Not all flagella look alike, at least if you zoom in close enough. Bui et al. have identified structural quirks in the Chlamydomonas flagellum that might account for the alga's swimming style.
When a sperm wriggles, its flagellum undulates symmetrically like a crawling snake. When Chlamydomonas zips along, by contrast, it appears to do the breast stroke, as its twin flagella reach forward and then pull back. Chlamydomonas' flagella sport the standard “9 + 2” structure of nine microtubule doublets surrounding a central pair. The trick has been to find a structural imbalance in these flagella that could explain why the movements of extension and retraction are asymmetric.
Bui et al. used electron cryotomography to take a close look at the flagellum and found that microtubule doublet 1 was the oddball. Dynein arms typically link neighboring microtubule doublets. When these arms pull, adjacent doublets slide past one another, and the flagellum flexes. But doublet 1's inner dynein arm is missing one of the molecules that is usually present in other doublets. Bui et al. hypothesize that this difference slows the sliding between doublet 1 and its neighbors, doublets 2 and 9.
The researchers also found more interconnections between microtubule pairs—including a strong bridge between doublets 1 and 2—than previously identified. The final picture, the team concludes, is that doublets 9, 1, and 2 differ from the three doublets that lie on the opposite side of the shaft. That disparity might yield asymmetrical motion when the alga swims.