268 SEX IN MICROORGANISMS 



which show considerable cytoplasmic inheritance; group B, com- 

 posed of varieties 2, 4, 6, and 8. Most of the study of mating types 

 has been concentrated on variety 1 in group A and variety 4 in 

 group B. The mating types in variety 1 are designated as I and II; 

 those in variety 4 as VII and VIII. 



The chief reason for Sonneborn's ( 1 945 ) conclusion that group 

 A and group B varieties have fundamentally similar systems of deter- 

 mination and inheritance of traits, including mating types, is their 

 possession of identical major cytogenetic processes. These consist 

 chiefly of conjugation, cytogamy, autogamy, and macronuclear re- 

 generation (Sonneborn, 1947). The details of these processes are dis- 

 cussed elsewhere in this symposium (Wenrich, 1954) and will be pre- 

 sented here only briefly. 



Conjugation occurs when cells of complementary mating types 

 are mixed under appropriate conditions. Initially clumps and even- 

 tually pairs are formed. Within each member of a conjugating pair 

 the micronuclei undergo meiosis, and all except one of the haploid 

 nuclei disintegrate. This remaining nucleus divides mitotically to pro- 

 duce the migratory and stationary nuclei of identical genetic consti- 

 tution. The migratory nuclei of the pair members are exchanged and 

 fuse with the stationary nuclei to form the syncarya. From the fact 

 that the nuclei fusing in the two pair-members are alike, it will be 

 clear that the syncarya must be of the same genotype. 



Cytogamy is essentially similar to conjugation except that an 

 exchange of gamete nuclei fails to take place and mutual self-fertiliza- 

 tion results. In this case, therefore, the syncaryon is formed by the 

 fusion of two sister haploid nuclei of identical genie constitution. The 

 syncarya must, therefore, be homozygous for all their genes. 



Autogamy occurs periodically in all the stocks studied. This 

 process is identical with cytogamy except that it occurs in single un- 

 paired cells. 



In all three of these reorganization processes the syncaryon nor- 

 mally gives rise to four mitotic division products, two of which 

 become micronuclei and two of which become macronuclei. The 

 anlagen of the two macronuclei are regularly separated into different 

 cells at the first postzygotic cell division and develop into mature 

 macronuclei which divide, apparently amitotically, at subsequent cell 

 divisions. The animals whose macronuclei are derived from a single 

 macronuclear anlage are called a caryonide. 



