Eukaryotes swim with coordinated flagellar (ciliary) beating and steer by fine-tuning the coordination. The model organism for studying flagellate motility, Chlamydomonas reinhardtii, employs synchronous, breaststroke-like flagellar beating to swim, and it modulates the beating amplitudes differentially to steer. This strategy hinges on both inherent flagellar asymmetries (e.g. different response to chemical messengers) and such asymmetries being effectively coordinated in the synchronous beating. In C. reinhardtii, the synchrony of beating is known to be supported by a mechanical connection between flagella; however, how flagellar asymmetries persist in the synchrony remains elusive. For example, it has been speculated for decades that one flagellum leads the beating, as its dynamic properties (i.e. frequency, waveform, etc.) appear to be copied by the other one. In this study, we combine experiments, computations, and modeling efforts to elucidate the roles played by each flagellum in synchronous beating. With a non-invasive technique to selectively load each flagellum, we show that the coordinated beating essentially only responds to load exerted on the cis flagellum; and that such asymmetry in response derives from a unilateral coupling between the two flagella. Our results highlight a distinct role for each flagellum in coordination and have implication for biflagellates’ tactic behaviors.@en

Synchronization of oscillators is an ubiquitous phenomenon that involves mechanical systems, like pendulum clocks, but also biological systems, like peacemaker cells in the heart or neural activity in the brain. If we consider biological systems at the microscale, namely at the scale of cells, we find that processes like locomotion and fluid transport often exploit synchronization of mechanical oscillators called flagella orcilia. These oscillators at the microscale are whip-like structures extending from the cell body. They are present in a number of micro-organisms like sperm cells, Paramecium or the algae C. reinhardtii. In human, cilia are found in the lungs, the respiratory tract and the middle ear. Cilia are activated in a coordinated way to effectively carry out their function, such as draining mucus. The mechanism behind this cilia coordination is still debated. It is not clear how very simple organisms lacking any feedback system have developed complex oscillatory patterns involving coordination among a multitude of cilia or flagella. There is high interest in understanding the fundamental principles ruling ciliary dynamics, since they would impact medical and engineering applications. The purpose of this thesis is to investigate the mechanisms regulating the synchronization of cilia and flagella.@en