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 o f ecdysone and eclosion hormone. We describe the degeneration of thora cic muscles in the flesh fly, Sarcophaga byllata, and experiments sugg esting that, apart from eclosion hormone, a new, unknown trigger signa l is involved in the degeneration. Flesh flies possess a set of 4 pair s of thoracic muscles which are used during eclosion behavior and late r degenerate. This programmed cell death occurs in two phases. First t he muscles lose their contractility within 30 min of eclosion, althoug h the membrane potential remains for 6 h, at which time EPSPs can stil l be detected. In addition, all eclosion muscles lose their high glyco gen content during the first 4 h despite being inactive. During the se cond degeneration phase, between 12 and 24 h after eclosion, the muscl e fibers lose their myosin ATPase and succinate dehydrogenase activiti es and become completely lysed, Experiments using ligation and muscle isolation in different stages suggest that two control signals are inv olved in the degeneration process. The first is released prior to eclo sion and is likely to be eclosion hormone. It causes a slow degenerati on process. Degeneration is then accelerated by the second signal, rel eased 20-30 min after eclosion. This signal causes the loss of muscle contractility. It is not bursicon and probably not a direct neuronal s ignal. Our data differ from previous reports on insect muscle degenera tion both in Diptera and Lepidoptera.