Fine structure of degenerating and regenerating flight muscles in a bark beetle, Ips confusus. II. Regeneration

Abstract

The flight muscles of the bark beetle Ips confusus regenerate by two means, by formation and differentiation of new myoblasts, and by the regeneration of the old flight muscle itself. Mononucleated myoblasts appear in beetles which have been in the inner bark of ponderosa pine logs for 5 days. These cells apparently fuse with other myoblasts to form multinucleated cells. By the end of the ninth day of regeneration the myofilaments become attached to an incoherent Z line. By the 11th day of regeneration these differentiating myoblasts appear very much like the fibers of the regenerating old flight muscle.Simultaneously the fibers of the old degenerate muscles show signs of regeneration. On the sixth day after the beetles entered the bark, rearrangement of the existing degenerate myofilaments takes place. The incoherent and diffused Z line shows some degree of reorganization. Numerous ribosomes are present between the filaments. Between the 7th to 11th days of regeneration the mitochondria appear to fuse to form giant mitochondria up to five sarcomeres in length. These mitochondria by subsequent divisions give rise to numerous mitochondria. Almost invariably the line of mitochondrial fission is aligned with the Z line. The presence of numerous ribosomes and polysomes in the fibers indicate a high protein synthetic activity. By the end of the 13th day regeneration of the flight muscle appears complete and the beetles are now ready to reemerge. These results further confirm our earlier observation (Bhakthan et al. 1970) that flight muscle degeneration in I. confusus is a reversible process.