The organization of microtubular systems in the quadriflagellate unicell Polytomella agilis has been reconstructed by electron microscopy of serial sections, and the overall arrangement confirmed by immunofluorescent staining using antiserum directed against chick brain tubulin. The basal bodies of the four flagella are shown to be linked in two pairs by short fibers. Light microscopy of swimming cells indicates that the flagella beat in two synchronous pairs, with each pair exhibiting a breast stroke-like motion. Two structurally distinct flagellar rootlets, one consisting of four microtubules in a 3 over 1 pattern and the other of a striated fiber over two microtubules, terminate between adjacent basal bodies. These rootlets diverge from the basal body region and extend toward the cell posterior, passing just beneath the plasma membrane. Near the anterior part of the cell, all eight rootlets serve as attachment sites for large numbers of cytoplasmic microtubules which occur in a single row around the circumference of the cell and closely parallel the cell shape. It is suggested that the flagellar rootlets may function in controlling the patterning and the direction of cytoplasmic microtubule assembly. The occurrence of similar rootlet structures in other flagellates is briefly reviewed.The presence of flagellar rootlets, constructed of microtubules in a characteristic grouping and/or striated fibers, originating near flagellar basal bodies, is a common feature of motile algal cells. In some of these algae, for example, Chlamydomonas (34) and the zoospores of Microthamnion (43), the microtubules comprising the flagellar rootlets appear to constitute the entire cytoplasmic microtubule system. In others, Ochromonas (2) and the zoospores of Schizomeris (1), large numbers of additional cytoplasmic microtubules appear to attach to the flageUar rootlets. In an earlier series of papers (2, 4, 5) it was shown that in the formation of this attachment in Ochromonas the flagellar rootlets, the rhizoplast and kineto-beak fiber in this organism function as nucleating sites (or microtubule-organizing centers, 29) for the initial assembly of cytoplasmic microtubules. These observations were made on cells in which the cytoplasmic microtubule system was regenerating after an exposure to hydrostatic pressure or antimitotic chemicals. To further clarify this proposed function, we suggested that it was important to examine the role of such sites during the normal development of the cytoplasmic microtubule system (e.g. in synchronously dividing cultures), and ultimately to analyze the polymerization capabilities of these structures in an in vitro 106

No abstract