The evolution of flagella is of great interest to biologists because the three known varieties of flagella (eukaryotic, bacterial, and archaebacterial) each represent an extremely sophisticated cellular structure that requires the interaction of many different and finely-tuned systems to function correctly. Jump to: navigation, search Charles Darwin, father of the theory of evolution by natural selection. ... A flagellum (plural, flagella) is a whip-like organelle that many unicellular organisms, and some multicellular ones, use to move about. ... A biologist is a scientist devoted to and producing results in biology through the study of organisms. ... Kingdoms Animalia - Animals Fungi Plantae - Plants Protista A eukaryote (also spelled eucaryote) is an organism with complex cells, in which the genetic material is organized into membrane-bound nuclei. ... Phyla/Divisions Actinobacteria Aquificae Bacteroidetes/Chlorobi Chlamydiae/Verrucomicrobia Chloroflexi Chrysiogenetes Cyanobacteria Deferribacteres Deinococcus-Thermus Dictyoglomi Fibrobacteres/Acidobacteria Firmicutes Fusobacteria Gemmatimonadetes Nitrospirae Omnibacteria Planctomycetes Proteobacteria Spirochaetes Thermodesulfobacteria Thermomicrobia Thermotogae Bacteria (singular, bacterium) are a major group of living organisms. ... Phyla / Classes Phylum Crenarchaeota Phylum Euryarchaeota     Halobacteria     Methanobacteria     Methanococci     Methanopyri     Archaeoglobi     Thermoplasmata     Thermococci Phylum Korarchaeota Phylum Nanoarchaeota The Archaea are a major group of prokaryotes. ...

The eukaryotic flagellum

There are two competing groups of models for the evolutionary origin of the eukaryotic flagellum (referred to as a cilium below to distinguish it from its bacterial counterpart). Kingdoms Animalia - Animals Fungi Plantae - Plants Protista A eukaryote (also spelled eucaryote) is an organism with complex cells, in which the genetic material is organized into membrane-bound nuclei. ... Cilium - Wikipedia, the free encyclopedia /**/ @import /skins-1. ...

Symbiotic/endosymbiotic/exogenous models

These models argue some version of the idea that the cilium evolved from a symbiotic spirochete that attached to a primitive eukaryote or archaebacterium (archaea). The modern version of the hypothesis was first proposed by Lynn Margulis (as Sagan (1967): Margulis was the first wife of the late Carl Sagan). 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. Common Clownfish (Amphiprion ocellaris) in their Magnificent Sea Anemone (Heteractis magnifica) home. ... Families Brachyspiraceae Leptospiraceae Spirochaetaceae The spirochaetes are a phylum of distinctive bacteria, which have long, helically coiled cells. ... Phyla / Classes Phylum Crenarchaeota Phylum Euryarchaeota     Halobacteria     Methanobacteria     Methanococci     Methanopyri     Archaeoglobi     Thermoplasmata     Thermococci Phylum Korarchaeota Phylum Nanoarchaeota The Archaea are a major group of prokaryotes. ... Lynn Margulis (born 1938) is a biologist and a professor at the University of Massachusetts Amherst. ... A respected astronomer and dogged critic of pseudoscience, Carl Sagan is best known for his enthusiastic efforts at popularizing science. ... Jump to: navigation, search The endosymbiotic theory concerns the origins of mitochondria and chloroplasts, which are organelles of eukaryotic cells. ...

The primary point in favor of the symbiotic hypothesis is that there are eukaryotes that use symbiotic spirochetes as their motility organelles (some parabasalids inside termite guts). While this is an example of co-option and the flexibility of biological systems, none of the proposed homologies that have been reported between cilia and spirochetes have stood up to further scrutiny. The homology of tubulin to the bacterial replication/cytoskeletal protein FtsZ is a major argument against Margulis, as FtsZ is apparently found native in archaea, providing an endogenous ancestor to tubulin (as opposed to Margulis' hypothesis, that an archaea acquired tubulin from a symbiotic spirochete). Jump to: navigation, search Schematic of typical animal cell, showing subcellular components. ... Orders Trichomonadida Hypermastigida The parabasalids are group of flagellate protozoa, most of which are symbiotic in animals. ... Families Mastotermitidae Kalotermitidae Termopsidae Hodotermitidae Rhinotermitidae Serritermitidae Termitidae A termite (also known as a white ant) is any member of the order Isoptera, a group of social insects that eat wood and other cellulose-rich vegetable matter. ... Tubulin is the protein which makes up microtubules. ... The cytoskeleton is a cellular scaffolding or skeleton contained, as all other organelles, within the cytoplasm. ... Phyla / Classes Phylum Crenarchaeota Phylum Euryarchaeota     Halobacteria     Methanobacteria     Methanococci     Methanopyri     Archaeoglobi     Thermoplasmata     Thermococci Phylum Korarchaeota Phylum Nanoarchaeota The Archaea are a major group of prokaryotes. ...

At present the symbiotic hypothesis for the origin of cilia seems to be limited to Margulis and a few of her associates. Margulis is, though, still strongly promoting and publishing a revised version of her hypothesis (Margulis' 1998 book Symbiotic planet: a new look at evolution has some frank autobiographical comments about her support of the symbiotic hypothesis for the origin of the cilium).

