Flagellar central pair complexes are helical when extruded.

In many organisms, the central pairs (CPs) of 9+2 cilia and flagella spin. Mitchell and Nakatsugawa (page 709) now claim that this spinning is an effect, not a cause, of flagellar bend propagation.

Motile 9+2 cilia and flagella owe their whip-like movement to motors in the outside barrel of fused doublet microtubules, with motors anchored to one doublet pushing on a neighboring doublet. But the more enigmatic part of this structure is the CP. This doublet of microtubules is connected to the outside barrel via radial spokes that are thought to modulate motor action.

That modulation requires a constant relationship between a particular face of the CP and those microtubule motors that are active at any one time—which is where spinning and twisting come in. Looking at electron micrographs of wild-type Chlamydomonas, Mitchell and Nakatsugawa see that the CP is twisted in straight, quiescent flagella. In mutants that lack the radial spoke heads, and therefore lack a physical connection between the outer and inner microtubules, the CP remains twisted, suggesting that the twist is inherent to the CP structure.

When this twisted structure is forced to bend, as during the beating of the flagella, the CP curves along with the bend. The C1 tubule (half of the CP) is always on the outside of the curve, as if it is longer and this greater length must be accommodated by either helical twisting or being on the outside of a curve. As the bend propagates down the length of the flagellum, it recreates this helix-to-curve transition in the CP and thus yields the rotation. The researchers speculate that the twist helps orient certain CPs: in flagella that change their bend direction the CP can rotate freely to accommodate the new direction, whereas in fixed-direction flagella such as sperm tails the CP is not twisted and does not rotate. ▪