Cilia and flagella are thin, rod-like organelles that protrude from the cell body and drive the motion of cells through fluids and drive fluids across the cell surface via the propagation of bending waves along their lengths. The core structure of the cilium, known as the axoneme, is composed of nine pairs of doublet microtubules, a central pair of single microtubules, and many other proteins, including the axonemal dynein motor proteins, which are coordinated in space and time to generate the ciliary beat. Although the structure of the axoneme has been well studied, the mechanism of coordination is not well understood. By studying the characteristics of the ciliary beat under different conditions, we attempt to derive a biophysical understanding of the beat.
In this study, we examine how the wavelength and other properties of the ciliary beat vary with ciliary length in strains of the motile biflagellate algae Chlamydomonas reinhardtii. In short cilia, it is generally accepted that the wavelength of the beat is proportional to the ciliary length. In longer cilia, such as those of sperm, however, propagation of multiple traveling waves is observed. These different wavelength/length ratios could be due to genetic or biochemical differences between species or length-dependent differences in the underlying physics of motility. To distinguish between these possibilities, we measured the beat wavelength in isolated, reactivated cilia from mutant strains of Chlamydomonas in which ciliary length is mis-regulated, leading to cilia that are shorter or longer than the wild type. This allowed us to probe the transition between short- and long-length behavior in a single organism rather than comparing different organisms.
The cover image of the September 16 issue of Biophysical Journal illustrates the ciliary beat in three specimens of mutant Chlamydomonas, each in a different length domain. Videos of reactivated axonemes were captured by using phase-contrast light microscopy, and the shape of the beat was extracted by using a custom Python image-processing pipeline. Each axoneme is represented in various coordinate spaces, which serve to illustrate how the wavelength of the beat begins to saturate after a critical length. In the left and middle columns, the wavelength increases as the length increases. This is clearly visible in the representation in the third row, which depicts the beat in X-Y space with the static curvature of the axoneme removed. In the right-most sample, an apparent “node” is clearly visible, marking the propagation of a simultaneous second traveling wave as the length begins to diverge from the wavelength.
We confirmed that for shorter cilia, the wavelength of the dynamic beat increased in proportion to ciliary length, as previously found. By contrast, in mutants whose cilia are up to 25 μm in length, the wavelength saturated at 15 μm. These findings probably suggest that the physics of motility is important for determining the wavelength. We propose that the saturating wavelength is a trade-off between maximizing swimming speed (by making the wavelength as short as possible) and minimizing power consumption (by making the wavelength as long as possible). We hope that our findings will inform further refinement of biophysical modeling of the ciliary beat.
To learn more about our research, visit https://howardlab.yale.edu
— Elijah H. Lee, Xiaoyi Ouyang, and Jonathan Howard