MONOPLOIDS IN MAIZE 399 



Such synthetic varieties should be 10-20 times more effective as sources of 

 new homozygous diploid lines than the better heterozygous stocks already 

 tested. 



About fifty homozygous sweet corn diploid stocks and about fifty homo- 

 zygous dent stocks have been developed at Ames during the past two years of 

 exploratory work. These are being tested for combining ability in comparison 

 with related inbred lines. Though there is no reason a priori to expect that 

 these lines will be better than average combiners, there is reason to think they 

 should carry well balanced genetic systems, since passage through the si)oro- 

 phyte phase as a monoploid involves drastic selection against lethal and sub- 

 lethal genes. In appearance the homozygous lines seem better than average 

 unselected inbreds in general vegetative vigor. 



CONCLUSIONS 



• It has been demonstrated that homozygous diploid stocks of maize can be 

 produced from monoploid sporophytes. The method as now developed is 

 practical from the point of view of the plant breeder as an alternate to in- 

 breeding for the production of homozygous lines. As a method of gamete se- 

 lection it offers unique possibilities. Improvements now being attempted 

 should increase the efficiency of the procedure very considerably. It is not 

 known yet whether the homozygous lines produced will prove to be better or 

 poorer or equal to unselected advanced generation inbred lines on the average 

 in respect to combining ability. 



