298 MUTATION AND PLANT BREEDING 



3,000 forms studied by Snowden were classified into 31 species on 

 morphological grounds; yet all of these may be freely interbred and, 

 on a genetic basis, can be considered as one species. Doctor Ouinby 

 has reviewed for this symposium the accomplishments in remodeling 

 this species from a too tall, too late, tropical forage grass into short, 

 early maturing, grain-producing varieties adapted throughout most 

 of the Great Plains of the United States. In this biological engineer- 

 ing program, the basic materials were mutations, particularly at three 

 loci controlling floral initiation, four loci controlling plant height, 

 and several loci which condition grain color, grain type, and disease 

 and insect reaction. Finally, a gene which reacts in the appropriate 

 cytoplasm to produce male sterility has provided the mechanics for 

 hybrid seed production, thus enabling the plant breeder to reap the 

 benefits of heterosis on a practical scale. In connection with this story 

 of the use of "macro" mutations in remodeling the sorghum crop, it is 

 important to be aware also of the innumerable minor genes of which 

 suitable combinations have provided for varietal superiority and 

 heterosis within the larger groups determined by the major genes. 



Variability of the magnitude found in sorghum is also encoun- 

 tered in other crops. Corn, wheat, and alfalfa are notable examples. 

 All students of plant breeding are familiar with the writings of 

 Vavilov based on most extensive plant explorations and collections 

 by him and other Russian scientists. A recent, posthumous publica- 

 tion (8) 2 dealing with world resources of cereals, grain, leguminous 

 crops, and flax appeared in 1957. Vavilov, as noted by Hayes, Immer, 

 and Smith (5), for example, defined eight principal regions of origin 

 of crop plants. It is in these regions that the greatest genetic diversity 

 in a crop is found. 



Many others, including Harlan (4), have also written about the 

 enormous natural variation found in most crop plants. It has been 

 estimated that domestication of most of the present-day crops began 

 50 to 70 centuries ago and the species from which domestication 

 started may well have arisen many centuries or millennia before that. 

 One could assume in the thousands of generations and vast popula- 

 tions of a crop that have existed that every possible genetic change 

 has occurred, not once but probably many times. To be sure, it is 

 probable that most of these spontaneous changes have been lost by 



2 See References, page 305. 



