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Differentiation of Chlamydomonas reinhardtii vegetative cells to gametes requires two environmental signals: nitrogen starvation and light. Vegetative cells incubated without nitrogen differentiate into pregametes. Pregametes can be converted into sexually mature gametes by irradiation with light. The action spectrum for the light-dependent step in gamete formation showed two maxima at 370 and 450 nanometers. This is similar to the spectrum of other blue light/ultraviolet light-A-absorbing photoreceptors.