Abstract
High-temperature (615°C) treated leaves of the mulberry tree, Morus alba L., contain a factor (mulberry factor, MF) that causes 5th instars of the silkmoth, Bombyx mori L. (Lepidoptera : Bombycidae), to become permanent larvae (SUDO and TANAKA, 1989). When allatectomized 5th instars were reared on a diet containing those leaves (HT diet), they became permanent larvae, as did sham-operated and intact controls. This suggests that the action of MF is not mediated through modifying the activity of corpora allata. Ecdysteroid titers in hemolymph, which began to increase within a week after molting to the 5th instar on a standard diet, remained at a low level longer than 2 weeks both in allatectomized and intact or sham-operated larvae kept on the HT diet. This reduced ecdysteroid titer may be the main cause for induction of permanent larvae. In organ culture, prothoracic glands isolated from 5-day old 5th instars reared on the HT diet were found competent to respond to prothoraciocotropic hormone (PTTH) by secreting ecdysone into the medium. It is thus likely that MF caused low ecdysteroid titers in hemolymph by suppressing secretion of PTTH from the brain. When 4th instars were reared on the HT diet, growth and the hemolymph ecdysteroid titer were not altered prior to the molt to the 5th instar.
