Extranuclear Genes and Nuclear Genes 



415 



hypothesis may be subject to test in another 

 way. What does one find in the cytoplasm 

 of cells located at the border between white 

 and green tissue? Here one finds cells which 

 contain both fully green and completely 

 white mature plastids, whereas cells within a 

 green sector contain all green mature plastids, 

 and cells in a white sector have only leuco- 

 plasts. (It should be noted that, when im- 

 mature, all plastids are smaller and colorless.) 

 Thus, even when the two kinds of mature 

 plastids are present in the same cell, they have 

 no influence upon each other, but develop 

 according to their innate capacities. If a 

 zygote (or other cell) contains both kinds of 

 plastids it will, by the accident of producing 

 daughter cells having only "white" or only 

 "green" plastids, give rise to stripes of white 

 and of green, respectively. We can conclude 

 from the results presented, amply supported 

 by others not mentioned, that plastids are 

 self-replicating, do not arise except from 

 plastids, and are capable of innate transmis- 

 sible changes. Accordingly, since plastids are 

 self-replicating, mutable, and capable of repli- 

 cating their mutant condition, they contain at 

 least one extranuclear cytoplasmic gene. We 

 have already mentioned that it was proven, 

 from other work, that the chlorophyll trait is 

 also influenced by nuclear cistrons. Thus, a 

 trait of a self-replicating cytoplasmic body is 

 subject to modification both by the extra- 

 nuclear gene(s) it contains, as well as by 

 nuclear genes. 



It has been found, after crossing two par- 

 ticular all-green corn plants, that some of the 

 progeny are green-and -white striped. The 

 striped plants prove to be homozygous for a 

 recessive nuclear gene, iojap (ij), for which 

 their parents were heterozygous. That the 

 striped phenotype is not due to some inter- 

 ference by ij ij in the biosynthetic pathway 

 leading to the production of chlorophyll 

 pigment (or, in other words, that it is not due 

 to some nuclear gene-caused error in metab- 

 olism) is demonstrated by the fact that the 



leucoplasts in albino cells remain colorless in 

 subsequent generations of corn plants, even 

 after the iojap recon is replaced by its normal 

 allele. The only simple explanation for this 

 effect is that, in the presence of ij ij, an 

 extranuclear gene, which is located in the 

 plastid and which is responsible for chloro- 

 phyll production, is caused to mutate to a 

 form no longer capable of performing this 

 function. This comprises proof that mutation 

 of an extranuclear gene can be induced by a 

 nuclear gene. A similar case, in which a 

 nuclear gene controls chlorophyll production 

 by mutating plastid genes, is known in the 

 catnip, Nepeta. 



We have already mentioned * that the 

 cytoplasmic particle kappa can be transmitted 

 from one generation of Paramecium to the 

 next. The distribution of kappa to the next 

 generation depends upon the way the new 

 generation initiates. Since a new generation 

 can be formed in several ways, we shall 

 understand kappa-transmission better after 

 a brief description of two such mechanisms. 



One method of producing the next genera- 

 tion of Paramecium is asexual. A typical 

 Paramecium contains a diploid micronucleus 

 and a highly polyploid (about lOOON) macro- 

 nucleus (or meganucleus) . The individual can 

 divide by fission to produce two daughter 

 paramecia comprising the next generation. 

 Both the micronucleus and macronucleus 

 replicate and separate, so that when fission is 

 completed, both daughter cells are chromo- 

 somally identical to each other and to the 

 mother cell from which they were derived. 

 Even though the cytoplasmic contents are not 

 equally apportioned to the daughters, a 

 killer mother will normally produce two 

 killer daughters, since both of these receive 

 some of the hundreds of kappa particles 

 distributed throughout the cytoplasm of the 

 parent cell. Should the daughters undergo 



* The previous and following discussion of Para- 

 mecium is based largely upon the work of T. M. Sonne- 

 born and coworkers. 



