no SEX IN MICROORGANISMS 



State resulting directly or indirectly from photosynthesis. It is sig- 

 nificant that sexual activation and photosynthesis are inhibited to the 

 same extent by 7 X 10^^ M phenylurethan. The effect of light can- 

 not be replaced by raised oxygen tension in the medium. However, 

 some indications of the nature of the active product have been ob- 

 tained and are discussed below. 



Period of Ilhmmiatiojj. Klebs (1896) observed that cells of 

 Chlamy dojnonas media capable of active mating in light lost their 

 sexual activity in darkness, under which condition he considered that 

 they returned to a vegetative phase. Since gametes only reappeared 

 in such cultures after a week of reillumination, it seemed likely that 

 "rejuvenation" of the culture by vegetative reproduction was needed 

 to restore sexual activity. Moewus (1933), on the other hand, noted 

 that cell suspensions of C eugametos which had been kept in dark- 

 ness could regain their sexual activity after only 30 to 45 minutes in 

 light, indicating that the same cells were concerned in the recovery. 



Smith (1946) found that as short a period as 5 minutes of illu- 

 mination markedly stimulated sexual activity in C. mimitissima. In 

 his later investigations with C reinbardi, which can be grown ab 

 initio in the complete absence of light, he found that sexual activity 

 began to be demonstrable after 90 minutes of illumination in bright 

 light. Using the same species, Wendlandt (see Smith, 1951b) showed 

 that, at lower light intensities, longer periods of illumination were 

 required before the first appearance of sexuality; whereas, for a con- 

 stant period of illumination, lower light intensities induced a less in- 

 tense sexual response (as measured by the number of clumps of 

 mating cells per unit volume). Once illuminated, cultures of this 

 species retained their sexual activity for at least a week in darkness 

 (Smith, 1948). 



Experiments with C. jnoewusii (Lewin, unpublished) have 

 shown that sexually active cell suspensions reversibly lose their activ- 

 ity in darkness and regain it in light, these changes taking place in 

 a matter of minutes, so that considerations of cell division do not 

 arise. The species is peculiarly well suited to studies of this mechan- 

 ism, since cell pairs, once united, remain active without further fusion 

 for some hours (see page 121), and their numbers provide a con- 

 venient direct assay of mating activity. By using this fact it has been 

 shown that cells, sexually inactivated by a period of darkness, may 

 completely regain activity after 10 to 30 minutes of illumination in 



