Control of microtubule pattern during the development of a large organelle in the ciliate Nassula

Abstract

Bundles of hexagonally arranged microtubules in the cytopharynx of Nassula develop abnormally during colchicine treatment. Some of the tubules undergo a microtubule-macrotubule transition. Portions of tubules that assemble during the treatment fail to grow along straight paths and are irregularly arranged. Portions of the same tubules which assemble after treatment are arranged hexagonally. Comparisons of normal development with that occurring during and after colchicine treatment indicate that several procedures are involved in defining microtubule arrangement. It seems that highly ordered arrays of microtubule nucleating sites form against the proximal ends of basal bodies. Each array of sites apparently determines the cross-sectional shape of a bundle and the orientation of the hexagonal lattice pattern in which its tubules are arranged by specifying the positions in which tubules start to assemble fairly exactly. The term microtubule-nucleating-template is proposed for arrays of sites that act in this way. A special close-packing-effect may maintain the hexagonal pattern as tubules lengthen until tubules are connected by links. Links mainly attach to particular binding points on tubule walls. Since links can project in a variety of directions from these points, links probably do not play an important role in establishing tubule arrangement in this organelle.