NONCHROMOSOMAL GENES 



259 



Male 



sterile 



fr fr (S) 



Fertile 

 Fr Fr (s) 



Fj plants 



all male 



fertile 



Fr fr (S) 



Fertile 

 Fr fr (S) 



Fertile 



Fr fr (S) 



50% 



c/ 



Fertile 

 fr fr (s) 



Male sterile 



fr fr (S) 



50% 



FIGURE 9.8. Inheritance of nonchromosoma! male sterility in maize. Female plants 

 carrying the male sterility factor (S) and recessive chromosomal genes fr (fertility 

 restorer) crossed with fertile males carrying the dominant gene Fr give rise to F j progeny 

 which are male fertile. When such Fj's are used as the female parent in crosses with 

 fertile males which are fr, the progeny in the next generation segregate one fertile: one 

 male sterile, indicating that the sterility factor was transmitted unexpressed through 

 the male fertile F j plants (after Jones, 1950, Genefics, 35:507). 



value in hybrid seed production, eliminating the most costly hand- 

 labor step, namely, the detasseling of the ear-bearing female parent. 



In summary then, nonchromosomal male sterility represents a hereditary 

 system which shows maternal inheritance and somatic segregation of 

 some unidentified determinant which is capable of mutation to more than 

 one alternative state. 



The nonchromosomal determination of streptomycin resistance in 

 Chlamydomonas is another system not associated with any known 

 structure. Although streptomvcin affects chloroplast inheritance, as 

 discussed above, no evidence has been obtained that the site of strepto- 

 mycin resistance is related to the chloroplast. No apparent morphologi- 

 cal differences between streptomycin-sensitive and streptomycin-resistant 

 strains have been detected in comparative electron microscope studies. 



