mac key: induced mutation in crop improvement 339 



Table 2. — Artificially Induced Decrease in Radio-resistance. 



Control 10,000 r 1 5.000 r 20,000 r 



No. of seeds sown 1 50 300 300 300 



No. of seedlings, % 



Skandia III winter wheat 86.0 86.3 84.0 80.7 



X-ray-induced deficiency-speltoid of 



Skandia III 92.0 79.7 78.3 52.7 



Difference against difference in control —12.6 —11.7 —34.0 



mdiff. 



±4.72 xx ±4.80 x ±5.14 xxx 



Seed moisture: 14.7 per cent Seed size: 2.50 - 2.75 mm 



only radio-resistance but also width of mutation spectrum may be 

 inHuenced. Unfortunately, no definite information is yet available on 

 this aspect on genotypic response to mutability. At present, there are 

 thus only some rather vague statements on the advantage of using 

 heterozygous parental material in mutation work (36, 40, 69), and the 

 problem can therefore only be pointed out here. Heterozygosity 

 implies, however, more unlike genes that can be changed, rearranged 

 in blocks, or split up. The fact that heterozygosity in itself implies an 

 enhanced pressure and chance of mutation (69) may also increase the 

 efficiency of the applied mutagen. Such an idea is in a way supported 

 by the fact that rare and new mutations become gradually more 

 frequent in experiments including radiation of successive generations 

 (48, 103). 



Phenotypic buffering, i.e., the ability to resist and absorb muta- 

 tion without severe deviations from type, is another genotypic factor 

 controlling rate and spectrum of progressive mutation. Duplication, 

 epistasis, polymery, and other cumulative or complementary gene 

 effects are added to allelism to form a very complex system of inter- 

 ference and interaction within the germ plasm. The extent, type, and 

 direction of this interdependence of genes, as well as their position 

 and stability, will vary with genera, species, and biotype and also 

 with character to form different patterns of buffering. 



The example I present is somewhat special, since it deals with 

 macromutations in a ploidy series, but I am sure of its wider applica- 

 tion. In Table 3, the relative prevalence of chlorophyll mutations 

 per plant progeny in relation to all other phenotypically distinct 

 mutations in M 2 of 2x monococcum , 4x dicoccum, and 6x vulgare 



