106 SEX IN MICROORGANISMS 



eugmnetos, were isolated from various soil samples by Gerloff (1940), 

 who described the species as C uioenjoiisii. Apart from a single homo- 

 thallic strain from the Cameroons, all these isolates conformed to 

 regular heterothallism. In 1948 Provasoli isolated complementary 

 mating types of a Chlamy doiitonas, the description of which closely 

 accorded with that of C. nioeumsn Gerloff. It has been found (see 

 Hutner and Provasoli, 1951, and Smith, in litt., November 1951) that 

 these isolates mate in reciprocal combinations with strains of C. eii- 

 gametos obtained directly or indirectly from Moewus, and it therefore 

 seems very probable that the two names are synonymous, as Gerloff 

 (1940) had originally suggested. 



Smith recognized the difficulty of finding sexually active strains 

 among haploid clones isolated at random from soil and accordingly 

 devised a procedure (1946, 1947) which yielded considerably bet- 

 ter results. Essentially, this consisted of the examination of freshly 

 flooded agar cultures for the formation of mating pairs, which would 

 indicate either homothallism or the presence of both mating types 

 of a heterothallic species, originating from a single diploid zygote. 

 He reported (1950b) the isolation by this means of fifteen hetero- 

 thallic, not interfertile, species of Chlmnydo7nonas, and (1951b) a 

 number of homothallic species. Using a modification of this method 

 involving a prior enrichment for zygospore-forming species, Lewin 

 (1951) isolated from ten soil samples three heterothallic and two 

 homothallic species. 



HYBRIDIZATION 



In 1916 Pascher reported the hybridization of two species of 

 Chlamy domo7ias and described the progeny emerging from the hy- 

 brid zygotes. Strehlow (1928-29) obtained zygotes by crossing C. 

 paradoxa {plus) with C botryoides {viimis), though he could not 

 effect the reciprocal cross, nor could he induce the zygotes to ger- 

 minate. A4oewus (1935b) reported success in crossing C. eugametos 

 with C. paupera, obtaining zygospores which showed a greater sensi- 

 tivity to chilling than did those of the parent types, and haploid 

 progeny exhibiting segregation of six morphological character pairs. 



He went on to describe extensive experiments in hybridization 

 of Folytoma spp. (1937), and of other species of Chlamy dojnonas. 

 In 1940 he reported that he had been able to isolate from 500 soil 



