PROGRAMMED DEGENERATION OF THORACIC ECLOSION MUSCLE IN THE FLESH FLY, SARCOPHAGA-BULLATA

Some muscles of holometabolous insects show a programmed degeneration process after metamorphosis which is thought to be under the control of ecdysone and eclosion hormone. We describe the degeneration of thoracic muscles in the flesh fly, Sarcophaga byllata, and experiments suggesting that, apart from eclosion hormone, a new, unknown trigger signal is involved in the degeneration. Flesh flies possess a set of 4 pairs of thoracic muscles which are used during eclosion behavior and later degenerate. This programmed cell death occurs in two phases. First the muscles lose their contractility within 30 min of eclosion, although the membrane potential remains for 6 h, at which time EPSPs can still be detected. In addition, all eclosion muscles lose their high glycogen content during the first 4 h despite being inactive. During the second degeneration phase, between 12 and 24 h after eclosion, the muscle fibers lose their myosin ATPase and succinate dehydrogenase activities and become completely lysed, Experiments using ligation and muscle isolation in different stages suggest that two control signals are involved in the degeneration process. The first is released prior to eclosion and is likely to be eclosion hormone. It causes a slow degeneration process. Degeneration is then accelerated by the second signal, released 20-30 min after eclosion. This signal causes the loss of muscle contractility. It is not bursicon and probably not a direct neuronal signal. Our data differ from previous reports on insect muscle degeneration both in Diptera and Lepidoptera.