Development of an indirect flight muscle in a muscle-specific mutant of Drosophila melanogaster.

Stripe (sr) is a highly specific mutant affecting only one of the indirect flight muscles, the dorsal longitudinal muscle (DLM). In the homozygous condition the DLM is reduced in size. In the hemizygous condition (sr/Df(3)sr) no DLM is present in the adult, though all other thoracic muscles are present. In the early stages of pupation, DLM development in sr/Df(3)sr is no different from that in wild type. Adult myocytes collect around target larval muscles and fuse to form myotubes; myofilaments are synthesized. Subsequently (35-hr pupa) the DLM commences to degenerate, forming random clumps of vacuolated muscle tissue. Adjacent muscles are unaffected and develop normally. In the adult a neuroma-like mass of nerve tissue is maintained where the DLM would normally be located. In this mass many abnormal synapses (hemisynapses) are seen: presynaptic specializations occur in the absence of any postsynaptic structure. Small remnants (less than 16-microns diameter) of muscle tissue are sometimes found in the neuroma-like mass. Such remnants resemble slow muscle, not the normal fast type of DLM. These data suggest a possible muscle origin from primary and secondary myotubes. The DLM motor axons are present in the neuroma-like mass, persisting even with the virtual degeneration of their end target. Thus, motoneurons and presynaptic specializations can survive independently of postsynaptic targets.