Endogenous/autogenous/direct filiation models

Contrasting with the symbiotic models, these models argue that cilium developed from pre-existing components of the eukaryotic cytoskeleton (which has tubulin, dynein, and nexin—also used for other functions) as an extension of the mitotic spindle apparatus. The connection can still be seen, first in the various early-branching single-celled eukaryotes that have a microtubule basal body, 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 somehow (scientists are unsure of the purpose of the centriole) 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. Dynein is a class of protein found in biological cells and is involved in their reproduction. ... The mitotic spindle is a structure of the eukaryotic cytoskeleton involved in mitosis and meiosis. ... Microtubules are protein structures found within cells. ... A basal body is a short cylindrical array of microtubules plus their associated proteins found at the base of a eukaryotic cell cilium or flagellum. ... Jump to: navigation, search A centriole in biology is a barrel shaped microtubule structure found in most animal cells, and cells of fungi and algae though not frequently in plants. ...

An obvious intermediate stage between spindle and cilium would be a non-swimming appendage made of microtubules with a selectable function like increasing surface area, helping the protozoan to 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. One can't argue that such a non-swimming appendage is merely convenient imagination or unlikely to be selectable, as modern protists with analogous non-swimming microtubular appendages do exist and find them perfectly useful, the axopodia of heliozoa being an example. Typical phyla Rhodophyta (red algae) Chromista Heterokontophyta (heterokonts) Haptophyta Cryptophyta (cryptomonads) Alveolates Pyrrhophyta (dinoflagellates) Apicomplexa Ciliophora (ciliates) Excavates Euglenozoa Percolozoa Metamonada Rhizaria Radiolaria Foraminifera Cercozoa Amoebozoa Choanozoa Many others; classification varies The Kingdom Protista or Protoctista is one of the commonly recognized biological kingdoms, including all the eukaryotes except for... Heliozoa, or sun animalcules, are roughly spherical amoeboids with many stiff, microtubule-supported projections called axopods radiating outward from the cell surface. ...

Regarding the origin of the individual protein components, an interesting paper on the evolution of dyneins (Gibbons, 1995; see also Asai and Koonce, 2001) shows that the more complex protein family of cilial dynein has an obvious ancestor in a simpler cytoplasmic dynein (which itself appears to be a result of a four-fold duplication of a smaller motif). Recently, 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.

The bacterial flagellum

An approach to the evolutionary origin of the bacterial flagellum is suggested by the fact that a subset of flagellar components can serve a function as a Type III transport system.

Admittedly, all currently known nonflagellar Type III transport systems are for injecting toxins into eukaryotic cells, and are therefore presumably descended from the flagellum, which is likely older than eukaryotes. For example, the bubonic plague bacterium Yersinia pestis has an organelle assembly very similar to a complex flagellum except that it functions as a needle to inject toxins into host cells. A toxin, in a scientific context, is a biologically produced substance that causes injury to the health of a living thing on contact or absorption, typically by interacting with biological macromolecules such as enzymes and receptors. ... Jump to: navigation, search Bubonic plague is an infectious disease that is believed to have caused several epidemics or pandemics throughout history. ... Phyla/Divisions Actinobacteria Aquificae Bacteroidetes/Chlorobi Chlamydiae/Verrucomicrobia Chloroflexi Chrysiogenetes Cyanobacteria Deferribacteres Deinococcus-Thermus Dictyoglomi Fibrobacteres/Acidobacteria Firmicutes Fusobacteria Gemmatimonadetes Nitrospirae Omnibacteria Planctomycetes Proteobacteria Spirochaetes Thermodesulfobacteria Thermomicrobia Thermotogae Bacteria (singular, bacterium) are a major group of living organisms. ... Yersinia pestis under fluorescent staining, 2000x. ... A toxin, in a scientific context, is a biologically produced substance that causes injury to the health of a living thing on contact or absorption, typically by interacting with biological macromolecules such as enzymes and receptors. ...

However, the Type III transport system still undergirds the hypothesis that the flagellum did not have to come about all at once, as a subset of components has a selectable function. That all known nonflagellar Type III transport systems are disease mechanisms is not shocking as the Type III secretion system was only discovered in 1994 and because scientific study of eubacteria is significantly biased towards disease-causing organisms. This provides another case of co-option, where a motility organelle has evolved into a "complex weapon for close combat." Jump to: navigation, search Natural selection is a process by which biological populations are altered over time, as a result of the propagation of heritable traits that affect the capacity of individual organisms to survive and reproduce. ... Jump to: navigation, search A disease is any abnormal condition of the body or mind that causes discomfort, dysfunction, or distress to the person affected or those in contact with the person. ...

A 2004 genetic study of Yersinia Pestis, however, reveals that it has all genes for a flagellum but lost the ability to express them. Thus its Type III transport system is rather not a precursor but a remain of the flagellum. (Scott A. Minnich and Stephen C. Meyer, Genetic Analysis of Coordinate Flagellar and Type III Regulatory Circuits in Pathogenic Bacteria.)

The archaeal flagellum

The recently elucidated archaeal flagellum is analogous, 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 Type IV pili (pili are filamentous structures outside the cell). Interestingly, some type IV pili can retract. Pilus retraction provides the driving force for a different form of bacterial motility called "twitching" or "social gliding" which allows bacterial cells to crawl along a surface. Thus type IV pili can, in different bacteria, promote either swimming or crawling. Type IV pili are assembled through the Type II transport system. So far, no species of bacteria is known to use its type IV pili for both swimming and crawling. A nanometre (American spelling: nanometer) is 1. ...

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—what are the most deeply branching organisms in each domain, and what are the interrelationships between the domains?