Evolution of flagella
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The evolution of flagella is of great interest to
biologist A biologist is a scientist who conducts research in biology. Biologists are interested in studying life on Earth, whether it is an individual Cell (biology), cell, a multicellular organism, or a Community (ecology), community of Biological inter ...
s because the three known varieties of flagella – (
eukaryotic Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
,
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
l, and archaeal) each represent a sophisticated cellular structure that requires the interaction of many different systems.


Eukaryotic flagellum

There are two competing groups of models for the evolutionary origin of the
eukaryotic Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
flagellum (referred to as
cilium The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike project ...
below to distinguish it from its bacterial counterpart). Recent studies on the
microtubule organizing center The microtubule-organizing center (MTOC) is a structure found in eukaryotic cells from which microtubules emerge. MTOCs have two main functions: the organization of eukaryotic flagella and cilia and the organization of the mitotic and meiotic spind ...
suggest that the most recent ancestor of all eukaryotes already had a complex flagellar apparatus.


Endogenous, autogenous and direct filiation models

These models argue that cilia developed from pre-existing components of the eukaryotic
cytoskeleton The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is com ...
(which has
tubulin Tubulin in molecular biology can refer either to the tubulin protein superfamily of globular proteins, or one of the member proteins of that superfamily. α- and β-tubulins polymerize into microtubules, a major component of the eukaryotic cytoske ...
and dynein also used for other functions) as an extension of the
mitotic spindle In cell biology, the spindle apparatus refers to the cytoskeletal structure of eukaryotic cells that forms during cell division to separate sister chromatids between daughter cells. It is referred to as the mitotic spindle during mitosis, a pr ...
apparatus. The connection can still be seen, first in the various early-branching single-celled eukaryotes that have a microtubule
basal body A basal body (synonymous with basal granule, kinetosome, and in older cytological literature with blepharoplast) is a protein structure found at the base of a eukaryotic undulipodium (cilium or flagellum). The basal body was named by Theodor Wi ...
, where microtubules on one end form a spindle-like cone around the nucleus, while microtubules on the other end point away from the cell and form the cilium. A further connection is that the centriole, involved in the formation of the mitotic spindle in many (but not all) eukaryotes, is homologous to the cilium, and in many cases ''is'' the basal body from which the cilium grows. An intermediate stage between spindle and cilium would be a non-swimming appendage made of microtubules with a function subject to
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Cha ...
, such as increasing surface area, helping the protozoan remain suspended in water, increasing the chances of bumping into bacteria to eat, or serving as a stalk attaching the cell to a solid substrate. Regarding the origin of the individual protein components, a paper on the evolution of dyneins shows that the more complex protein family of ciliary dynein has an apparent ancestor in a simpler cytoplasmic dynein (which itself has evolved from the AAA protein family that occurs widely in all archea, bacteria and eukaryotes). Long-standing suspicions that tubulin was homologous to FtsZ (based on very weak sequence similarity and some behavioral similarities) were confirmed in 1998 by the independent resolution of the 3-dimensional structures of the two proteins.


