Ultrastructure of juvenile hormone-induced degenerating flight muscles in a bark beetle,Ips paraconfusus

Summary Topical application of 5 g of a juvenile hormone analogue (JHA), ZR-615, to female callow adults of Ips paraconfusus induced degeneration of the dorsoventral flight muscles. Within 24h after JHA-treatment the diameter of the myofibrils was reduced to almost half due to the lysis of the peripheral myofilaments. Mitochondria showed conspicuous degenerative changes like swelling, dissolution of the matrix or presence in the matrix of dense filamentous material or myelin-like figures. Degeneration of the mitochondria seemed to take place inside isolation membranes derived from sarcoplasmic reticulum. A number of granular osmiophilic bodies appeared in the sarcoplasm. Three days after JHA-treatment the muscles were very thin and sheath-like. Most of the mitochondria had already degenerated. The dense sarcoplasm contained numerous crystalline bodies. The granular dense bodies were also more frequent. The myofibrils were comprised of only occasional small bundles of myofilaments. The tubules of the T system enclosed an amorphous material. The nuclei and the tracheal system remained intact but they were crowded due to the decreased volume of the muscle. In some specimens degeneration of the myofibrils and mitochondria was completed by the third day. No sign of degeneration was observed in the flight muscles of acetone treated control insects.Supported by the National Research Council of Canada grant A4669.We thank Dr. G.B. Staal, Zoecon Corp., Palo Alto, California, for the generous gift of ZR 615, and Messrs A. Syed and M. Horta for rearing the insects