Toroidal bands in polytene chromosomes of Drosophila.

Results obtained from the thin-section electron microscopy of salivary gland chromosomes of Drosophila melanogaster mainly support the concept of cable-like organization of polytene chromosomes, with disk-like bands composed of parallel bundles of homologous chromomeres. Outward orientation of loop fibres may generally cause a toroidal bending in the chromomere bundles. Both longitudinal and transverse sections of polytene chromosomes indicate that the bands may contain toroidal subunits. Torus-shaped bands were only found in thin sections of the most distal and most proximal regions, as well as in certain heavy bands at the late-replicating regions of polytenized interphase chromosomes. This suggests that an incomplete duplication of chromomeres may be a reason for torus formation, by preventing the separation of sister chromatids at the earliest phases of the polytenization process. The appearance of more numerous, but smaller, subunits in thin-sectioned faint bands is interpreted as a consequence of more complete segregation of sister chromatids in those bands during polytenization.