Symbiotic/endosymbiotic/exogenous models

These models argue that the cilium evolved from a symbiotic
Gracilicutes Gracilicutes (Latin: ''gracilis'', slender, and ''cutis'', skin, referring to the cell wall) is a clade in bacterial phylogeny. Traditionally gram staining results were most commonly used as a classification tool, consequently until the advent o ...
(ancestor of
spirochete A spirochaete () or spirochete is a member of the phylum Spirochaetota (), (synonym Spirochaetes) which contains distinctive diderm (double-membrane) gram-negative bacteria, most of which have long, helically coiled (corkscrew-shaped or s ...
and
Prosthecobacter ''Prosthecobacter'' is a genus of bacteria from the phylum Verrucomicrobiota with a distinctive characteristic; the presence of tubulin-like genes. Tubulins, which are components of the microtubule, have never been observed in Gracilicutes bef ...
) that attached to a primitive eukaryote or archaebacterium ( archaea). The modern version of the hypothesis was first proposed by
Lynn Margulis Lynn Margulis (born Lynn Petra Alexander; March 5, 1938 – November 22, 2011) was an American evolutionary biologist, and was the primary modern proponent for the significance of symbiosis in evolution. Historian Jan Sapp has said that "Lynn Ma ...
. The hypothesis, though very well publicized, was never widely accepted by the experts, in contrast to Margulis' arguments for the symbiotic origin of mitochondria and chloroplasts. Margulis did strongly promote and publish versions of this hypothesis until the end of her life. One primary point in favor of the symbiotic hypothesis was that there are eukaryotes that use symbiotic spirochetes as their
motility Motility is the ability of an organism to move independently, using metabolic energy. Definitions Motility, the ability of an organism to move independently, using metabolic energy, can be contrasted with sessility, the state of organisms th ...
organelles (some
parabasalid The parabasalids are a group of flagellated protists within the supergroup Excavata. Most of these eukaryotic organisms form a symbiotic relationship in animals. These include a variety of forms found in the intestines of termites and cockroache ...
s inside
termite Termites are small insects that live in colonies and have distinct castes (eusocial) and feed on wood or other dead plant matter. Termites comprise the infraorder Isoptera, or alternatively the epifamily Termitoidae, within the order Blatto ...
guts, such as '' Mixotricha'' and '' Trichonympha''). This is an example of co-option and the flexibility of biological systems, and the proposed homologies that have been reported between cilia and spirochetes have stood up to further scrutiny. Margulis' hypothesis suggests that an archaea acquired
tubulin Tubulin in molecular biology can refer either to the tubulin protein superfamily of globular proteins, or one of the member proteins of that superfamily. α- and β-tubulins polymerize into microtubules, a major component of the eukaryotic cytoske ...
proteins from a eubacter ancestor of
Prosthecobacter ''Prosthecobacter'' is a genus of bacteria from the phylum Verrucomicrobiota with a distinctive characteristic; the presence of tubulin-like genes. Tubulins, which are components of the microtubule, have never been observed in Gracilicutes bef ...
. However, the homology of
tubulin Tubulin in molecular biology can refer either to the tubulin protein superfamily of globular proteins, or one of the member proteins of that superfamily. α- and β-tubulins polymerize into microtubules, a major component of the eukaryotic cytoske ...
to the bacterial replication and
cytoskeletal The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is compo ...
protein
FtsZ FtsZ is a protein encoded by the ''ftsZ'' gene that assembles into a ring at the future site of bacterial cell division (also called the Z ring). FtsZ is a prokaryotic homologue of the eukaryotic protein tubulin. The initials FtsZ mean "Filam ...
(see Prokaryotic cytoskeleton), which was apparently native in archaea, suggesting an endogenous origin of tubulin rather than a symbiotic transfer.


Bacterial flagellum

There is good evidence that the bacterial flagellum has evolved from a Type III secretory and transport system, given the similarity of proteins in both systems. All currently known nonflagellar Type III transport systems serve the function of exporting (injecting)
toxin A toxin is a naturally occurring organic poison produced by metabolic activities of living cells or organisms. Toxins occur especially as a protein or conjugated protein. The term toxin was first used by organic chemist Ludwig Brieger (1849 ...
into eukaryotic cells. Similarly, flagella grow by exporting flagellin through the flagellar machinery. It is hypothesised that the flagellum evolved from the type three secretory system. For example, the bubonic plague
bacterium Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were amon ...
''
Yersinia pestis ''Yersinia pestis'' (''Y. pestis''; formerly '' Pasteurella pestis'') is a gram-negative, non-motile, coccobacillus bacterium without spores that is related to both ''Yersinia pseudotuberculosis'' and ''Yersinia enterocolitica''. It is a facult ...
'' has an organelle assembly very similar to a complex flagellum, except that is missing only a few flagellar mechanisms and functions, such as a needle to inject toxins into other cells. As such, the type three secretory system supports the hypothesis that the flagellum
evolved Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
from a simpler bacterial secretion system. However, the true relationship could be the reverse: recent phylogenetic research strongly suggests the type three secretory system evolved from the flagellum through a series of gene deletions.


