Section 5 — Mutagenesis 



equilibrated over drying agents) of mustard, 

 tobacco and Agrostis stolonifera were irradiated 

 with different doses of X-rays and the number of 

 long lived radicals produced was determined 

 employing ESR technique. A linear dose effect 

 relationship was obtained in all the three cases 

 at different moisture levels. The yield of radicals 

 in super dry seeds was much higher than in moist 

 seeds. At 5 per cent level, the yield of radicals 

 was least in mustard for a given dose as compared 

 to tobacco and Agrostis. Agrostis yielded a 

 greater number of radicals than tobacco. 

 However, at 2 per cent moisture content, this 

 relationship was altered in so far as mustard 

 yielded the maximum number of radicals per 

 gram for a given dose and no difference was 

 found between tobacco and Agrostis. 



The part played by oils and melanin in mustard 

 will be discussed in explaining the radical yield 

 at different moisture levels. The results on the 

 effect of NO, pretreatment with heat and 

 irradiation at low temperature will be discussed 

 in relation to the observed biological effects. 



5.110. Naturally Occurring and Artificially Induced 

 Mutations in Maize. Angelo Bianchi (Milano, 

 Italy). 



Analysis of the genetic variability detectable 

 in open-pollinated populations of maize reveals 

 that mutations occurring in nature are almost 

 exclusively point events. On the contrary the 

 greatest majority of the artificially induced 

 mutations turn out to be of chromosome type. 

 So far this picture has not been changed greatly 

 with physical and chemical treatments performed 

 under different experimental conditions, even in 

 experiment designs of high resolving power. In 

 maize such a requirement has been met, with 

 increasing reliability, using: (a) endosperm 

 markers; (b) heterozygotes for suitable plant 

 characters; (c) pollinator stocks possessing 

 translocations between A and B chromosomes; 

 and (d) the waxy locus affecting pollen grain 

 composition. In the first and in the last case, the 

 treatments are performed preferably on the 

 pollen mother cells and/or on the mature pollen, 

 in the other cases the target is usually the seed. 

 In all cases, several factors (as temperature, 

 moisture content, storage time, type of X-rays, 

 dose fractionation) have been varied with the 

 consequence of changing at times significantly 

 the total effect, but hardly the type of response. 

 In the chemical mutagenesis some compound 

 (ethylmethansulphonate) turned out to be 

 highly efficient when administered to seed, much 

 less when applied to pollen, while the contrary is 



true of diepoxibutane. For the latter, moreover, 

 the isomeric forms show significantly different 

 mutagenic activity. While chemical mutagenesis 

 is affecting the hereditary material of maize in 

 some fundamental aspects similar to those of 

 physical agents, some difference has also been 

 detected. 



X-ray treatment of very sensitive microsporo- 

 genesis stages, such as to induce more than 90 per 

 cent of pollen abortion, have caused an appreci- 

 able increase of wx ->- Wx events, on the basis 

 of pollen grains phenotypes. Since this appears 

 the first case artificially obtained back mutation 

 in maize, confirmatory data are necessary. 



5.111. Radiation Induced Modification of Paramuta- 

 tion Expression. Duane B. Linden (Mayaguez, 

 Puerto Rico). 



Paramutation as it is expressed at the R locus 

 in maize involves an interaction between the 

 gene undergoing a change (R r ) and a source 

 capable of inducing this alteration (R st or R mb ). 

 The components of this system were subjected 

 to radiation before being brought in contact to 

 determine their radiosensitivity and to obtain 

 additional information regarding the nature of 

 the paramutation phenomenon. 



When the site for paramutation change (R 1 ) 

 was irradiated before crossing to R st or R mb the 

 resulting progeny fall into three categories: one 

 group in which there is no apparent effect of 

 the radiation; a second group in which there is 

 apparent complete inactivation of the para- 

 mutation process; a third group with reduced 

 paramutation alteration. The frequency of the 

 second and third categories suggest that radiation 

 results in an inactivation type event rather than 

 inducing a gene mutation alteration at the R r 

 locus. 



Irradiation of the regulator stocks revealed 

 differences between the two sources. When the 

 R r source was irradiated prior to crossing with 

 R r and then testcrossed there was a very low 

 incidence of complete inactivation of paramuta- 

 tion but the partial alteration to a less complete 

 change occurred very frequently. The alteration 

 was always in one direction toward a less efficient 

 paramutation alteration. However when the R mb 

 source was tested in this manner the results differ- 

 ed. There was appearent complete inactivation in 

 some crosses. In other crosses there was an in- 

 crease in efficiency of inducing paramutation 

 alterations, as the degree of change was greater 

 than the controls. 



95 



