Extrcmuclear Genes 



373 



figure 29-3. Normal (above) 

 and kappa-containing (right) Par- 

 amecium. (Courtesy of T. M. 

 Sonneborn.) 



be concluded that plastids do not arise except 

 from plastids. Consequently, since they are 

 self-replicating, mutable, and capable of rep- 

 licating their mutant condition, plastids ap- 

 parently contain at least one cytoplasmic 

 gene. Although DNA is present in chloro- 

 plasts, 5 this substance has not yet been 

 proved to be the basis for the genetic alterna- 

 tives under discussion. As already men- 

 tioned, the chlorophyll trait is also influenced 

 by nuclear genes. Thus, this trait is con- 

 trolled by both the plastid and the nuclear 

 genotypes. 



In another study, a cross of two all-green 

 corn plants gives some progeny which are 

 green-and-white striped. The striped plants 



"See R. Sager and M. R. Ishida (1963). and 

 M. Edelman, C. A. Cowan. H. T. Epstein, and 

 J. A. Schiff (1964). 



prove to be homozygous for a recessive nu- 

 clear gene, iojap (//), for which their parents 

 were heterozygous. Since colorless plastids 

 in ova of striped plants remain colorless in 

 subsequent generations, even in homozygotes 

 for the normal allele, the colorless plastid is 

 not due to interference by if if in the biosyn- 

 thetic pathway leading to the production of 

 chlorophyll pigment. The only simp'e ex- 

 planation for this effect is that, in the pres- 

 ence of if if, an cxtranuclear gene located in 

 the plastid and essential for chlorophyll pro- 

 duction is somehow induced to mutate to a 

 form no longer able to perform this function. 

 The results convincingly demonstrate that 

 mutation of an cxtranuclear gene ecu be in- 

 duced 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. 



