When present, the cell has just one flagellum or a few flagella. Prokaryotes sometimes have flagella, but they are structurally very different from eukaryotic flagella. Prokaryotes can have more than one flagella. They serve the same function in both prokaryotes and eukaryotes to move an entire cell. They are short, hair-like structures that are used to move entire cells such as paramecium or move substances along the outer surface of the cell for example, the cilia of cells lining the fallopian tubes that move the ovum toward the uterus, or cilia lining the cells of the respiratory tract that move particulate matter toward the throat that mucus has trapped.
Cilia are not found on prokaryotes. Intriguingly, high levels of expression of some of the retained genes in pollen suggest that these gene products still have gamete-associated functions, and the experimental characterization of others indicates that they are required for microtubule-associated cytoskeletal processes Hodges et al.
The former is required for the organization of the cortical cytoskeleton and is therefore a determinant of cell morphology Camilleri et al. Regarding the latter, the phragmoplast is a plant cell—specific microtubule array formed during mitosis, and FUSED is required for cytokinesis Oh et al. A protein that was not reported as a candidate ciliary protein conserved in angiosperms but that is nonetheless clearly homologous to a protein associated with cilium assembly in flagellate eukaryotes is TONNEAU1.
TONNEAU1 is most closely related to a centrosomal protein from animals called FOP, and failure to identify TONNEAU1 as a ciliary protein conserved in angiosperms perhaps merely serves to highlight the common limitation of using automated pipelines to identify candidate orthologous proteins i. It also suggests that further flagellum-associated proteins may await discovery in land plants that do not build flagella. In other work, a screen for TONNEAU1-interacting proteins identified a novel family of 34 proteins that have in common six short sequence motifs; the conservation of some of these motifs, including their order of appearance, is evident in another animal centrosomal protein, CEP Drevensek et al.
Collectively, recently published data are therefore rapidly revealing previously hidden evolutionary commonalities between the centrosome in animal cells and MTOCs of the plant cortical cytoskeleton. It is plausible that not all of the ciliary proteins retained in angiosperms function in cytoskeletal processes.
If they are not, the question would be how any gain of new function occurred. Here, we provide two alternative views. First, if we consider another obvious example of multicellularity among eukaryotes, the animals, their phenotypic complexity is not explained simply by an increase in the number of protein-coding genes relative to near, unicellular relatives.
In plants, alternative splicing has also emerged as an important mechanism for generating protein diversity Reddy et al. Therefore, alternative splicing could have readily provided the mechanism by which the function or localization of some ancestrally ciliary proteins changed in an angiosperm ancestor; following the loss of the ability to build flagella during land plant evolution, alternatively spliced gene products conferring significant fitness benefits would provide a selective pressure for the retention of a ciliary gene.
Alternatively, the retention and neofunctionalization in plants of proteins otherwise associated with ciliary function can also be explained by the concept of protein moonlighting. Therefore, a classic reductionist view is that individual proteins typically have a single function, but examples of proteins fulfilling multiple—often very different—roles inside cells are becoming increasingly common Copley The concept of moonlighting is different from the functional diversity that results from alternative splicing in that it describes the functionality of an individual polypeptide in multiple distinct cellular processes.
For moonlighters, functionality in different processes may occur as a consequence of conformational change induced by posttranslational modification or the use of different regions of a protein surface.
For instance, moonlighting functions were first observed for abundant, soluble enzymes, such as those involved in the glycolytic pathway, in which the active site used for catalysis represents only a small part of a protein surface.
The prevalence of protein moonlighting is now such that it is no longer considered an oddity; indeed, some consider that protein multifunctionality is likely to contribute very significantly to cellular and organismal complexity Copley If any ciliary proteins had assumed moonlighting functions in an angiosperm ancestor, the fitness benefits conferred by some of those moonlighting activities could again explain the retention of genes encoding typically ciliary proteins in land plants such as Arabidopsis.
These protists belong to the phylum Apicomplexa. Over apicomplexan species have been described. Some exist as cysts in the environment, and others undergo an obligatory sexual stage in order to complete often-complex life cycles, but the apicomplexans are more generally thought of as a family of obligate intracellular parasites.
They are thought to have evolved from free-living marine algae; among their closest extant relations are the recently discovered chromerid algae from the Australian Great Barrier Reef and the predatory colpodellids Kuvardina et al.
As with many other eukaryotic lineages some apicomplexans have secondarily lost the ability to build a flagellum, but, as a group, the apicomplexans owe their name to the unique, polarity-defining MTOC called the apical polar ring , from which subpellicular microtubules that define cell shape radiate.
Structures homologous to the apical polar ring are also evident in the chromerids and colpodellids, and recent studies suggest a flagellum-based origin for a structure that in the apicomplexans is involved in cell invasion, as well as in the definition of cell shape Portman and Slapeta In some apicomplexans, such as Toxoplasma gondii , an extremely globally successful parasite of vertebrates and opportunistic pathogen of the immunocompromized, a spiral-like arrangement of tubulin sheets folded in the shape of a cone and known as the conoid is also associated with the apical polar ring figure 4.
Its genetic tractability and easy visualization by microscopy make T. In the animal host, extracellular T. Recently, it has been observed that, following the duplication of the parasite centrosome, which is associated with the nucleus, a fibrous connection extends from the duplicated centrosome to the apical polar ring, providing a hard-wired link that helps orchestrate daughter cell budding.
With regard to a possible connection with eukaryotic flagella, the essential components of this fibrous connection appear homologous to proteins that form the rootlet fibers that link the flagellar basal bodies in C. Apical polar ring—flagellum connections? The centriole-containing centrosome is located close to the nucleus N and distant from the apical polar ring APR ; the closed conoid CC is topped by preconoidal rings PCR , and the rhoptries are secretory organelles that release their contents during cell invasion.
