During the ovarian cycle of the Hawaiian cockroach, Diploptera punctata, a mitotic wave occurs in the corpora allata (CA) of mated females, before an increase in gland volume and juvenile hormone synthesis. Chilling females of the Hawaiian cockroach, a day after mating, effectively suppresses proliferation of CA cells, and, in insects chilled for 3h, the mitotic wave is practically abolished (Pszczolkowski and Chiang, J. Insect. Physiol. 46, 923-931). We studied the potential role of ecdysteroids in suppression of CA cell proliferation by chilling.
In comparison to intact controls, ecdysteroid levels in haemolymph of chilled cockroaches were elevated shortly after termination of chilling. Cellular divisions were suppressed in a dose- dependent manner by topical applications of 20-hydroxyecdysone (20E) 24h after mating. Also, application of 0.4 microgram of 20E per insect 24h after mating suppressed the mitotic wave in the CA. The same dose of 20E had no effect when applied 48h or 72h after mating. Severance of the ventral nerve cord had no effect on CA cell proliferation, whereas application of 20E suppressed CA cellular divisions. In the same experiment, a combination of ventral nerve cord severance with 20E application had no effect on the dynamics of CA cell mitotic divisions.
We conclude that elevation of 20E levels in response to chilling suppresses mitotic divisions in the corpus allatum of Diploptera punctata. The signal from 20E is transmitted via the ventral nerve cord.
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