SUBSTRUCTURES IN THE CORE OF THICK FILAMENTS - ARRANGEMENT AND NUMBERIN RELATION TO THE PARAMYOSIN CONTENT OF INSECT FLIGHT MUSCLES

Transverse sections (100-140 nm thick) of solid myosin filaments of th e flight muscles of the honeybee, Apis mellifica, the fleshfly, Phormi a terrae-novae and the waterbug, Lethocerus uhleri, were photographed in a JEM-200 electron microscope at 200 kV. The images were digitized and computer processed by rotational filtering. The power spectra of t he images of each of these filaments showed six-fold symmetry for the outer wall region and three-fold symmetry for the inner wall region. I mages of the honeybee additionally showed three-fold symmetry for the center of the filament. Considering both paramyosin content of the myo sin filaments and the results of the rotational filtering, we suggest the existence of 3 paramyosin strands in the myosin filaments of the f leshfly, 6 paramyosin strands in the honeybee filaments and 5 strands in the myosin filaments of the waterbug. In the case of the honeybee, the 3 paramyosin strands of the inner wall are positioned directly opp osite the myosin subfilaments, while the 3 strands of the center seem to be arranged opposite the gaps between the myosin subfilaments. The paramyosin filaments of the fleshfly wobble between 2 myosin subfilame nts, without loosing their three-fold symmetry arrangement in the inne r wall. The 3 paramyosin strands in the inner wall of the waterbug myo sin filaments are either arranged opposite the myosin subfilaments or opposite the gaps between the subfilaments. Finally, we were able to g enerate a 3-dimensional reconstruction of the myosin filament of the h oneybee, showing the parallel arrangement of both, myosin subfilaments and paramyosin strands, relative to the long filament axis.