Eubacterial flagellum

Eubacterial flagellum is a multifunctional organelle. It’s also one of a range of motility systems in bacteria. The structure of the organelle appears like a motor, shaft and a propeller.Young, Matt & Edis, Taner (2004). ''Why Intelligent Design Fails: A Scientific Critique of the New Creationism'' Rutgers University press New Brunswick, New Jersey, and London.72-84. However, the structure of eubacterial flagellae varies based on whether their motor systems run on protons or sodium, and on the complexity of the flagellar whip. The evolutionary origin of eubacterial flagellae is probably an example of indirect evolution. A hypothesis on the evolutionary pathway of the eubacterial flagellum argues that a secretory system evolved first, based around the SMC rod- and pore-forming complex. This is presumed to be the common ancestor of the type-III secretory system and the flagellar system. Then, an ion pump was introduced to this structure which improved secretion. The ion pump later became the motor protein. This was followed by the emergence of the proto-flagellar filament as part of the protein-secretion structure. Gliding-twitching motility arose at this stage or later and was then refined into swimming motility.


Archaeal flagellum

The recently-elucidated archaeal flagellum, or ''
archaellum The archaellum (; formerly archaeal flagellum) is a unique structure on the cell surface of many archaea, that allows for swimming motility. The archaellum consists of a rigid helical filament that is attached to the cell membrane by a molecula ...
'', is
analogous Analogy (from Greek ''analogia'', "proportion", from ''ana-'' "upon, according to" lso "against", "anew"+ ''logos'' "ratio" lso "word, speech, reckoning" is a cognitive process of transferring information or meaning from a particular subject ...
—but not homologous—to the bacterial one. In addition to no sequence similarity being detected between the genes of the two systems, the archaeal flagellum appears to grow at the base rather than the tip, and is about 15 nanometers (nm) in diameter rather than 20. Sequence comparison indicates that the archaeal flagellum is homologous to bacterial
Type IV pili A pilus (Latin for 'hair'; plural: ''pili'') is a hair-like appendage found on the surface of many bacteria and archaea. The terms ''pilus'' and '' fimbria'' (Latin for 'fringe'; plural: ''fimbriae'') can be used interchangeably, although some r ...
, filamentous structures outside the cell. Pilus retraction provides enables a different form of bacterial motility called "twitching" or "social gliding" which allows bacterial cells to crawl along a surface, They are assembled through the Type II secretion system. They can also promote swimming, but no species of bacteria is known to use its Type IV pili for both swimming and crawling.


Further research

Testable outlines exist for the origin of each of the three motility systems, and avenues for further research are clear; for prokaryotes, these avenues include the study of secretion systems in free-living, nonvirulent prokaryotes. In eukaryotes, the mechanisms of both mitosis and cilial construction, including the key role of the centriole, need to be much better understood. A detailed survey of the various nonmotile appendages found in eukaryotes is also necessary. Finally, the study of the origin of all of these systems would benefit greatly from a resolution of the questions surrounding deep phylogeny, as to what are the most deeply branching organisms in each domain, and what are the interrelationships between the domains.


See also

*
Last universal ancestor The last universal common ancestor (LUCA) is the most recent population from which all organisms now living on Earth share common descent—the most recent common ancestor of all current life on Earth. This includes all cellular organisms; th ...


References


Further reading

* * * * * *


External links


Flagellum Evolution at The Panda's Thumb
* {{DEFAULTSORT:Evolution Of Flagella
Flagellum A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates. A microorganism may have f ...