The myofibrillar structure of the thoracic muscles is completely disrupted in homozygous adults; no sarcomeres are seen and only loosely organised thick filament skeins can be discerned by electron microscopy.

Heterozygotes are flightless with a normal wing posture. Homozygotes are flightless with an abnormal wing posture. Approximately half of up² Mhc⁵ double heterozygotes have an abnormal wing posture. Approximately one-third of heterozygotes also carrying rsd¹ Act88F⁸ have an abnormal wing posture.

Muscle fibers are disorganized and Z discs show electron dense structures.

up² flies show 60% viability.

up² flies are flightless and jumpless. Some up² flies show folded mesothoracic legs after recovery from etherization.

The flight musculature is mostly present in up² flies but is grossly abnormal. Flight muscles lack cross-striations but contain disorganized thick and thin filaments and disorganized mitochondria. The anterior part of the flight muscles is more severely affected than the posterior part and the tergal depressor of the trochanters (TDTs) are generally absent or very small. At no time during pupal development is the muscle present in normal quantities or with normal morphology.

up²/+ females have a normal complement of well-organized fibres but lack 50% of the Z-band material.

Flight muscles lack cross-striations (Hotta and Benzer, 1972). 'jump' muscle absent or very small. Extracts of indirect flight muscles do not show a 54kD polypeptide that may be a Z-band component (Deak et al., 1982). Shows electron-dense structures instead of normal Z-bands (Deak et al., 1982). homozygote with abnormal wing phenotype heterozygote wing phenotype normal homozygote unable to fly heterozygote unable to fly