Control of Sexual Activity in Chlamydomonas by Liǵht

SUMMARY: Chlamydomonas moewusii is a unicellular, haploid, heterothallic alga. Suspensions containing a high proportion of active gametes are readily obtainable. The species is obligately photo-autotrophic. Organisms grown in light lose sexual activity when incubated in darkness, and only regain it when illuminated. Organisms of both mating-types require light for sexual activation. This reaction appears independent of the presence or absence of CO2. It was not found possible to replace the effect of light by the addition of any chemical or natural extract to the medium. By the use of a simple device, changes in the sexual activity of a given suspension were followed experimentally. The sensitivity of organisms to light remained approximately constant during an experimental run of 1–2 hr. Mating activity of organisms transferred to darkness remained constant for a few minutes (‘dark lag’) before decreasing. The length of the dark lag was a function of the duration of previous illumination. The dark lag of plus organisms was shorter than that of minus organisms under comparable conditions. After the lag period, decline of activity in darkness was approximately exponential. Rates of loss of activity between 10° and 3° were measured, and half-life values calculated, for each mating-type. The rate of loss of activity was approximately the same for both mating-types. Loss of sexual activity was somewhat accelerated by anaerobiosis. Resumption of sexual activity on re-illumination followed a short period of persistent inactivity (‘light lag’). The length of the light lag was not dependent on the duration of the preceding dark period. At 25°, the lower threshold of white light was 10 f.c. for minus organisms, 50 f.c. for plus organisms. At 300 f.c., minus organisms were reactivated between 12·5° and 35°, plus organisms between 17·5° and 30°. The action spectrum of photoactivation exhibited two peaks, around 450 and 680 mμ., thereby resembling the absorption spectrum of the chloroplast. Phenylurethan (6 × 10−4 m) reversibly inhibited photosynthesis and photoactivation of sexuality, while respiration and the mating process itself were affected to a much lesser degree. It is postulated that mating activity is controlled by an intracellular hormone, activated at the plastid during illumination, and operative at the flagella.