178 The Nature of Biological Diversity 



a fragment of the prezygotic macronucleus (homozygous for marker 

 genes) ; and, if the latter, whether from a prezygotic macronucleus of 

 a singlet or doublet conjugant (homozygous for different alleles). 

 This is possible in spite of the fact that all clones have heterozygous 

 micronuclei, because the micronuclear genotype has no detectable 

 effect on the phenotype; the latter corresponds regularly to the geno- 

 type of the macronucleus (Sonneborn, 1946) . 



The experiment was designed to reveal whether the singlet-doublet 

 alternative would prove to be correlated with the source of the macro- 

 nucleus. The answer should be most clearly provided by two of the 

 six obtainable types of clones, a to /, shown in Fig. 5. These two, 

 marked with asterisks, are types b and e. Clones of type h are de- 

 scended from singlet conjugants but possess macronuclei regenerated 

 from a fragment of the prezygotic macronucleus of doublet con- 

 jugants. Clones of type e, reciprocally, are descended from doublet 

 conjugants but possess macronuclei regenerated from a fragment of 

 the prezygotic macronucleus of a singlet conjugant. Both types b and 

 e should change character, from singlet to doublet and from doublet 

 to singlet, respectively, if the basis of the difference between singlets 

 and doublets lies in a fixed macronuclear differentiation. That did not 

 happen. Type b clones were composed of singlets in spite of possessing 

 macronuclei derived wholly from doublets: type e clones were com- 

 posed of doublets in spite of possessing macronuclei derived wholly 

 from singlets. Macronuclear differentiation thus seems to be excluded 

 as the basis of the singlet-doublet difference, unless macronuclear 

 fragments can be redifferentiated, and that has not been found to 

 occur in any previous work. 



The same conclusion is indicated by the character of clones of types 

 c and / in Fig. 5. These are descended from singlet and doublet con- 

 jugants, respectively, that had exchanged cytoplasm (as indicated by 

 transfer of macronuclear fragments, which can pass across only 

 through a broad cytoplasmic bridge), and that had developed new 

 macronuclei from products of syncarya. During this development, 

 macronuclei are known to be subject to differentiation by the action 

 of nuclear products in the cytoplasm (Sonneborn, 1954a). But in the 

 case of clones of types c and /, the cytoplasms present in the cells of 

 both types were thorough mixtures of the cytoplasms of doublets and 

 singlets. Such mixed cytoplasms, in all previously studied cases, often 

 differentiate the new macronuclei in accord with the newly introduced 

 cytoplasm. But this failed to happen in the present experiment. All 

 clones of types c and / remained true to their parental conjugant's 

 character; i.e., those from singlet conjugants (c) produced singlet 



