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CHAPTER 45 



gene. It becomes clear, therefore, that the 

 occurrence and control of cytoplasmic mixing 

 during conjugation provides a powerful tool 

 for detecting and proving the occurrence of 

 extranuclear genes in Paramecium. 



With this background regarding the differ- 

 ential transmissive behavior of nuclear and 

 extranuclear genes in Paramecium, consider 

 the results obtained from the study of the 

 genetic basis for two dijferent mating types, 

 calling one alpha and the other beta. It can 

 be proven that there is a gene basis for these 

 mating types in the macronucleus. However, 

 exconjugants from a mating of alpha by beta 

 form clones of different mating type only if 

 there is no cytoplasmic mixing, and clones of 

 the same mating type only if the mates mix 

 cytoplasms. There is clearly, then, also an 

 extranuclear gene basis for the mating-type 

 trait. The extranuclear genes involved in 

 this case are invisible and seem to be a 

 normal component of the cell. So, just as is 

 the case for chlorophyll production in corn, 

 the mating type phenotype in Paramecium is 

 affected both by nuclear and extranuclear 

 genes. 



Additional experiments clarify the role of 

 both types of gene. Recall that the macro- 

 nucleus degenerates during conjugation, and 

 that one daughter nucleus produced by a 

 mitosis of the fertilization nucleus forms the 

 new macronucleus, which thereafter divides 

 at every fission and goes to all daughter cells 

 of a clone. At first this new macronucleus 

 can be referred to as a young macronucleus, 

 and later as an adult macronucleus. 



Experiments demonstrate that if a young 

 macronucleus is located in cytoplasm con- 

 taining the alpha extranuclear gene, then as 

 an adult macronucleus it comes to determine 

 the alpha mating type; a genetically identical 

 young macronucleus placed in cytoplasm 

 containing the beta extranuclear gene becomes 

 an adult macronucleus determining beta type. 

 Clearly the young macronucleus carries the 

 potentiahty of producing either alpha or beta 



mating type in the form of one or more 

 macronuclear genes. Which alternative 

 comes to phenotypic expression is dependent, 

 however, upon the type of extranuclear gene 

 carried. Once the macronucleus is mature, 

 that is, is determined as an alpha or beta type, 

 thereafter it and its daughters will persist in 

 this condition. So this fixation of the 

 macronucleus is irreversible. 



On the other hand, it is found by suitable 

 experimentation that a mature, fixed macro- 

 nucleus produces, or determines the function- 

 ing of, extranuclear genes of the same mating 

 type. For example, an adult alpha macro- 

 nucleus causes the alpha cytoplasmic effect 

 to be produced, which, in turn, is ready to 

 determine the mating type of the young 

 macronucleus produced in the next sexual 

 generation. This mutual, circular, depend- 

 ency between extranuclear and nuclear genes 

 is an example of what may be referred to as 

 a feed-back system. The extranuclear gene 

 feeds instructions to the nuclear gene, which 

 in turn feeds back instructions to the extra- 

 nuclear gene, which feeds back instructions 

 to the nuclear gene, etc. 



Other traits in Paramecium are also known 

 to be controlled by nuclear and extranuclear 

 genes operating in feed-back systems. It 

 should be noted, however, that when such 

 systems operate, they may not always result 

 in an irreversible fixation in the type of 

 phenotypic alternative which either of the two 

 kinds of genes may express. Moreover, it is 

 not known whether the one kind of gene acts 

 directly on the other kind of gene or on its 

 products. Finally, it should be pointed out 

 that in none of the cases mentioned, ignoring 

 kappa, has it been proved that the extra- 

 nuclear and the nuclear genes affecting the 

 same trait differ except in location and in the 

 consequences expected from this difference. 

 Accordingly, we should not exclude the pos- 

 sibility that extranuclear genes may some- 

 times prove to be episomes or derivatives 

 of episomes. 



