258 CELL HEREDITY 



exhibiting maternal inheritance. The most widespread of these traits is 

 pollen sterility, which has been described in some 20 to 30 different 

 genera, beginning with ('orrens first example reported in 1909 in Cir- 

 sium. To illustrate the principles involved, as well as some of the 

 complications, let us consider the most extensively investigated systems, 

 those in the corn plant, Zea mays. 



Cvtoplasmicallv inherited male sterility in maize was first reported by 

 Rhoades in 1933, on the basis of maternal transmission. Plants of the 

 male-sterile line, when outcrossed with normal pollen, gave almost en- 

 tirely male-sterile plants. Repeated backcrosses with fertile pollen did 

 not restore fertility. Occasionally some fertile pollen was produced 

 in such crosses, and this provided an opportunity to test transmission of 

 male sterility through the pollen. As predicted from the absence of 

 segregation among the progeny of male-sterile female parents, the trait 

 was found to be transmitted only from the mother and not through the 

 exceptional fertile pollen grains formed on the male-sterile plants. 



More recently, a number of new male-sterile strains of independent 

 origin exhibiting nonchromosomal inheritance have been studied. Most 

 of these appeared as sterile tassels on single plants in large commercial 

 fields of inbred corn, strongly suggesting an origin by spontaneous muta- 

 tion. The effect of the mutation appears only after meiosis, interfering 

 with maturation of the pollen grain. 



Somatic segregation of the pollen-sterility determinant can be seen 

 simply by the appearance of fertile and sterile sectors on the tassel, 

 and represents evidence of the particulate nature of the determinant. 

 A most interesting example of sectored pollen sterility is that appearing 

 in plants carrying the iojap gene, which also induces plastid mutations. 

 In recessive iojap plants, both cytoplasmic pollen sterility and plastid 

 mutations occur in striped sectors, but the sectors do not coincide, indi- 

 cating that different particulate segregating elements, both susceptible 

 to mutation by products of the iojap gene, are involved in plastid 

 heredity and in pollen development. 



In recent studies of cytoplasmic male-sterility in maize, some chromo- 

 somal genes have been found which are fertility restorers ( fr). There 

 is specificity of response, because particular genes can restore fertility 

 only with particular male-sterile lines. In all cases, the gene-restorer 

 action is solely phenotypic; it does not interfere with the replication or 

 maintenance of the nonchromosomal determinant. As shown in Figure 

 9.8, a male-sterile strain crossed with a fertility restorer gave all male- 

 fertile plants, but when the fr gene was subsequently crossed out of the 

 stock, the male sterility reappeared. Both nonchromosomal male- 

 sterility and the chromosomal-restorer genes are of great commercial 



