae was higher (51%, n = 41) than that of the intact (16%, n = 75), sham-operated (24%, n = 88), control-operated larvae (5%, n = 40), and larvae with type-II cells removed (11%, n = 27). Thus, removal of type-Ia₁ cells can impede induction of diapause. These results indicate that the type-Ia₁ neurosecretory cells have an important role in the induction of pupal diapause.