Cilia and flagella are motile cellular appendages found in most microorganisms and animals, but not in higher plants. In multicellular organisms, cilia function to move a cell or group of cells or to help transport fluid or materials past them. 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.
On the other hand, flagella are longer and there are fewer flagella per cell usually one to eight. Though eukaryotic flagella and motile cilia are structurally identical, the beating pattern of the two organelles can be different. The motion of flagella is often undulating and wave-like, whereas the motile cilia often perform a more complicated 3D motion with a power and recovery stroke.
This video explains the difference between cilia and flagella, as well as the function and structure of these cell organelles. Eukaryotic motile cilium and flagellum are structurally identical.
Each is a bundle of nine fused pairs of microtubule doublets surrounding two central single microtubules. The movement of both cilia and flagella is caused by the interactions of these microtubules. In non-motile or primary cilia the two central single microtubules are absent. In prokaryotes cells the flagella are filamentous protein structures composed of flagellin.
There two types of cilia - motile and non-motile or primary cilia. Lack of proper functioning of cilia and flagella can cause several problems in human beings. For example,. Share this comparison:. Evolutionary constraints applied to the axoneme structure are likely to be extended to the CBBs. For example, C. Similarly, Drosophila embryonic centrioles, which are not associated with cilia, are composed of MT doublets, whereas during motile flagella formation in spermatogenesis, CBBs that nucleate motile cilia are composed of MT triplets Fig.
In the extreme case, many species have completely lost their CBBs and cilia, such as Angiosperms, some fungi, and certain amoebas Fig. Canonical and de novo CBB assembly can also coexist in the same organism, such as in parthenogenic insects Riparbelli and Callaini, ; Ferree et al.
Because in animals and Chlamydomonas experimentally induced de novo biogenesis lacks control on the place, time, and number of CBBs assembled, it has been suggested that the presence of a parental structure provides a scaffold for the regulation of new centriole assembly Marshall et al. It is thus striking how organisms such as the amoeboflagellate Naegleria and sperm cells of Plasmodium , ferns, or diatoms that form CBBs de novo show impressive number, spatial, and time control Fig.
Perhaps in all organisms regulation of the location and numbers is achieved by controlled availability and localization of assembly components, which is often but not always specified by a parental CBB. It has been suggested that Chlamydomonas reabsorb their flagella due to the presence of a cell wall, which inhibits the migration of the basal bodies into opposite poles Parker et al.
In human cycling ciliated cells, where cilia disassembly is also observed, it seems plausible that this mechanism requires regulatory coordination with the cell cycle, though currently little is known about the mechanisms regulating this process. In animals, the centriole, as part of the centrosome, has important functions in embryonic development, asymmetric divisions, and male meiosis Rodrigues-Martins et al.
A centrosome composed of centrioles and PCM is primarily found in animals and Fungi Opisthokonts ; however, other examples are found outside of this taxon, such as in brown algae and Plasmodiophorids plant parasites belonging to the Cercozoa group; Fig.
During cell division, the spindle ensures equal separation of the sister chromatids into the two daughter cells. The localization of CBBs at or close to the poles of the spindle is often achieved through the interaction of MTs nucleated by the CBB and the spindle itself. It has been suggested that this localization is a strategy to ensure equal inheritance of these structures so that both daughter cells can form cilia Pickett-Heaps, , However, even if this was originally the case, the CBB has important functions in cell division, suggesting possible cooption of the structure to actively participate in the process and coordinate it with other cell functions.
The use of that module extends to different tissues within the same organism, as we and others have shown that canonical, de novo and assembly in multiciliated cells use the same molecular pathway to assemble the centriole structure Rodrigues-Martins et al.
Perhaps one of the most striking features of the evolution of CBBs is the almost perfect correlation between the absence of the structure and absence of that ancestral gene module that is strictly required to assemble the structure Figs. Exceptions to this rule may provide interesting examples of eukaryotes that are in the process of losing the structure.
That is the case of the smallest eukaryotic cell, the algae Ostreococcus , which does not seem to have a CBB but encodes some of its components in the genome Figs. Despite the conservation of structural components, proteins that contribute additional regulatory steps, upstream or downstream of CBB assembly, appear to have been added in a stepwise, taxon-specific manner throughout evolution Carvalho-Santos et al. CP and CEP97, two centriolar proteins that have been proposed to coordinate ciliogenesis with the cell cycle, are confined to the Metazoans Fig.
Protein divergence among orthologues may also contribute to species-specific adaptations in CBB assembly Carvalho-Santos et al. Comparative genomics, powered by the rapid increase in the availability of complete genome sequences from all branches of the eukaryotic tree, is revealing the nature of our eukaryotic ancestor and its evolution.
