Abstract
The interphase flagellar apparatus of the green alga Chlorogonium elongatum resembles that of Chlamydomonas reinhardtii in the possession of microtubular rootlets and striated fibers. However, Chlorogonium, unlike Chlamydomonas, retains functional flagella during cell division. In dividing cells, the basal bodies and associated structures are no longer present at the flagellar bases, but have apparently detached and migrated towards the cell equator before the first mitosis. The transition regions remain with the flagella, which are now attached to a large apical mitochondrion by cross-striated filamentous components. Both dividing and nondividing cells of Chlorogonium propagate asymmetrical ciliary-type waveforms during forward swimming and symmetrical flagellar-type waveforms during reverse swimming. High-speed cinephotomicrographic analysis indicates that waveforms, beat frequency, and flagellar coordination are similar in both cell types. This indicates that basal bodies, striated fibers, and microtubular rootlets are not required for the initiation of flagellar beat, coordination of the two flagella, or determination of flagellar waveform. Dividing cells display a strong net negative phototaxis comparable to that of nondividing cells; hence, none of these structures are required for the transmission or processing of the signals involved in phototaxis, or for the changes in flagellar beat that lead to phototactic turning. Therefore, all of the machinery directly involved in the control of flagellar motion is contained within the axoneme and/or transition region. The timing of formation and the positioning of the newly formed basal structures in each of the daughter cells suggests that they play a significant role in cellular morphogenesis.
Highlights
A large body of evidence has implicated the basal body as a necessary precursor to flagellum formation, even in cases where no basal bodies or centrioles exist in nonflagellated stages of the life cycle [11, 24, 31, 48, 51]
We report here that a similar situation occurs in the biflagellate green alga Chlorogonium.Prior to division, the basal bodies detach from the parental flagella, which remain motile in the absence of associated basal bodies, microtubular rootlets, and striated fibers
Chlorogonium elongatum is a biflagellate green alga closely related to Chlamydomonas [6, 23]
Summary
A large body of evidence has implicated the basal body as a necessary precursor to flagellum formation, even in cases where no basal bodies or centrioles exist in nonflagellated stages of the life cycle [11, 24, 31, 48, 51]. We have examined flagellar waveform and beat frequency in interphase and dividing cells of Chlorogoniumand find that there is virtually no difference in the behavior of the flagella in the two cell types Both interphase and dividing cells swim in helices, undergo the photophobic response, and exhibit strong negative phototaxis. These results indicate that, at least in Chlorogonium,the basal bodies, striated fibers, and microtubular rootlets are not required for, and probably have no role in, the initiation or control of flagellar movement, or in the signal transduction and processing required for the photophobic response or phototaxis. All elements directly involved in the control of flageUar movement must be located in the flagellar shaft distal to the basal body
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