NONCHROMOSOMAL GENES 251 



ear consists of rows of fertilized eggs, arising by mitotic divisions and 

 spatially oriented, so that cell lineage relations are more or less clearly 

 maintained. Each fertilized egg develops into a separate kernel. If an 

 ear developing in a striped region is pollinated by a normal green male 

 and the resulting kernels are planted in the form of a map, the germinat- 

 ing seedlings are again of three types, but spatially oriented by sectors, 

 as shown in Figure 9.5. Thus hereditary determinants of green and of 

 bleached chloroplasts are segregated among the egg cells, and this segre- 

 gation is reflected in the distribution of green, white, and striped progeny 

 seedlings in sectors. 



In this system, both maternal inheritance and somatic segregation indi- 

 cate the presence of a nonchromosomal determinant which influences 

 chloroplast development. However, it does not indicate whether or not 

 the determinant is actually located in the chloroplast; it may be any- 

 where in the cell except on a chromosome. 



If the appearance of green and white stripes were the result of sorting 

 out of normal and mutant plastids, one might expect to find cells with 

 mixtures of plastid types at the borderline between sectors. Consider- 

 able effort by a number of investigators to study plastid segregation 

 has not provided definitive results. In general, cells with mixed plastid 

 types are not found, but rather there is an increasing abnormality of 

 appearance of all plastids in cells close to the white sectors. These ob- 

 servations are more in line with the hypothesis of a cytoplasmic in- 

 hibitor of plastid formation than of plastid segregation. 



A detailed study of the iojap mutants of maize by Rhoades was de- 

 signed to investigate this question further, lojap is a chromosomal 

 gene, which gives rise to striped plants when homozygous recessive. In 

 crosses of the resulting striped plants reciprocally with green ones, the 

 progeny seedlings show maternal inheritance of striping, as in the ex- 

 amples illustrated in Figure 9.4. In many generations of backcrossing 

 striped females with green males, there is no observed effect of the male 

 parent upon the progeny, despite the absence of the iojap gene after out- 

 crossing. Planting ears in the form of a map, from sectored plants 

 pollinated by normal green males, gives evidence of somatic segregation 

 as in Figure 9.5. Thus, in this system, the gene iojap produces a muta- 

 tion, that is, an irreversible change, in a nonchromosomal determinant. 



Indirect evidence that this determinant is not in the chloroplast itself 

 comes from the following observations: 



1. Cytologically one does not see cells with mixed green and mutant 

 plastids at the boundaries of striped areas. 



2. The extent of striping can be varied greatly by changing the grow- 



