198 MUTATION AND PLANT BREEDING 



to be a genetic male-sterile that gave male-sterile offspring when 

 crossed to some varieties and fertile offspring when crossed to others. 

 The proposed procedure for the production of hybrid seed by a 

 3-way cross was published by Stephens, et al. in 1952 (40). The Day 

 male-sterile was distributed to plant breeders by the Texas Agri- 

 cultural Experiment Station in 1951. Because cytoplasmic male- 

 sterility was found, the Texas Station did not put any hybrids into 

 production using the Day male-sterile. However, one seed company 

 did produce hybrids for a year or two using the Day male-sterile. 



Beginning in 1949, work was done that established the exist- 

 ence of cytoplasmic male-sterility. In 1950, F 2 populations of recip- 

 rocal crosses involving Sooner Milo and Texas Blackhull Kafir 

 were grown and partial male-sterility was found in the popula- 

 tions whose female parent was milo. Holland (10) reported the 

 early data from this study in a thesis. It soon became apparent that 

 male-sterility in this case was due to the interaction between milo 

 cytoplasm and kafir nuclear factors as reported by Stephens and 

 Holland in 1954 (39). The degree of male-sterility was increased 

 as the proportion of kafir chromosomes in milo cytoplasm was 

 increased. The number of genes involved was not determined by 

 Stephens because of the pressure of work to get sorghum hybrids into 

 production and because unfavorable weather in 1952 and 1953 at 

 Chillicothe confused the expression of genetic sterility. Maunder 

 and Pickett (21) reported cytoplasmic male-sterility to be depend- 

 ent on a single pair of recessive genes, ?ns c ms c , interacting with 

 sterile cytoplasm. Plants in their fertile classes in segregating pop- 

 ulations varied in seed set from 5 per cent upward. Obviously, 

 important modifying genes must exist. Finding fertility restorers 

 was no problem in sorghum. All milos and milo derivatives with 

 sterile cytoplasm carry the dominant allele, Ms c ; otherwise they 

 would be male-sterile. The dominant restorer also exists in many 

 varieties with normal cytoplasm. 



The genetic male-steriles that arose as mutations are not impor- 

 tant today in the production of hybrid sorghum seed, but work 

 with them contributed to the discovery of cytoplasmic male-ste- 

 rility. J. C. Stephens had been working with mutant male-steriles 

 in sorghum for more than 20 years when he and R. F. Holland 

 announced, in 1954, the attainment of cytoplasmic male-sterility. 



