T ical genetic effects or not has been up for discussion several times.

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1 ON THE DFFERENTAL ACTON OF MUTAGENC AGENTS BALSGARD FRUT By NLS NYBOM BREEDNG NSTTUTE, FJ~LKESTAD, SWEDEN HE question whether different mutagenic agents bring about ident T ical genetic effects or not has been up for discussion several times. The common answer, and the one usually found in text books, is that the mutations originate zat random)) and that they are the same whether spontaneous or induced by various means. But there have also been indications of differences between induced and spontaneous mutations. Also, recent evidence points to rather great divergences in action both between various chemical mutagens and between chemicals and radiations. This has been recorded either at the cytological level (cf., e.g., GRAY, 1954) or as qualitative differences in mutation yield (e. g., K~LMARK, 1953 and MACKEY, 1954). The nonionizing radiations, i. e., in the first place ultraviolet light, have long been known to deviate from the ionizing ones in cytological effects. Moreover, some authors held the opinion that only ultraviolet light would be able to induce positive (progressive) genetical changes, whereas ))Xrays do not give rise to true gene mutations but only to destructive changes)) (LEFEVRE et al., 1953). These ideas are difficult to reconcile with the fact that undisputable progressive mutations have been induced by xrays in cultivated plants (cf., e. g., GUSTAFSSON and TEDN, 1954) and also with the author s own studies on the viability of translocation hoinozygotes in barley to be described below. GLES et al. (1955) have also found that ))xrays can induce dominant, positive mutations resulting in the restoration of specific biochemical syntheses )) in hreurosporn. The ionizing radiations may suitably be grouped and treated according to the linear ionization density they bring about along the track of the ionizing particle in the tissue; yrays with ca. 8 such ionizations per p particle track, xrays usually with some 100, neutrons of various energies with and, finally, aparticles and fission fragments from about 2000 to several ionizations per [i. Comparisons between the genetical action of these various radiations

2 ~ ~~ ~~~ NLS NYBOM usually have revealed quantitative differences in efficiency rather than qualitative differences in effects. But, there are recorded specific differences in cytological effects, which together with accumulated information on the general characteristics in action between, e. g., xrays and neutrons on living organisms give reasons for further investigations (cf., e. g., EHRENBERG and NYBOM, 1954). From a theoretical point of view, but also with regard to the production of mutations for plant breeding purposes it is of great importance to find out whether the distribution of mutation types varies with the kind of irradiation, and especially whether there is a higher proportion of nondestructive or positive mutations induced by any special treatment. A few such cases on differential mutation induction have been published (cf. K~~LMARK, 1. c., and MACKEY, 1. c.). Within our own research group (cf. also GUSTAPSSON and NYBOM, 1949) we have drawn attention to the higher frequency of socalled erectoides mutations in barley after neutrons (EHRENBERG and NYBOM, 1954). New data on this question have been collected confirming the earlier findings. Altogether, for the years , 82 morphological and physiological barley mutations, selected according to common principles and without regard to the kind of radiation used, may be grouped as follows: Type of radiation Sparsely ionizing: (yrays, xrays and fast protons)! ~ Ei'i'cloides mutations 10 ~ Olllrr ' \ iatrlr niutations Densely ionizing: (Fast and thermal neutrons, arays) x* for the difference in distribution is 10,~~ and the corresponding 1) close to 0,001. As already mentioned, the question whether genetical changes induced by means of different radiations are characterized by different degrees of viability is also of considerable interest. We are, therefore, accumulating data for studying this in detail, but it will take another few years before the results are ready. The aim of this paper is to describe some other experiments bearing upon this problem. The effect on viability of chromosome changes induced by xrays and neutrons. One of the hypotheses for interpreting the position

3 ON THE DFFERENTAL ACTON OF MUTAGENC AGENTS 213 ef fecf assumes that simultaneously with the chromosome breakage there is also damaged or changed a locus adjacent to the breakage surface, thus leading to a phenotypical effect of the changed chromosome. Considering that the breakage surfaces produced by different radiations do behave somewhat differently (e. g., with regard to restitution and reunion, cf. CATCHESDE, 1948), it was considered worth while to investigate the effect on viability of chromosome structural changes brought about by different radiations. n an earlier paper (NYBOM, 1954) the author studied the productivity of a number of barley lines homozygous for xray induced chromosome translocations. They had been selected in irradiated material only on sterility indications, excluding such types as were apparently phenotypically deviating (e. g., the chlorophyll change described in the same paper) t was found that all of the 19 translocation lines tested, with one exception, possessed a next to normal or just slightly changed vigour (measured as the grain yielding ability). The average yielding abilities of all lines was 97,s % of that of the original line. One might have expected that such a refurnishing of the chromosome structure would impair the wellbeing more seriously. t is true, however, that many such changes might have been quite lethal and thus eliminated at an early stage. Now, a new similar material has been collected but this time after neutron irradiation. The neutrons were produced in the 80cm cyclotron at the Nobel nstitute for Physics, Stockholm. An average proton energy of around 3MeV gave an ionization density of ca. 800 ion pairslp.l The preparation of the material and the planning of the field trial was analogous with that described in the previous paper (NYBOM, 1954). t should be mentioned, however, that the changes worked with have not been verified cytologically. t is not excluded that some of them might consist of some other structural change, fertile when homozygous but leading to semisterility after crossing with the mother line. But, in analogy with the xray material, these sterility causes are in all cases most probably reciprocal translocations. Eleven cases of neutron induced changes had been identified through crossing with the mother line. For comparison 11 xray induced translocations, chosen at random, were inserted into the field trial, alternat The same strain of tworow barley (Hordeum distichurn, var. Bonus) was used as before.