Note the very different morphologies of tachyzoites versus biflagellate microgametes and the absence of the APR and associated structures in the latter. SAS-6 is found only within the centrioles, and SASlike protein is present only at the apical end of the cell; the central position of the nucleus is also shown.
Source: Reprinted with permission from de Leon and colleagues under the terms of a Creative Common Attribution 3. Further evidence of a possible ancient relationship between the flagellar cytoskeleton and the cytoskeletal apparatus that facilitates cell invasion by T. Despite the large evolutionary distance between trypanosomes and apicomplexans which spans major eukaryotic groups; Tekle et al.
If the view of a flagellum-based origin for the apical polar ring is correct, not only does it provide an unexpected example of co-opting flagellar proteins into an alternative structure, but it also provides an intriguing comparison with the retention and function of genes encoding ciliary proteins in acentriolar plants—or, indeed, in any other eukaryotes.
How many proteins with known or inferred flagellum functions will turn out to have homologs or orthologs in other eukaryotes that do not build flagella i. For instance, miniature green alga belonging to the genus Ostreococcus are abundant oceanic picoplankton and are believed to lack the capacity to build flagella, but around 40 proteins associated with flagellum function are still encoded within the nuclear genomes of different Ostreococcus species Merchant et al. In contrast, using the same pipeline as for their identification of ciliary profile proteins in land plants, Hodges and colleagues predicted that the number of flagellar proteins in fungi will be lower than that in aflagellate land plants.
It is during the last 30 or so years that the molecular bases underpinning the diversity in form and function of eukaryotic flagella and their importance to human health have become apparent. As with other aspects of modern cell biology, our understanding of flagellum origins, function, and variation and the ensuing paradigm revisions have been hugely informed by advances in molecular biology and DNA techniques.
Indeed, it is only recently apparent that the versatility of eukaryotic flagella as organelles of motility and sensory perception is exploited across the breadth of eukaryotic evolution Bloodgood , Brown and Witman Further surprises regarding flagellar or ciliary versatility continue to be revealed; for instance, the immunological synapse—a specialized surface membrane region formed when cytotoxic T lymphocytes recognize target cells and from which cytotoxic granules are secreted—has very recently been proposed to represent a highly modified cilium de la Roche et al.
With the new view of the flagellum as a secretory organelle Wood et al. JM was supported by Doctoral Training Grant studentship no. Google Scholar. Oxford University Press is a department of the University of Oxford. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide.
Sign In or Create an Account. Sign In. Advanced Search. Search Menu. Article Navigation. Close mobile search navigation Article Navigation. Volume Article Contents Abstract. Eukaryotic flagella: Ancient organelles unrelated to bacterial flagella. The origin of ninefold symmetry. Axoneme assembly. Initially an organelle of motility or sensory perception?
Conservation of the protein inventory in motile flagella. Species-specific variation of regulatory networks. Variation on a grander scale: Extraaxonemal structures. Moderation of axoneme structure. Ciliary proteins in other guises. References cited. Jonathan Moran , Jonathan Moran. McKean p. Ginger m. Oxford Academic. Paul G.
Michael L. Split View Views. Select Format Select format. Permissions Icon Permissions. Abstract The microtubule axoneme is an iconic structure in eukaryotic cell biology and the defining structure in all eukaryotic flagella or cilia.
Open in new tab Download slide. Google Scholar Crossref. Search ADS. Flagellar motility is required for the viability of the bloodstream trypanosome. De la Roche. De Leon. A SASlike protein suggests that the Toxoplasma conoid complex evolved from flagellar components.
The respiratory tract in humans is lined with cilia that keep inhaled dust, smog, and potentially harmful microorganisms from entering the lungs.
Among other tasks, cilia also generate water currents to carry food and oxygen past the gills of clams and transport food through the digestive systems of snails. Flagella are found primarily on gametes, but create the water currents necessary for respiration and circulation in sponges and coelenterates as well. For single-celled eukaryotes, cilia and flagella are essential for the locomotion of individual organisms.
Protozoans belonging to the phylum Ciliophora are covered with cilia, while flagella are a characteristic of the protozoan group Mastigophora. In eukaryotic cells, cilia and flagella contain the motor protein dynein and microtubules, which are composed of linear polymers of globular proteins called tubulin.
The core of each of the structures is termed the axoneme and contains two central microtubules that are surrounded by an outer ring of nine doublet microtubules. One full microtubule and one partial microtubule, the latter of which shares a tubule wall with the other microtubule, comprise each doublet microtubule see Figure 1. Dynein molecules are located around the circumference of the axoneme at regular intervals along its length where they bridge the gaps between adjacent microtubule doublets.
A plasma membrane surrounds the entire axoneme complex, which is attached to the cell at a structure termed the basal body also known as a kinetosome. Basal bodies maintain the basic outer ring structure of the axoneme, but each of the nine sets of circumferential filaments is composed of three microtubules, rather than a doublet of microtubules.
Thus, the basal body is structurally identical to the centrioles that are found in the centrosome located near the nucleus of the cell. In some organisms, such as the unicellular Chlamydomonas , basal bodies are locationally and functionally altered into centrioles and their flagella resorbed before cell division. Eukaryotic cilia and flagella are generally differentiated based on size and number: cilia are usually shorter and occur together in much greater numbers than flagella, which are often solitary.
The structures also exhibit somewhat different types of motion, though in both cases movement is generated by the activation of dynein and the resultant bending of the axoneme. The movement of cilia is often described as whip-like, or compared to the breast stroke in swimming.
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