4 If) MUTATION AM) PLANT BREEDING 



relations of chromosomal strands in en- vs. hetero-chromatic regions, 

 or to different placement with reference to the nuclear membrane. 

 Significant information may be forthcoming from experiments under- 

 way utilizing radioactive-labeled chemical mutagens. Such experi- 

 ments should provide answers to questions regarding incorporation 

 and possible localization of the mutagen in the chromosome and 

 whether the mutagenic agent acts directly or indirectly with the 

 chromosomal constituents. Callaghan and Grun (10). using C 14 

 labeled maleic hydrazide, observed that this compound is incorpo- 

 rated into chromatin material, is localized in the nucleoli, and is 

 uniformly distributed in eu- and hetero-chromatin. 



An example of induced mutation leading to directional genetic 

 change in populations is found in investigations on radiation-induced 

 polygenic variation. Experiments with Drosophila will serve as an 

 illustration. Utilizing lines that no longer responded to selection for 

 number of sternopleural hairs, Scossiroli (59, 60) and Clayton and 

 Robertson (1 1) were able to induce, with radiation, new genetic vari- 

 ability. Selection was practiced for both higher and lower number of 

 hairs, but response was realized only in the high lines. In our labora- 

 tory, Daly (12) induced genetic variability for flowering time in 

 Arabidopsis thaliana with y-radiation and selected for early and late 

 flowering. In the R 3 generation, 80 lines were developed from selected 

 R 2 plants. Up to 50 per cent of the lines selected for earliness were 

 found to be significantly earlier than nonirradiated selected control 

 populations. None of the late-flowering lines differed significantly 

 from the corresponding controls. We have had similar experiences 

 with a radiation-induced quantitative character in Nicotiana tabac- 

 um. Although it cannot be stated unequivocally that the mutations 

 per se are biased in one direction, it is nevertheless clear that the 

 ultimate result of induced variability in quantitative characters may 

 frequently, and perhaps characteristically, be a directed change in the 

 population. Further information on this phenomenon is urgently 

 needed for clarification of its genetic basis as well as its evolutionary 

 and practical significance. It is not evident that the "easy direction" 

 is necessarily a detrimental one, as e.g., earliness in Arabidopsis; but 

 rather it appears to be characteristic of the genotype to respond to a 

 mutagen by a greater yield of genetic variability in one direction from 

 the parental mean compared to another. 



