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Functions Associated with the Flagellar Apparatus

Three major functional modules, all expressing a facet of cell polarity, are associated with the basal body/axoneme: cell locomotion in which it acts as a swimming organelle due to its beating activity, sensory reception in which it acts as a sensory organ due to the concentration of receptors on the ciliary plasma membrane which cannot diffuse beyond a barrier at the base of the cilium where the basal body is anchored, and cell division in which, in agreement with the early views of Boveri [5], the basal body/axoneme acts in coordinating karyokinesis and cytokinesis, due to the duplication process governing basal body/centriole biogenesis and, as we learned after Boveri, to its critical role in the cell-wide organization of the microtubule network.

Cell Locomotion

Although the details of the generic dynein-dependent sliding of doublet microtubules in the axoneme have long been known, the overall control of the flagellar movement is far from being understood. We will only note here that the mechanisms of flagellum-dependent motility can be quite different. For instance, in the euglenid taxon there are examples of the classical beating movement of the flagel-lum (e. g. Euglena) or the movement of an undulating membrane by the flagellum (e.g. trypanosomes). But the flagellum can also produce a very efficient unidirectional gliding motion powered by flagellar surface motility as is the case with Per-anema trichophorum [6]. The surface motility which can be uncoupled from ciliary beating and may be driven by a retrograde motor along the axoneme seems to move transmembrane mastigonemes. This surface motility serves additional functions in this organism, such as prey capture, a feature which has also been documented in other taxa. The stramenopile Epipyxis pulchra, can use its two different flagella as an efficient pair of pliers to catch, engulf, or release, any prey passing at grasping distance [7, 8]. A striking feature of this unicellular organism is that once contact has been made between the different types of prey and the flagellar surface, the organism is able to select the preferred prey, suggesting that a sensory mechanism is also at work in this process. The strong vortex created by the beating of numerous cilia in the Paramecium gullet is an alternate and powerful method of capturing prey. These few examples show how the basal body/axoneme and its membrane can be used in various ways to ensure basic functions for cell survival such as polarity, motility, sensory reception, and moreover, can be adapted to various challenges including the capture of prey.

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