MYERS: DISCUSSION OF SESSION III 305 



It is known that treatment with gamma- or X-rays or with high 

 temperature results in small increases in frequency of crossing-over at 

 least in some regions of the chromosome, notably in regions proximal 

 to the centromere (10). Most chemical mutagenes that have been 

 tested cause some crossing-over in the male Drosophila as also does 

 radiation, according to Doctor Auerbach in personal conversation. 

 The possibility, as yet virtually unexplored, exists that certain chemi- 

 cals or other treatments (not necessarily mutagenic) have the capacity 

 greatly to increase crossing-over. The natural variability that could 

 be "unlocked" by radiation, chemical, or other treatment might equal 

 or exceed any new variability that could be induced by mutagenic 

 agents. 



In conclusion the following points might be made: 



1 . There is a growing body of information pointing to the possibili- 

 ties of inducing favorable mutations in crop plants. 



2. Most plant breeders, working with crops in which an enormous 

 amount of genetic variability exists, should concentrate at present 

 on more effective use of that natural variability. 



3. Plant breeders should not, with available information and tech- 

 niques, use mutagenic agents in their breeding programs with 

 only the vague objective of increasing variability. 



4. Plant breeders who have specific reasons for increasing genetic 

 variability may find mutagenic agents are useful tools. An exam- 

 ple might be the case where resistance to a specific race of a disease 

 organism is not known and is needed. Another example would 

 be with crops which seem to have limited natural genetic variation 

 such as peanuts. (See Gregory, this symposium, for example.) 



5. Research on the questions listed above and others relating to the 

 efficacy of induced mutation versus use of naturally occurring 

 genetic variation should be carried out intensively by plant 

 breeders, geneticists, and cytogeneticists who have the facilities 

 and opportunities to do a competent job of such research. 



References 



1. Anderson, Edgar. 1949. Introgressive Hybridization. New York: John 



Wiley ir Sons, Inc. 



2. Caldecott, Richard S., Stevens, Harland, and Roberts, Bill J. 1959. 



Stem rust resistant variants in irradiated populations — mutations 

 or field hybrids. Agron. Jour., 51: 401-^03. 



