Beat Frequency is Reduced but Waveform Shape is Conserved in Chlamydomonas Flagella at High Viscosity

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Motivation: Cilia and flagella are thin subcellular organelles that produce fluid movement and cell motility. Coordinated activity of the motor protein dynein causes sliding between adjacent microtubule doublets and subsequent bending of the cilium. Here, the role of mechanical feedback in dynein regulation is investigated by studying the effects of increased viscosity on the flagellar waveform.Methods: Flagellar waveforms of the unicellular green alga Chlamydomonas reinhardtii were measured in media of different viscosities. Genetic mutants missing inner dynein arms (ida1) or outer dynein arms (oda2) were used to investigate the force-dependent contributions of these molecular motors to the flagellar waveform. Media viscosity was increased from 1.5 cP to 4.7 cP using Ficoll 10% w/v. High speed videos (350 fps) of periodically-beating uniflagellate cells were processed in MATLAB to extract waveform parameters sensitive to external viscosity, and to compare mutant cells to wild-type cells.Results: Beat frequency decreased with increasing viscosity in wild-type (59.5±7.0 Hz to 31.7±9.1 Hz) and mutant cells (ida1: 50.4±6.1 Hz to 27.3±11.1 Hz; oda2: 24.9 ±4.0 Hz to 7.2±1.8 Hz). Both wild-type and oda2 cells exhibited greater maximum curvature at the higher viscosity (wild-type: 0.212±0.042 rad/μm to 0.287±0.025 rad/μm; oda2: 0.163±0.033 rad/μm to 0.230±0.014 rad/μm). Curvature differences were smaller in the ida1 mutant (0.266±0.024 rad/μm to 0.288±0.089 rad/μm). Shear rate and bend propagation speed decreased with viscosity, and the amplitude of shear was reduced slightly. Notably the qualitative shape of the waveform and relative differences between wild-type, ida1, and oda2 were maintained at the higher viscosity.Discussion: At higher viscosity, beat frequency decreases to maintain viscous force amplitudes and flagellar waveforms similar to baseline, which supports the role of mechanical feedback in dynein coordination.