STUDIES ON THE EXPERIMENTAL CONTROL OF THE MUTATION PROCESS 



increase of the relative frequency of these erectoid mutants with the increase 

 in ionization density of the radiation. In fact, with thermal neutrons and 

 dry seeds they distinctly predominate, although with X-rays they comprise 

 no more than twenty per cent of all viable mutations found in the second 

 generation. If this important finding holds true also in repeated experimen- 

 tation, we may state that the problem of directed mutation is solved and that 

 man is now, in principle, capable of producing new hereditary factors (genes) at 

 will, beneficial to himself and his interests. 



Here it ought to be added that Demerec^* and Kaplan ^'^ have presented 

 evidence from microbial genetics and Giles ^' from studies on nenrospora that 

 such a planned direction of the mutation process is no longer a Utopian 

 idea but, in fact, a reality. 



REFERENCES 



1 GusTAFSsoN, A. et al. Acta Agric. Scand. 1954, IV 359. 



2 GusTAFSsoN, A. Liitids Ujitv. Arsskr. jV. 7, Avd. 2, 1940, Bd. 36, Nr. 11, 1. 

 ^ GusTAFSsoN, A. Hereditas, 1947 33 1. 



* GusTAFSSON, A. and Mac key, J. Hereditas, 1948, 34 371. 

 s Mao key, J. Acta Agric. Scand. 1954, IV 419. 



6 D'Amato, F. Caryolgia, 1950, 3 211. 



7 Kaplan, R. W. Z- Pflanzenzucht, 1953, 32 121. 



» Anderson, E. H. Proc. Nat. Acad. Sci. 1 95 1 , 37 340. 

 ^ Ehrenberg, L. et al. Hereditas, 1953 39 493. 

 1" Ehrenberg, L. and Andersson, G. Nature, Lond. 1954, 173 1086. 



11 Ehrenberg, L. and Saeland, E. J. Nucl. Energy, 1954, 1 150. 



12 Wettstein, D. von. Acta Agric. Scand. 1954, FV 491. 



13 Ehrenberg, L. and Nybom, N. Acta Agric. Scand. 1954, IV 396. 

 " Demerec, M. Evolution, 1953, 7 43. 



15 Kaplan, R. W. Ann. Rev. Microbiol. 1952, 6 49. 



i« Muller, H. J. Radiation Biology, 1 954, 1 375-626. 



1^ Giles, N. H. Cold Spring Harbor Symp. Qiiant. Biol. 1952, 16 283. 



DISCUSSION 



F. H. Sobels : I would be very much interested to hear Gustafsson's interpretation 

 of this directed mutagenesis. How does he visualize the specific response of certain 

 loci to a particular type of treatment ? 



A. Gustafsson : With regard to what we call the group mutability or the changed 

 spectrum of mutation, described first in barley and Antirrhinum, we have considered 

 that certain types of genes giving a parallel pattern of mutation have a characteristic 

 in common, some distinct fine-structure (for instance specific dissociation constants, 

 Kaplan). The change of cell environment (e.g. by increasing hydration) should lead 

 to an activation of certain types of genes irrespective of their position in the genome. 



Some types of mutations, like the viridis chlorophyll mutations in barley, seem to 

 increase in frequency when chromosome disturbances accumulate. Then the viridis 

 type of mutation may, for its mode of origin, be less dependent on changes in indi- 

 vidual loci (gene mutations) than on simultaneous changes in several loci (deficiencies 

 or duplications) or on chromosomal rearrangements (weak position effects). 



Here, the parallel or directed formation of distinct types of mutation, under the 

 influence of specific chemical or physical mutagens, may be explained by postulating 

 a specific fine-structure common to certain types of genes. The fine-structure is 

 then rearranged or broken down in a parallel manner, for instance by the presence 

 of oxygen at the time of y- or X-irradiation, by indirect rather than by direct effects, 

 by increased hydration of the cell, or — as the case may be — by densely rather than 

 by sparsely ionizing agents. 



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