The younger flagellum coordinates the beating in C. reinhardtii

TL;DR

This study reveals that in C. reinhardtii, the younger (cis) flagellum primarily drives coordination during synchronous beating, with unilateral coupling playing a key role in flagellar asymmetry and steering.

Contribution

It demonstrates that flagellar coordination depends mainly on load on the cis flagellum and uncovers the unilateral coupling mechanism maintaining asymmetry.

Findings

Coordination responds mainly to load on the cis flagellum.

Unilateral coupling sustains flagellar asymmetry.

Implications for flagella-driven steering behaviors.

Abstract

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.