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The flagellum is the result of successive gene duplications.

Elizabeth H. White / CDC

A Complex Tail, Simply Told

One of evolutionary biology's greatest challenges is deciphering the origins of complex structures. Now, scientists have unraveled the steps in the evolution of the bacterial flagellum, a tiny, whiplike structure used in swimming and host invasion. A new study shows the flagellum is the result of successive duplications of a single gene in the ancestor of today's bacteria, a finding that not only answers an important question about the evolution of complex structures but also provides additional ammunition to counter arguments from evolution's foes.

A suite of more than 50 genes builds and operates the flagellum. Several hypotheses address its origins, but none adequately explained at the genetic level how this organelle might have arisen. To find out, evolutionary biologist Howard Ochman and postdoc Renyi Liu of the University of Arizona, Tucson, obtained the complete genomes of 41 flagellated bacteria species and identified 24 flagella-related genes common to all the microbes.

In each species, the 24 genes were very similar to each other but not to any other genes in the genome. This finding, coupled with the observation that this complete set of genes exists in all flagella-bearing bacteria, suggests the genes arose by duplication of a single gene in the ancestor of all bacteria, Ochman says. Slight changes in the genes then generated new functions. Each gene is responsible for a different aspect, such as producing the proteins that make up the flagellar motor, filament, and other structural components. In addition, an evolutionary tree constructed by the researchers suggests that the order in which the genes appeared matches the sequence of steps in the assembly of the flagellum. Ochman and Liu report their findings online this week in Proceedings of the National Academy of Sciences.

The study underscores several important tenets of evolution, says evolutionary biologist Michael Lynch of the University of Indiana in Bloomington. "Complexity builds out of simplicity, and this is a well-documented argument for how that can happen," he says. It also provides a straightforward counterexample to claims from "intelligent design" proponents that the flagellum could not have evolved from a single gene, adds cell biologist Ken Miller of Brown University (ScienceNOW, 18 October 2005). "By testing the hypothesis of common ancestry of the flagellum in so many different species, the researchers clearly show these genes were derived from one another through gene duplication."

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