278 SEX IN MICROORGANISAIS 



mating types at conjugation in selfing caryonides (Nanney, unpub- 

 lished), it was observed that the mating type VIII individuals in such 

 caryonides change to type VII with high frequency. All selfing 

 caryonides studied, regardless of the amount of selfing observed, 

 showed this instability. In some caryonides one of the mating types 

 was so infrequent that routine observation for selfing was not suffi- 

 cient to demonstrate that these were in fact selfers and observations 

 on several subcultures for a period of days were required to detect 

 selfing. Since most such selfing caryonides appear superficially to be 

 pure type VIII cultures, but revert largely to type VII at reorganiza- 

 tion, it appears possible that many of the unstable VIII's studied by 

 Sonneborn were of this kind. If so, the reversion is in these cases in- 

 timately connected with the question of the nature of selfing caryo- 

 nides and is not due simply to the fact that these clones have recently 

 changed mating type. 



Sonneborn's original interpretation is also called into question 

 in so far as it focuses attention upon the occasional aberrant clones 

 and ignores the behavior of the majority of clones which have 

 changed mating types. Usually when changes occur from one pure 

 mating type to another pure mating type, not only are the mating 

 types changed but the changed types are perpetuated normally 

 through nuclear reorganization. Even when sister caryonides, sharing 

 a common source of cytoplasm, show different mating types, the 

 type VII caryonide produces almost exclusively type VII progeny 

 and the type VIII caryonide produces almost exclusively type VIII 

 progeny (Nanney, unpublished). One finds the usual high correlation 

 between the mating type of the parent and that of its sexual progeny, 



Sonneborn (1953) has recently provided a similar but more 

 elegant test for the hypothesis of nuclear control, and the results 

 seem conclusive. This test involves inducing at conjugation (a) cyto- 

 plasmic exchange between the mates and (b) macronuclear regenera- 

 tion. The cytoplasmic exchange results in the change of mating type 

 in one member of the pair, so that new macronuclei controlling one 

 mating type share the same cytoplasm with fragments of a macronu- 

 cleus controlling a different mating type. The conditions under which 

 macronuclear regeneration was induced were such as to suppress 

 division of the new macronuclei, but not to abort them. This sup- 

 pression of division of the new macronuclei results in the formation 

 of diverse lines of descent from a single cell, some containing a new 



