280 SEX IN MICROORGANISMS 



caryonides. These caryonides are clones deriving their macronuclei 

 from single macaronuclear anlage; yet within such clones are found 

 cells of diverse mating types (Kimball, 1939; Jennings, 1941; Nanney 

 and Caughey, 1953), Since the macronuclei control the mating types, 

 it is presumably some macronuclear instability that permits cells 

 within a single clone to manifest different mating types. The fact that 

 diverse pure types may be derived from the selfers indicates that the 

 unstable macronuclei may "stabilize," though it is not clear what the 

 stabilization involves. 



A recent formal explanation for the selfers in P. aurelia (Nanney, 

 1953) was based on the idea of structural inhomogeneity. Since the 

 most widely held interpretation of macronuclear structure considers 

 that the macromicleus is compound, i.e., contains many sets of nuclear 

 genes associated into subnuclei with a certain degree of integrity, it 

 appears plausible to postulate differences between mating types as 

 due to some characteristic of the subnuclei. If this were correct, some 

 macronuclei might contain diverse types of subnuclei, and such 

 "mixed" macronuclei could provide a basis for the observed vegetative 

 segregation of mating types. 



In an attempt to test this interpretation (Nanney, unpublished), 

 macronuclei which would ordinarily control different mating types 

 were allowed to fuse. This fusion was induced through starving 

 exconjugants (Sonneborn, 1947). Stock 90 in variety 1 was used in 

 this study because in this variety mating types are determined at ran- 

 dom at reorganization, giving many cells with diverse sister nuclei, 

 and because selfing is rare in this stock. After macronuclear fusion 

 many clones were found which behaved like the spontaneous selfers 

 in other stocks of the same variety. This production of selfers by the 

 fusion of diverse macronuclei seemed strongly to support the hypoth- 

 esis of structural inhomogeneity for the spontaneous selfers, but did 

 not exclude other interpretations. 



Further information from another source has cast considerable 

 doubt upon this interpretation. The hypothesis offered hope that an 

 analysis of the pattern of segregation of mating types in an unstable 

 clone would be profitable. It seemed reasonable to assume that the 

 probability of origin of a "stable" macronucleus would depend upon 

 the original number of subnuclei, the relative numbers of the differ- 

 ent kinds and the number of divisions the macronuclei had undergone. 

 The analysis in P. aurelia was, however, complicated by a relatively 



