The eukaryotic flagellum beats with apparently unfailing periodicity, yet responds rapidly to stimuli. Like the human heartbeat, flagellar oscillations are now known to be noisy. Using the alga textit{C. reinhardtii}, we explore three aspects of nonu
niform flagellar beating. We report the existence of rhythmicity, waveform noise peaking at transitions between power and recovery strokes, and fluctuations of interbeat intervals that are correlated and even recurrent, with memory extending to hundreds of beats. These features are altered qualitatively by physiological perturbations. Further, we quantify the recovery of periodic breastroke beating from transient hydrodynamic forcing. These results will help constrain microscopic theories on the origins and regulation of flagellar beating.
In a multitude of lifes processes, cilia and flagella are found indispensable. Recently, the biflagellated chlorophyte alga Chlamydomonas has become a model organism for the study of ciliary coordination and synchronization. Here, we use high-speed i
maging of single pipette-held cells to quantify the rich dynamics exhibited by their flagella. Underlying this variability in behaviour, are biological dissimilarities between the two flagella - termed cis and trans, with respect to a unique eyespot. With emphasis on the wildtype, we use digital tracking with sub-beat-cycle resolution to obtain limit cycles and phases for self-sustained flagellar oscillations. Characterizing the phase-synchrony of a coupled pair, we find that during the canonical swimming breaststroke the cis flagellum is consistently phase-lagged relative to, whilst remaining robustly phase-locked with, the trans flagellum. Transient loss of synchrony, or phase-slippage, may be triggered stochastically, in which the trans flagellum transitions to a second mode of beating with attenuated beat-envelope and increased frequency. Further, exploiting this algas ability for flagellar regeneration, we mechanically induced removal of one or the other flagellum of the same cell to reveal a striking disparity between the beating of the cis vs trans flagellum, in isolation. This raises further questions regarding the synchronization mechanism of Chlamydomonas.
Groups of beating flagella or cilia often synchronize so that neighboring filaments have identical frequencies and phases. A prime example is provided by the unicellular biflagellate Chlamydomonas reinhardtii, which typically displays synchronous in-
phase beating in a low-Reynolds number version of breaststroke swimming. We report here the discovery that ptx1, a flagellar dominance mutant of C. reinhardtii, can exhibit synchronization in precise antiphase, as in the freestyle swimming stroke. Long-duration high-speed imaging shows that ptx1 flagella switch stochastically between in-phase and antiphase states, and that the latter has a distinct waveform and significantly higher frequency, both of which are strikingly similar to those found during phase slips that stochastically interrupt in-phase beating of the wildtype. Possible mechanisms underlying these observations are discussed.