4 ~ ~~ 214 NLS NYBOiM TABLE 1. The relative grain yields of homozygous translocotioris in borley. Mother variety = 100. ~ R e p 1 i c a t i o 11 1,inr A\eragc ' V ~ Neutron induced: N N N X 95 N N N N N N 2G 119 N N 32 X X $)1,N 101,a 91,n 105,n 84,3 92,U 104,o J 92.u Ayerage of all lines 98,o 2,iis Xray induced: t li li i li t ' J 1Oi ,a X6.o u ,~ ,s ,u (il,a ,H h 118 X9 105.~ iiig with the neutron lines. The mother line was put as each third number, as a standard. All lines were precullivated together the year before in order to obtain uniform seed quality. As in the previous paper, the grain yield of each little plot (ca. 205 g) was expressed in % of the neighbouring mother plot. The results are presented in Table 1. The 11 neutron lines gave an average yield OP 98,o % of that of the mother. None of them, with the possible exception of N 14 with 84,3 %, deviated appreciably from the average. The yielding ability of the xray induced translocations even sur

5 ON THE DFFERENTAL ACTON OF MUTAGENC AGENTS 215 passed the mother slightly. The difference was not significant, however, nor was there any certain difference between the two materials. Of the xray translocations, the line H 297 deviated through its low yielding ability, just as it did in the e,arlier experiments. t is also phenotypically slightly different from the mother line (NYBOM, 1954). t might, thus, constitute the only apparent example of what was mentioned in the beginning of this paragraph, viz. a viabilitydepressing effect somehow following the production of a structural change. The total impression of the experiment is that it is an exception rather than the rule that structural changes are associated with such phenotypicnl effects in bnrley. Moreover, it seems that the breakage surfaces produced by the more densely ionizing neutrons do not devinte from those of the xrays in this respect. The viability of xruy and nputron induccd morphological nnd physiological mutations. As mentioned in the introduction, neutrons seem to induce a higher proportion of socalled erectoides mutations in barley than do xrays. These mutations are characterized by reduced spike internode lengths, but have usually also increased strawstiffness and sometimes even better yielding ability. They constitute, thus, an important mutation category from plant breeding point of view. Now, one may wonder whether the erectoides mutations induced by xrays are somehow different from those originating with higher probability after neutron irradiation. To test this another yield trial was arranged comprising all erectoides mutations isolated during the last years (when both xray and neutron treatments were performed parallelly). Altogether, there were 7 erectoides cases isolated afi er sparsely ionizing radiations (among which one after yirradiation) and 11 after densely ionizing radiations (one after airradiation from radon). This trial was arranged in a similar way as the translocation materials. The mother line was used as a standard on each third plot. The yields, expressed in % of that of the mother line, are presented in Table 2. The yields of different mutants varied from less than 10 % to more than 100 %. However, there seems to be no tendency that the neutron induced mutations are characterized by either especially low or especially high vigour (measured as yielding ability). Still, another similar material was included in these trials and recorded in Table 2. t consisted of a number of morphological mutations, chosen according to common principles and without regard to origin. Actually, there are certain heterogeneities between the materials coming from xray and neutron irradiation; e. g., all cases of longawned glumes were to be found after xrays whereas all intermediumlike mutations had

6 21 6 NLS NYBOM TABLE 2. Relative gruin yields of morphological and physiological mutations in barley. Mother varieiy= 100. ~ _ Neutron induced Yield Average of erectoitles 48,r & 12,z % Longawned glunies 4.i 6 Soon Early 4 Erectoides , , 59,, 60., 61., Average of erectoides Waxless 16 ntermediumlike Sixrowed 3 rregular spikes Average yield 3, Average yield originated after neutrons. Taken together, the results have some interest, though. There are no clear differences in vbability between those mutations induced by means of xrays and those coming after neutron irradiation. The total distribution of all analysed morphological and physiological mutants according to grain yield and origin is as follows: Grain yield in 9; of the niotlier line (Bonuu). n GO 80 OU Xrays, yrays Neutrons, urays

7 ON THE DFFERENTAL ACTON OF MUTAGENC AGENTS 217 Summing up, the experiments described show that mutants or individuals carrying chromosomal changes induced by densely ionizing radiations, e. g., neutrons, do not differ appreciably with regard to viability from corresponding changes induced by the more common, sparsely ionizing radiations, e. g., xrays. From practical plant breeding point of view there seems to be no cause to use or to avoid any special kind of irradiation, if not for other reasons, e. g., facility or biological efficiency. Literature cited CATCESDE, D. G Genetic effects of radiations. Adv. in Genetics 2: EHRENBERG, L. and NYBonl, N on density and biological effectiveness of radiations. Acta Agr. Scand. 4: 30G418. GLES, N. H., DE SERRES, F. J., and PARTRDGE, C. W. H Comparative studies of x rayinduced forward and reverse mutations. Ann. New York Acad. Sci. 59: GRAY, L. H Some characteristics of biological damage induced by ionizing radiations. Radiation Res. 7: GUSTAFSSON, A. and NYROM, N Colchicine, Xrays and the mutation process. Hereditas 35: GUSTAFSSON, A. and TEDN, Plantbreeding and mutations. Acta Agr. Scand. 4: KOLMARK, G Differential response to mutagens as studied by the Neurospora reverse mutation test. Hereditas 39: 55'055'6. LEFEVRE, G. JR., RATTY, F. J. JR., and HANKS, G. D Frequency of Notch mutations induced in normal, duplicated and inverted xchromosomes of Drosophila melanoguster. Genetics 38: MACKEY, J The biological action of mustards on dormant seeds of barley and wheat. Acta Agr. Scand. 4: 4l1J4Z!l. NYBOM, N Karyotype and viability in barley. Acta Agr. Scand. 4: