Bacterial flagella and secretory systems

Bacterial flagella are complicated, multi-component structures that allow bacteria to move through their environment. An updated version of William Paley's pre-Darwinian Watchmaker argument often invokes the flagellum because of its machine-like appearance, but we can demonstrate that the evolution of this structure can be explained as a stepwise process and is thus a model of reducible complexity.

Mark J. Pallen & Nicholas J. Matzke (2006). From The Origin of Species to the origin of bacterial flagella. Nature Reviews Microbiology 4, 784-790.

This diagram shows the components of a flagellum that must all work together to give a bacterium motility. There are 20 major protein components that compose the flagellum itself, with another 20-30 required for it to grow and function.

A protein-based molecular motor at its base composed of many different proteins working together rotates the flagellum in the bacterial membrane using the proton motive force. Protein rings support the flagellum in the cell membranes and cell wall. A secretory system works to export the flagellin monomers that the hollow filament is composed of, allowing it to grow from its tip. Bacterial flagella continuously grow into their environment, and their length is controlled stochastically: if they get too long they break.

The difficulty in explaining the evolution of complicated cell structures such as the flagellum comes from the fact that if a single protein in the complex ceases to function, the entire structure ceases to function. Thus, it is difficult to imagine a priori a stepwise evolutionary path to the current structure, a problem originally conceived from looking at the complexity of the mammalian eye.

How did the complex structure of the flagellum come about? If we look closely we can see that the flagellum can be broken down structurally into components that are found elsewhere in the cell and have been conserved across bacteria. The following diagram illustrates a possible stepwise evolutionary path for the bacterial flagellum from a simple pore (1) to the F1F0-ATP synthetase, a rotary motor closely homologous to the flagellar motor, that also uses the proton motive force. Unlike the flagellar motor, the F1F0-ATP synthetase uses this rotation to generate ATP rather than provide motility. Next, the theoretical path moves through an active transport system that allows bacteria to pump toxins into the environment (2), the Type III secretion system which is conserved across many types of pathogenic bacteria. This active transport system could have developed into the bacterial pilus (4) which allows bacteria to attach to surfaces or other cells and inject toxins directly into other cells. These structures are very similar to the bacterial flagellum, and in a few more steps (5), the pilus could progress through a protoflagellum to a modern flagellum.

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Although it is not clear yet that this exact path is the one taken to get from a simple pore to a full-fledged flagellum, it does provide a possible stepwise evolution that could bridge this large gap. Importantly, this hypothetical evolutionary pathway challenges the argument that only a complex structure that emerges complete with all of its parts is capable of functioning: here we can see that many components that compose the flagellum could, and do, function independently.

Further reading: 

Mark J. Pallen & Nicholas J. Matzke (2006). From The Origin of Species to the origin of bacterial flagella. Nature Reviews Microbiology 4, 784-790.