340 



MUTATION AND PLANT BREEDING 



Table 3. — The Relative Prevalence of Chlorophyll and Other Phenotvpically 

 Distinct Mutations in M> of X-ray- and Neutron-treated 2x, 4x, and 6x Wheat. 



The relative prevalence in M 2 of 

 Species N 



Chlorophyll Other phenotvpically 



mutations, % distinct mutations, % 



2x monococcum wheat 282 66.3 33.7 



4x dicoccum wheat 389 43.7 56.3 



6x vulgare wheat 983 0.7 99.3 



Test of heterogeneity 



x 2 d.f. P 



2x - 4x wheat 32.7 1 <0.001 



4x-6x wheat 687.7 1 <0.001 



2x - 6x wheat 721 .2 1 <0.001 



wheat is calculated from a set of X-ray and neutron experiments 

 (72, 73). 



A sharp decrease in the relative rate of chlorophyll mutations 

 with increasing level of ploidy is associated with the fact that the 

 total mutation rate increased twofold in the X-ray experiments and 

 threefold in the neutron experiments from diploid to hexaploid 

 wheat. What is interesting from our present approach is that the data 

 demonstrate that the ability to produce chlorophyll, and presumably 

 also other characteristics essential for the ability to complete life 

 and to reproduce, are better buffered in the polyploid wheats. The 

 induced macromutations in these wheats center more in characteris- 

 tics which are not decisive for life or death but rather interfere with 

 ecological adaptation, etc. 



As evident from Table 4, the same trend can be observed even 

 within the category of mutations influencing the genetic control of 

 the chlorophyll apparatus. Mutations harmful to the carrier already 

 at the young seedling stage, like albina, xantha-types and viridoal- 

 bina, decrease in relative frequency with increasing level of ploidy in 

 contrast to types like tigrina and viridis where the production of 

 chlorophyll is less completely blocked (38). 



This observation that the majority of mutations in one species 

 can be centered on more harmful changes than in another is not only 

 observed in ploidy series. If we still keep to the cholorophyll muta- 



