272 SEX IN MICROORGANISMS 



10° C. (Sonneborn, 1939). Under these conditions very little change 

 of mating type occurred at conjugation, each mate producing a clone 

 predominantly of its own mating type. These observations were 

 made before the discovery of macronuclear regeneration, and since 

 mating types do not change at macronuclear regeneration, it seems 

 possible that some, perhaps all, of this failure to change mating types 

 at low temperature is due to the occurrence of macronuclear regen- 

 eration. 



In sum, the evidence available on mating type determination in 

 group A agrees in indicating macronuclear control of the differences 

 in mating types. The problem is then raised as to how different 

 macronuclei arise from genetically identical micronuclei. Pair mem- 

 bers are identical in their nuclear genes after fertilization, as required 

 by the cytogenetic details and as shown by genetic analysis (Sonne- 

 born, 1939, 1947), and yet pair members may differ in their mating 

 types. Even more remarkable, sister macronuclei developed from 

 presumably identical mitotic products of a single fertilization nucleus 

 may determine different mating types. The fact that mating types 

 may change at any autogamy, despite the fact that complete homozy- 

 gosis is established by a single autogamy, argues that no micronuclear 

 differences or genie recombination can account for the differences in 

 mating types. This conclusion is reinforced by an additional fact set 

 forth in the next paragraph. Several tentative hypotheses have been 

 advanced to account for macronuclear differences, but none is satis- 

 factory. This then remains the chief unexplained phenomenon of 

 mating type determination in group A, the manner in which unlike 

 macronuclei arise from identical micronuclei. 



The additional fact referred to in the previous paragraph is of 

 special importance, although the reason for its importance will not 

 become evident until later. The mating type of an animal before 

 reorganization is not correlated in any way with the mating type of 

 its progeny. An animal of mating type I gives rise to the same kinds 

 of progeny and in the same proportions as animals of mating type II 

 (Sonneborn, 1937; Kimball, 1937). In other words, animals in the 

 two-type stocks of group A which differ in mating type do not 

 differ from each other with regard to either the types of progeny 

 which they produce or the relative frequencies of these progeny. 



The preceding account shows that the micronuclei in animals of 

 different mating types in group A are alike, while the macronuclei 



