MATING TYPE DIT ERiMINATION 281 



short period of vegetative growth before autogamy intervened and 

 replaced the macronuclei being studied, \\1ien it was discovered 

 that selfing caryonides occurred in Tetrahymefia pyriformis and that 

 autojTaniy did not (Nanney and Caughey, 1953), it seemed that 

 an analvsis would be profitable here. Hence, studies were undertaken 

 w'nh this end in view. In the course of these studies (Nanney and 

 Caughey, unpublished) it was found that the mating types in selfing 

 clones can be stabilized readily simply by starving the cells. This was 

 not anticipated on the structural hypothesis, since starvation of mac- 

 ronuclei with stable but diverse subnuclei could not result in the 

 establishment of macronuclei with only one type of subnucleus. 

 Hence, the hypothesis is of no value for the selfers in T. pyriforinis, 

 and since the systems of determination in P. aiirelia and T. pyriformis 

 are so much alike, the hypothesis for P. mtrelia is severely discredited. 

 These observations indicate that, while the systems leading to the 

 manifestation of the different mating types are mutually antagonistic, 

 the\' may under certain circumstances be maintained simultaneously 

 for long periods of growth. The failure of the structural hypothesis 

 suggests that the differences characterizing macronuclei controlling 

 different mating types are to be sought in physiological rather than 

 structural features. 



SUMMARY AND CONCLUSIONS 



A survey of the patterns of mating type determination and in- 

 heritance in Paramecium aiirelia is presented with an examination 

 of the significance of these patterns in an understanding of cellular 

 heredity. The major conclusions may be summarized as follows: 



1. Nuclei containing the same genetic materials may be differ- 

 entiated in regard to the phenotypes (mating types) that they control. 



2. The differentiated nuclei normally breed true in vegetative 

 growth, i.e., the nuclear characteristics are hereditary. 



3. An important factor in determining the manner in which the 

 nuclei develop (in the group B varieties) is the kind of cytoplasm 

 in w hich they develop. 



4. The significant cytoplasmic conditions are in their turn 

 determined by the kind of nucleus that previously occupied the cell. 



Thus, mating type perpetuation through both vegetative and 

 sexual reproduction is shown to be due to a series of nucleocytoplas- 



