108 SEX IN MICROORGANISMS 



completely in darkness, and Moewus attributed these effects to the 

 formation and destruction of "sex substances" (see page 115). In an 

 attempt to confirm some of Moewus' results, though unable at the 

 time to obtain cultures of C. eugametos, Smith (1946) examined the 

 sexual behavior of a number of Chlamy domonas species isolated from 

 Californian soils. He found that all three heterothallic species tested 

 were capable of mating in darkness, though the activity was appre- 

 ciably stimulated by light. Lerche (1937) reported that light was not 

 essential for the initiation of mating in Dunaliella, and Maher (1946- 

 47) showed that Frotosiphon was capable of completing its sexual 

 cycle in complete darkness. 



Quality of Light. The experiments reported by Smith in 1946 

 were made on suspensions of cells which had been initially grown in 

 light and were subsequently darkened prior to being tested for mating 

 ability. In 1948, having isolated the facultatively heterotrophic Chla- 

 mydomonas reinhardi, he was able to extend his observations to cul- 

 tures which had been grown in darkness on nutrient media contain- 

 ing sodium acetate. He found that such cells, although motile, were 

 incapable of sexual activity unless they had been subjected to a pe- 

 riod of illumination. Red light (6150 to 5900 A) or blue light (4357 

 A) was tested in place of white light, and in all cases the sexual 

 clumping of cells could be induced. In this respect C. reinhardi ap- 

 parently differs from C. eugametos, which, according to Moewus 

 (1939b), can only receive sexual stimulation from light at the blue 

 end of the spectrum (4300 to 5000 A). 



In his description of C. chlaviydogama, Bold (1949) stated that 

 copulation occurred only in illuminated cultures. Using the strains 

 isolated by Bold, Cadoret (1949) was able to confirm the observation 

 and to investigate the character of light responsible for the physio- 

 logical effect. His experiments indicated that for C. chlaviydogama, 

 unlike C. reinhardi, white light could not be replaced by either red 

 or blue Hght, but that only a combination of both e]ualities of light 

 would induce sexual clumping and pairing. Cadoret suggested that, 

 for the manifestation of sexual activity, light must be absorbed by 

 two distinct substances, perhaps green and orange in color respec- 

 tively, and that chlorophyll and a carotenoid may both be involved. 

 The role of carotenoids in the sexual life of C. eugametos has been 

 given some weight in the publications of Moewus and co-workers 