As we sequence the genomes of more diverse eukaryotes, we also predict an increasingly complex LECA Figs. Because we have not yet found intermediate structures, we can only speculate how CBBs and cilia could have emerged from simpler, preexisting components.
We hypothesize that the creation of diffusion barriers coupled with gene duplication and divergence may have provided a mechanistic driving force for the emergence of eukaryotic subcellular structures. Diversification of these structures and their function in different eukaryotes and different tissues likely proceeded through stepwise addition, duplication, and divergence of molecular components.
CBBs and derived structures are unequivocally identified by electron microscopy, in particular their characteristic ninefold symmetry, which has permitted their identification in different branches of the eukaryotic tree.
This coarse correlation has been widely used for predicting new components of these structures Avidor-Reiss et al. As phylogenetic patterns for structural parts of CBBs and derived structures emerge, it will be important to investigate whether they correlate with certain molecular and morphological signatures Fig. For example, it is of key importance to piece together the evolution of the transition zone, the axonemal region where the transformation from basal body to axoneme occurs, because its components are often involved in many human diseases.
This type of analysis should help us to identify new molecular components of this structure as well as having a better understanding of their mechanism of action.
The identification of pairs of closely related species where these structures were maintained in one organism and lost from the other e. Phaeodactylum [flagellated vs. Ostreococcus [flagellated vs.
Theileria [flagellated vs. Finally, in the future it will also be important to understand how different cellular mechanisms have coevolved, in particular complexes determining cell polarity, protein trafficking, and the MT cytoskeleton. The centriole and its derived structures are thus an ideal paradigm to study the mechanisms involved in the evolution of the eukaryotic cell. Pedersen, as well as Bernhard Schermer and Thomas Benzing.
Carvalho-Santos is recipient of a fellowship from FCT. Structure and biogenesis of centrosomes and cilia. A On the left, a schematic and EM micrograph reproduced from Vorobjev and Chentsov, of an animal prometaphase centrosome composed of mother MC and daughter DC centriole arranged in an orthogonal fashion.
The mother centriole harbors subdistal and distal appendages. B Key regulatory and structural components in CBB biogenesis canonical [top] and de novo [bottom]; Azimzadeh and Marshall, EM cross section of tracheal motile cilia top: reproduced from Satir and Dirksen in Handbook of Physiology with permission from the American Physiology Association and renal nonmotile primary cilia bottom: image courtesy of H.
EM longitudinal section of the Chlamydomonas flagellum adapted from Pedersen et al. Simplified taxonomic tree representing major eukaryotic groups in different colors these groups contain a common ancestor and all its descendants; adapted from Hedges and Baldauf Unikonts include eukaryotic cells that, for the most part, have a single emergent flagellum and are divided into Opisthokonts propel themselves with a single posterior flagellum; Metazoans, Fungi, and Choanoflagellates and Amoebozoa Cavalier-Smith, Bikonts include eukaryotic organisms with two emergent flagella Cavalier-Smith, Branch color code: purple, Opisthokonts; blue, Amoebozoa; green, Plants; yellow, Alveolates; orange, Stramenopiles; rose, Rhizaria; brown, Excavates and Discicristates.
Finally, we represent the pathways used for CBB assembly, canonical, de novo, or both. In Drosophila melanogaster , the gray centriolar MTs represent the fact that certain tissues present centrioles with doublets whereas others show triplets. The asterisk in D. In Acerentomon microrhinus , the CBBs formed during spermatogenesis have a different symmetry and the cartwheel is also represented in these structures.
In the remaining cases, gray structures are used when evidence pointing to their presence is not robust poor EM data or data from a related species. For references please check Table S1. Simplified taxonomic tree representing major eukaryotic groups in different colors using the same color code as in Fig. A The eukaryotic cell where the proto-cilium evolved likely had a cytoskeleton composed of actin and MTs that converged in the MTOC, a nucleus, and an endomembrane system.
This protrusion would evolve to become a specialized structure with specific membrane composition maintained by diffusion barriers. C The evolving proto-cilium was likely capable of environmental sensing and gliding, which might have driven the implementation of this organelle.
D Further on, the bundle of MTs would evolve in order to create a specialized arrangement of closed and open MTs forming a ninefold symmetric structure capable of bending due to the presence of molecular motors. The basal body gave support to the motile axoneme at the cell membrane. Adapted from Satir et al. Sign In or Create an Account. Advanced Search. User Tools. Sign In. Skip Nav Destination Article Navigation.
Review Evolution July 25 This Site. Google Scholar. Juliette Azimzadeh , Juliette Azimzadeh. Author and Article Information. Zita Carvalho-Santos. Juliette Azimzadeh. Abbreviations used in this paper:.
Received: November 30 Accepted: June 29 Online Issn:
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