NONCHROMOSOMAL GENES 249 



and, consequently, with chlorophyll. During vegetative growth and 

 multiplication, chloroplasts can be seen to increase in size and to divide. 

 In plants, chloroplast fission has been described in young leaf cells, 

 but chloroplasts are not transmitted as such through the zygote. In 

 young seedlings, plastids develop from structures about the size of 

 mitochondria, called preplastids. The origin of preplastids is unknown, 

 so that at the morphological level it is not clear whether these organ- 

 elles have a genetic continuity. If so, one would expect the preplastid 

 to be a replicating unit and, presumably, to contain a nucleic acid. 

 There is some cytochemical evidence suggesting the presence of RNA in 

 chloroplasts, based upon staining reactions, but not confirmed by any 

 other method. 



Despite the lack of critical evidence, there is a body of indirect evi- 

 dence implicating chloroplasts and, to a similar degree, mitochondria, 

 in their own heredity. This evidence comes from the genetic literature, 

 where some of the best examples of nonchromosomal heredity involve 

 these organelles. 



CHLOROPLAST HEREDITY 



Since the chloroplast is such a complex organelle containing many pig- 

 ments, enzymes, and perhaps enzyme-forming sites as well, it should be 

 anticipated that normal chloroplast development may be interfered with 

 in a large number of ways. For example, many chromosomal mutations 

 are known which alter chloroplasts. In maize alone over 50 different 

 loci of this sort have been mapped. In addition to Mendelian factors, 

 instances of nonchromosomal inheritance of chloroplast mutations have 

 been reported in a large number of plants. 



Although many kinds of chloroplast abnormalities have been observed, 

 those which are easiest to study genetically are the ones in which the 

 plants are sectored, containing normal green as well as mutated stripes. 

 The green areas provide enough products of photosynthesis to keep the 

 plants alive until maturity and seed production occur. Both chromo- 

 somal and nonchromosomal factors induce striping. 



The common type of nonchromosomally induced striping is called 

 status albomaculatus; the results of reciprocal crosses between green 

 and striped plants are as shown in Figure 9.4. The striped phenotype is 

 not transmitted by the male parent, and, when the female parent is 

 striped, three classes of progeny, green, white, and striped, are produced 

 with variable frequencies. The same pattern of inheritance recurs gen- 



