580 ERNST J. SCHREINER 



Synthetic Varieties of Maize 



As early as 1919, Hayes and Garber suggested "synthetic production 

 of a variety by self-fertilization, crossing, and subsequent selection" 

 as one of three possible means of utilizing increased vigor from crosses 

 (Hayes and Garber, 1919). They recognized that: 



"...before combining selfed lines for the purpose of 

 producing improved varieties, it is necessary to determine the 

 yielding ability of all Fj combinations. Selfed lines which 

 combine favorably with all others that are to be used should 

 then be used for the recombinations." 



Hayes (1926) also was one of the earliest breeders to report research with 

 synthetic corn varieties. He recombined selfed strains selected for 

 yield by pollinating several plants in each strain of a variety with a 

 mixture of pollen from other strains of the same variety. Yields of five 

 synthetics, each made up of lines derived from a single commercial variety, 

 ranged from -11.7 to +16.6 percent of the parental varieties from which 

 they were derived. Hayes concluded that it might be difficult to obtain 

 a variety that will be as vigorous as certain F^ crosses, but he did 

 predict that "...if resistance to some particular disease is a major 

 importance and resistant lines can be obtained, there is every reason to 

 expect that improved synthetics can be secured. 



Sprague and Jenkins (1943) presented data on the performance of 5 

 synthetic varieties and 16-line multiple crosses. The synthetic varieties 

 gave approximately the same yield as adapted open-pollinated varieties. 

 Multiple crosses compared favorably with available standard double crosses, 

 Hayes, Rinke, and Tsiang (1944) produced a synthetic variety from 8 inbred 

 lines that represented rather wide genetic diversity and gave relatively 

 satisfactory performance in all single-cross combinations with each other. 

 The synthetic was almost equal to Minhybrid 403, and both the synthetic 

 and Minhybrid were greatly superior to open-pollinated varieties. 



In the opinion of Lonnquist and McGill (1956) , the general conclu- 

 sion derived from earlier synthetic varieties, that they were little 

 better than commonly-grown open-pollinated varieties, was probably due 

 to the fact that the component lines used in such synthetics were not 

 chosen for their combining ability. They concluded from their own 

 research that synthetic varieties of corn produced by intercrossing 

 selected S]^ lines of high combining ability, as determined in top-cross 

 combinations, can be expected to maintain their improved productivity 

 in advanced generations through normal mass-selection procedures. Yields 

 of 4 second-cycle synthetics averaged 96 percent of hybrid U.S. 13 in 

 yield, as compared with 82 percent for the first -cycle populations, over 

 a 2-year period of testing. 



Wernham (1960) has discussed the uses and advantages of disease- 

 resistant synthetics of maize with particular emphasis on the method used 

 to acquire the adaptability of local varieties and maintain the resis- 

 tance of the original synthetic. According to Hallauer and Eberhart 

 (1966) , synthetic corn varieties have been used extensively in recent 

 years as populations for the extraction of selection lines. The main 

 objective has been to increase the gene frequency for specific attributes 

 because higher frequency of either better or more desirable genotypes 

 would be expected in these synthetic varieties. 



