  Eukaryotes swim with coordinated flagellar (ciliary) beating and steer by
fine-tuning the coordination. The model organism for studying flagellate
motility, C. reinhardtii (CR), employs synchronous, breast-stroke-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 CR,
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 responds to only 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.
