For phototactic steering, Chlamydomonas detects environmental light conditions with a photoreceptor (eyespot) while rotating the cell body around its body axis. Because of the bodily rotation and the directionality of the eyespot sensitivity, the light signal perceived by the eyespot must alternate between a light period and a dark period. It is an interesting question how cells can correctly change its swimming direction while the light signals change periodically. In this study, we examined the timing of the change in cells' swimming direction with respect to the timing of the light/dark cycle occurring at the photoreceptor. Most of the cells that displayed positive phototaxis had the eyespot facing the outside of the helical swimming track. We found that when phototactic light was applied from the direction perpendicular to the swimming direction of a cell, a phototactic response was initiated when the eyespot faced the light source. This was constantly observed irrespective of the phase at which the phototactic light was turned on. The initial change observed after the light stimulation was a decrease in the pitch angle of the helical swimming path, which caused the cell to swim less farther away from the light source than when unstimulated. This change was followed by a large turn toward the light source, which occurred when the eyespot faced away from the light. These observations indicate that the dominance of the cis-flagellum (the flagellum nearest to the eyespot) over the trans-flagellum (the flagellum farthest from the eyespot) decreases during the light phase and increases during the dark phase. Thus, both light reception (on response) and the cessation of light perception (off response) by the eyespot are important for producing phototactic turns.
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