Two to three day old Mhc¹⁰ mutant females expressing Mhc^R759E.t12.4 exhibit impaired flight ability compared with control flies. The mutant flies are able to take off appropriately, however, they are unable to remain airborne for more than a few seconds and land promptly. About twice as many mutant as control flies either are unable or refused to beat their wings when flight tested. Compared with controls, mutant flies display decreased muscle length amplitude, power generation and optimum frequency of muscle oscillation. No significant differences are observed for active, passive or net tension between mutant and control fibers. Complex stiffness of the mutant muscle fibers is reduced relative to wild-type. The mutants also display some decrease in certain components of mechanical rate constants.

At the late pupal stage, myofibril structure appears normal in homozygous Mhc¹⁰ mutants expressing Mhc^R759E.t12.4, with normal thick and thin filament arrangements and intact sarcomeres. This is true for adults at 2 h after eclosion. At 2 days of age, sarcomere structure is generally normal, but occasional cracks in the mutant myofilament lattice occur. At 1 week after eclosion, mutant myofibrils show moderate to severe disruption. Hexagonal packing of filaments is not as regular as it is in wild-type, with gaps in the myofilament lattice. Thus, while mutant muscle assembly appears normal, myofibrils degenerate as flies age. Flight ability is severely

impaired in young Mhc¹⁰ mutants expressing Mhc^R759E.t12.4, and it deteriorates as flies age.