302 MUTATION AND PLANT BREEDING 



are selected from such a variety following mutagen treatment, there 

 is real danger that the derived lines, being more nearly "pure lines", 

 will have a narrower range of adaptation than the original variety. 



A second precaution required is the prevention of contamination 

 due to cross-pollination. Some natural cross-pollination occurs in 

 almost every self-pollinated crop and the amount of such contamina- 

 tion is sharply increased by the sterility in the first generation follow- 

 ing treatment with the mutagene. 



Caldecott, et al. (2) have suggested that many of the variant types 

 found in populations following radiation may actually have been of 

 this origin. Subsequent studies by Ausemus (unpublished) have, in 

 fact, shown that the rust-resistant segregates found in wheat popula- 

 tions following radiation (7) were probably the result of cross- 

 pollination with other rust-resistant varieties, even though the X x 

 generation had been spatially isolated in the breeding nursery. 



A third precaution has to do with the validity of plot trials at one 

 or two locations in 1 or 2 years. Such preliminary data provide 

 a very uncertain basis for concluding that true increases have been 

 obtained in yield or other characters having low heritability. 



An example of efficacy of "mutation breeding" that might be 

 viewed with some reservation because of the precautions stated is the 

 case reported by Doctor Gaul of Jutta winter barley obtained as an 

 induced mutant from Kleinwanzlebener. The new variety is said to 

 have improved winterhardiness, straw-stiffness, and yielding ability. 

 This seems to be a large order of favorable effects to have occurred in 

 a single line from treated material. 



Doctor Gaul has suggested that particular emphasis be given to 

 "micro" mutations involving the genes of polygenic systems. In this 

 connection it should be noted that so-called micro mutations might 

 result either from a change in a minor gene (polygene) or from a 

 small change in a gene having ordinarily major effect. As Doctor Gaul 

 has pointed out, most of the characters of major importance in plant 

 breeding are quantitative in nature and dependent upon polygenes. 

 Whether or not there is more likelihood of favorable mutations and 

 less of strongly deleterious mutations in these numbers of polygenic 

 systems and whether or not, because of the number of polygenes 

 involved, the chances of mutations significantly affecting quantitative 

 characters are greater, are questions which must be settled by further 



