Phototropin plays a crucial role in controlling changes in chemotaxis during the initial phase of the sexual life cycle in Chlamydomonas.

During sexual differentiation, Chlamydomonas reinhardtii changes its chemotactic behavior in response to ammonium. Just like gamete formation, the change in chemotaxis mode is controlled by the sequential action of two environmental cues, removal of ammonium or nitrate from the medium and light. Thus, vegetative cells and mating incompetent pre-gametes, the latter being generated by nitrogen starvation in the dark, exhibit chemotaxis towards ammonium. Irradiation of pre-gametes results in a loss of chemotaxis and the gaining of mating competence. Incubation of these gametes in the dark resulted in their regaining chemotactic activity; re-illumination again resulted in its loss. Blue light was shown to be most effective in switching-off chemotaxis. RNA-interference strains with reduced levels of the blue-light receptor phototropin showed an attenuated inactivation of chemotaxis that could be partially compensated by the application of higher fluence rates, suggesting that these light responses are mediated by phototropin. The sharing of photoreceptor and signal transduction components as well as similar temporal patterns observed for changes in chemotaxis towards ammonium and gametic differentiation suggest an integration of the signaling pathways that control these two responses.