Evolution of flagella

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The evolution of flagella is of great interest to biologists because the three known varieties of flagella (eukaryotic, bacterial, and archaeal) each represent a sophisticated cellular structure that requires the interaction of many different systems.

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Eukaryotic flagellum

There are two competing groups of models for the evolutionary origin of the eukaryotic flagellum (referred to as cilium below to distinguish it from its bacterial counterpart).

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.[1] The hypothesis, though very well publicized, was never widely accepted by the experts,[citation needed] in contrast to Margulis' arguments for the symbiotic origin of mitochondria and chloroplasts.[citation needed]

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, such as Mixotricha and Trichonympha). 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).[citation needed]

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