ABSTRACT
For organisms that respond to environmental stimuli using taxes, reversal of the tactic sign should be tightly regulated for survival. The biciliate green alga Chlamydomonas reinhardtii is an excellent model for studying reversal between positive and negative phototaxis. C. reinhardtii cells change swimming direction by modulating the balance of beating forces between their two cilia after photoreception at the eyespot; however, it remains unknown how they reverse phototactic sign. In this study, we observed cells undergoing phototactic turns with a high-speed camera and found that two key factors determine the phototactic sign: which of the two cilia beats stronger for phototactic turning and when the strong beating starts. We developed a mathematical model to explain this sign-reversal with a single equation, which suggests that the timing of the strong ciliary beating is regulated by switching between the light-on and light-off responses at the eyespot.
Competing Interest Statement
The authors have declared no competing interest.