274 SEX IN MICROORGANISMS 



ent mating types. Marked temperature effects on mating type deter- 

 mination have also been shown for the B varieties under certain con- 

 ditions and again, as in the A varieties, the higher temperatures favor 

 the even mating types (Nanney, unpublished). 



In only one essential feature have differences between the A 

 and B groups been noted (Sonneborn, 1947). In group A there is no 

 correlation between the mating types of parents and their progeny, 

 nor between the mating types of sister caryonides. In group B, 

 changes occur rarely at reorganization, and when they do occur they 

 usually involve both sister caryonides. A strong correlation is thus 

 found between the mating type of the cytoplasmic parent and the 

 mating type of the progeny. Similarly, the mating types of sister 

 caryonides are strongly correlated. At conjugation this parent-prog- 

 eny correlation usually results in one of the cytoplasmic parents giv- 

 ing rise to two caryonides of one mating type and the other giving 

 rise to two caryonides of the other mating type. Since exconjugants 

 of a single pair are known to be alike in regard to the kinds of genes 

 which they possess, Sonneborn concluded that the differences which 

 characterize the mating types, though clearly due to differences in 

 the macronuclei, are inherited through some cytoplasmic mechanism. 

 This interpretation was supported by the observation that pair mem- 

 bers which are allowed to exchange massive amounts of cytoplasm 

 usually give rise to the same kinds of progeny. 



The presence of a cytoplasmic component in the group B system 

 of mating type determination introduces special problems, and diver- 

 sities of interpretation have existed in regard to its significance. Be- 

 cause of the special features of the cytoplasmic involvment, we will 

 return later to a more extended consideration of nucleocytoplasmic 

 interactions in the B system. 



The general features of the B system may be summarized as 

 follows: 



1. As in group A, mating types are determined by alternative 

 macronuclear conditions. 



2. Higher temperatures, as in group A, favor the formation of 

 macronuclei controlling the even mating types. 



3. In contrast to the group A pattern, mating types in group B 

 show a strong parent-progeny correlation. Mating types are not 

 redetermined at random at nuclear reorganization, but tend strongly 

 to be maintained through reorganization. 



