290 SEX IN MICROORGANISMS 



morphological changes occurs in conjugation and autogamy. The 

 autogamous animal undergoes loss of mating activity, meiosis, macro- 

 nuclear breakdown, pronuclei formation, and paroral cone formation 

 (Diller, 1936). It appears, then, that the autogamous animal, like the 

 conjugant, is activated and that both undergo the same physiological 

 cycle. However, autogamy differs from conjugation in three impor- 

 tant essentials. (1) Autogamy occurs spontaneously, but only under 

 certain physiological conditions, in single isolated animals. No contact 

 or union with a mate is involved. (2) Autogamy results in self-ferti- 

 lization. The two pronuclei fuse to produce an homozygous syn- 

 caryon. (3) Autogamy and conjugation are initiated through different 

 mechanisms (Metz, 1948; Metz and Foley, 1949). 



It should be noted that exchange of pronuclei fails to occur in 

 a number of abnormal conjugation processes. The usual genetic result 

 in such instances is identical with natural autogamy, that is, self-ferti- 

 lization. All such phenomena are sexually induced and are considered 

 to be natural or experimental variants of normal conjugation. They 

 are conveniently termed cytogamy (Wichterman, 1940). They are 

 to be distinguished from natural autogamy, which occurs spontane- 

 ously and without sexual contact between animals of opposite mating 

 type. 



Cellular Adhesion and Mating-Type Substances in 

 Par ante ciu7n 



The initial phase of the sexual reaction in Farainecmm is adhe- 

 sion of potential conjugants through the mating reaction union. There 

 is no evidence for action at a distance, no chemotaxis leading to this 

 reaction. Adhesion occurs only upon random contact. Furthermore, 

 it is not mediated by any agent from the medium. Thus culture fil- 

 trates or supernates have no effect upon the mating behavior of Para- 

 meciimi (Sonneborn, 1937, 1939b; Metz, 1947; Kimball, 1943), and 

 repeated washing does not alter their mating reactivity (Metz, 1947 

 and unpublished). Therefore the initial adhesion, the mating reaction, 

 must result from direct interaction of surfaces. Indeed it may be 

 attributed to the interaction of surface substances. Evidence for such 

 surface substances is derived from several sources, Sonneborn (1937, 

 1942b, c) has observed that an animal of one mating type can adhere 

 briefly to an animal of the same mating type if it has first clumped 



