Analysis of serial cross-sections of the Chlamydomonas flagellum reveals several structural asymmetries in the axoneme. One doublet lacks the outer dynein arm, has a beaklike projection in its B-tubule, and bears a two-part bridge that extends from the A-tubule of this doublet to the B-tubule of the adjacent doublet. The two doublets directly opposite the doublet lacking the arm have beak-like projections in their B-tubules. These asymmetries always occur in the same doublets from section to section, indicating that certain doublets have consistent morphological specializations. These unique doublets give the axoneme an inherent structural polarity. All three specializations are present in the proximal portion of the axoneme; based on their frequency in random cross-sections of isolated axonemes, the twopart bridge and the beak-like projections are present in the proximal one quarter and one half of the axoneme, respectively, and the outer arm is absent from the one doublet >90% of the axoneme's length. The outer arm-less doublet of each flagellum faces the other flagellum, indicating that each axoneme has the same rotational orientation relative to the direction of its effective stroke. This strongly suggests that the direction of the effective stroke is controlled by a structural component within the axoneme. The striated fibers are associated with specific triplets in a manner suggesting that they play a role in setting up or maintaining the 180 ° rotational symmetry of the two flagella.In forward swimming Chlamydomonas, the two fagella beat with an effective stroke in approximately the same plane, but in opposite directions (17,32,34). Although the fine structure (6, ll, 14, 30, 32, 39, 40), biochemistry (10, 15, 21, 27, 28, 31, 40), and waveform (3,5,17,32,34) of the Chlamydomonas flagellum have been extensively investigated, little information is available on the mechanism by which the direction of the effective stroke is established. The isolated flagellar apparatuses of Chlamydomonas appear to retain the same beat pattern as in intact cells (16,17), so the direction of beat must be controlled by a structural component within the flagellar apparatus. However, it is not known whether this component is internal or external to the axoneme. Structural differences have been observed in some of the outer doublets of Chlamydomonas (15,38), but it has not been determined whether these asymmetries always occur in the same doublets along the axoneme, nor whether they are oriented in a way that is related to the plane of beat. The central pair of Chlamydomonas rotates during flagellar movement (19,20), so it can not give polarity to the axoneme.To determine whether the axonemes of Chlamydomonas have an intrinsic polarity related to the plane of beat, we made a detailed examination of the fine structure of the flagellar axoneme in serial thin sections having a known orientation relative to the two basal bodies. The results indicate that: (a) there are consistent morphological differences between several ou...

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The Chlamydomonas mutant vfl-3 lacks normal striated fibers and microtubular rootlets. Although the flagella beat vigorously, the cells rarely display effective forward swimming . High speed cinephotomicrography reveals that flagellar waveform, frequency, and beat synchrony are similar to those of wild-type cells, indicating that neither striated fibers nor microtubular rootlets are required for initiation or synchronization of flagellar motion . However, in contrast to wild type, the effective strokes of the flagella of vfl-3 may occur in virtually any direction . Although the direction of beat varies between cells, it was not observed to vary for a given flagellum during periods of filming lasting up to several thousand beat cycles, indicating that the flagella are not free to rotate in the mature cell. Structural polarity markers in the proximal portion of each flagellum show that the flagella of the mutant have an altered rotational orientation consistent with their altered direction of beat. This implies that the variable direction of beat is not due to a defect in the intrinsic polarity of the axoneme, and that in wild-type cells the striated fibers and/or associated structures are important in establishing or maintaining the correct rotational orientation of the basal bodies to ensure that the inherent functional polarity of the flagellum results in effective cellular movement . As in wild type, the flagella of vfl-3 coordinately switch to a symmetrical, flagellar-type waveform during the shock response (induced by a sudden increase in illumination), indicating that the striated fibers are not directly involved in this process .Striated fibers are associated with most cilia and flagella (26,36), including the modified cilia of sense organs (3) . Their ubiquitous presence suggests that these fibers play an important role or roles in flagellar and ciliary morphogenesis or function. Several possible functions have been proposed for the striated fibers, including (a) an active role in the initiation or coordination of flagellar movement (7, 15,19,31,32,42), (b) a passive role in anchoring the cilia and flagella to resist the forces and moments resulting from ciliary or flagellar beating (1,25,26,37,38), and (c) a role in the development or maintenance of the proper positioning of the basal bodies (9, 11,25,26). However, there has previously been no experimental evidence for any of these functions.Recently, a mutant of Chlamydomonas was described that is defective in striated fibers and microtubular rootlets (43). This mutant provides an opportunity for learning more about the roles of these structures. We studied flagellar waveform and orientation in forward and reverse swimming cells ofthis $~s mutant; the flagella have a waveform similar to that of wild type in both forward and reverse beating modes. However, the direction of beat is highly variable, and thin-section analysis shows that the mutant's flagella are present in abnormal rotational orientations . These results suggest that the striated f...

Summary. The colonial Volvocales are often said to be composed of Chlamydomonas-like cells, but there are substantial differences in motility and flagellar apparatus construction between the unicellular forms and the individual members of a colony or spheroid. These changes appear to be required for effective organismal motion and might possibly limit the rate at which new colonial forms evolve from unicellular ones. The flagellar-beat envelopes in colonial members are modified such that they beat in the same direction and in parallel planes with their effective strokes at right angles to the cellular anterior-posterior axis. These changes result from a series of developmental events of the flagellar apparatus of the colonial forms while the colony is still an embryo. Differences in the flagellar-apparatus structure in the members of the Goniaceae and Volvocaceae are not obviously correlated with the traditional placement of these algae in a simple volvocine lineage. Effective colonial motion clearly requires precise positioning and rotational orientation of the cells within the colony. Almost any arrangement where the cells are placed with rotational symmetry within the colony results in colonial progression with rotation. Such rotational symmetry is present from the time of embryogenesis. The mechanism that leads to organismal steering in behavioral responses (e.g., phototaxis) must likewise differ between colonial and unicellular forms. In at least some cases, this appears to result from changes in beat frequency in some parts of the spheroid, but changes in beat direction cannot be ruled out for all forms.

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