Death commitment in the anterior silk gland of the silkworm, Bombyx mori

The insect steroid hormone, 20-hydroxyecdysone (20E) triggers the programmed cell death (PCD) of the anterior silk glands (ASGs) of the silkworm, Bombyx mori. We tried to determine the time of commitment to die (death commitment) by examining ASG responses to 20E and juvenile hormone analogue (JHA) in vivo as well as in vitro. The ASGs obtained late on day 6 of the fifth instar completed PCD when cultured with 20E, while the ASGs obtained on day 4 and cultured with 20E did not undergo PCD. The ASGs became competent to respond to 20E at mid-day 5. The ASGs with responsiveness to 20E were not sensitive to JHA, indicating that the ASGs were committed to die before becoming capable of responding to 20E. Topical application of JHA on day 4 suppressed 20E-induced PCD, but that on day 5 failed to do so, indicating that the death commitment might occur between day 4 and 5. We also determined the time of death commitment after allatectomy of the fourth instar larvae, a procedure that induced the precocious PCD. Timed application of JHA and culture of ASGs with 20E in the presence of JHA showed that the ASGs had lost their sensitivity to JHA between 72 and 96 h after allatectomy, i.e. 24–48 h before precocious gut purge in the allatectomized larvae. This result is similar to that obtained in the fifth instar. We conclude that the cellular commitment to die takes place one day before the ASGs become competent to respond to 20E.