Editing Flagellum (section)

====Evolution====
{{Main|Evolution of flagella}}

At least 10 protein components of the bacterial flagellum share homologous proteins with the [[type three secretion system]] (T3SS) found in many gram-negative bacteria,<ref name="ReferenceA">{{cite journal |last1=Pallen |first1=Mark J. |last2=Matzke |first2=Nicholas J. |title=From The Origin of Species to the origin of bacterial flagella |journal=Nature Reviews Microbiology |date=October 2006 |volume=4 |issue=10 |pages=784–790 |doi=10.1038/nrmicro1493 | pmid = 16953248 | s2cid = 24057949 }}</ref> hence one likely evolved from the other. Because the T3SS has a similar number of components as a flagellar apparatus (about 25 proteins), which one evolved first is difficult to determine. However, the flagellar system appears to involve more proteins overall, including various regulators and chaperones, hence it has been argued that flagella evolved from a T3SS. However, it has also been suggested<ref name="Saier">{{cite journal |last1=Saier |first1=M |title=Evolution of bacterial type III protein secretion systems |journal=Trends in Microbiology |date=March 2004 |volume=12 |issue=3 |pages=113–115 |doi=10.1016/j.tim.2004.01.003 |pmid=15001186}}</ref> that the flagellum may have evolved first or the two structures evolved in parallel. Early single-cell organisms' need for [[motility]] (mobility) support that the more mobile flagella would be selected by evolution first,<ref name="Saier"/> but the T3SS evolving from the flagellum can be seen as 'reductive evolution', and receives no topological support from the [[phylogenetic]] trees.<ref name="Gophna2003">{{cite journal |last1=Gophna |first1=Uri |last2=Ron |first2=Eliora Z. |last3=Graur |first3=Dan |title=Bacterial type III secretion systems are ancient and evolved by multiple horizontal-transfer events |journal=Gene |date=July 2003 |volume=312 |pages=151–163 |doi=10.1016/S0378-1119(03)00612-7 | pmid = 12909351}}</ref> The hypothesis that the two structures evolved separately from a common ancestor accounts for the protein similarities between the two structures, as well as their functional diversity.<ref>{{cite journal |last1=McCann |first1=Honour C. |last2=Guttman |first2=David S. |title=Evolution of the type III secretion system and its effectors in plant–microbe interactions |journal=New Phytologist |date=January 2008 |volume=177 |issue=1 |pages=33–47 |doi=10.1111/J.1469-8137.2007.02293.X | pmid = 18078471 |doi-access=free|bibcode=2008NewPh.177...33M }}</ref>