270 SEX IN MICROORGANISMS 



with a very low frequency. However, Sonneborn (1942) increased 

 its frequency greatly by exposing cells to high temperatures during 

 the period when the syncaryon undergoes its divisions and the new 

 macronuclear anlagen are developing. If the exposure is begun earlier 

 in the fertilization processes, the micronuclei may be lost. Macro- 

 nuclear regeneration may also be induced by exposure to very low 

 temperatures at the same period (Sonneborn, unpublished). 



Group A Pattern 



In some varieties of group A a few stocks have been found 

 which are pure for one mating type (Sonneborn, 1938; Sonneborn 

 and Dippell, 1946). These few stocks are of the odd mating type. 

 In all other stocks any isolated animal can, according to a definite 

 pattern, produce both mating types. To these "two-type" stocks we 

 will turn first. The main features of inheritance of mating types in 

 such stocks were first reported by Sonneborn (1937) and Kimball 

 (1937). 



Animals of these two type stocks usually undergo no change of 

 mating type during vegetative reproduction. However, changes of 

 mating type may occur at nuclear reorganization. The kind of nu- 

 clear reorganization (conjugation or autogamy) has little or no influ- 

 ence on the frequencies of the mating types produced. A single 

 reorganized animal may give rise to either or both mating types. If 

 both are produced, segregation of the mating type determiner nearly 

 always takes place at the first postzygotic cell division, and the two 

 products of this division are, during subsequent vegetative reproduc- 

 tion, pure for mating type. It is at this same division that the inde- 

 pendently developing macronuclear anlagen separate. Hence, the 

 unit of mating type inheritance is the caryonide, and this sort of 

 inheritance is said to be caryonidal. This fact at once suggests that 

 the macronuclei may be differentiated in such a way as to determine 

 different mating types. In further support of this view, Sonneborn 

 (1937, 1938, 1939, etc.) marshalled a number of lines of evidence 

 which will be briefly reviewed. 



When macronuclear regeneration occurs the mating type of the 

 parent cell is perpetuated and no change is observed (Sonneborn, 

 1942). This same result is obtained whether macronuclear res^enera- 

 tion occurs as the result of treatment with high temperatures or as 



