NILAN AND KONZAK: MUTATION INDUCTION EFFICIENCY 449 



from studies of the erectoid mutations of barley. Treatment with 

 densely ionizing neutrons will induce more erectoid mutants than 

 treatments with sparsely ionizing X and gamma rays. Furthermore, 

 neutron treatments will affect only some individual erectoid loci; 

 with X-ray, other loci will be affected (16). Whether these results are 

 due to differential sensitivities of specific loci, to changes in modify- 

 ing genes at other loci, to environmental or other modifying factors, 

 or to differential selection of certain mutations, the practical impor- 

 tance of these findings still remains — a specific changed type can be 

 more readily selected from one kind of treatment than from another. 



Some of our investigations on the influence of moisture and 

 heat shock on radiation damage also show a shift in the mutation 

 spectrum among M 2 chlorophyll-deficient seedlings. From dry seeds 

 but not from moist, the proportions of albina and viridis to total 

 mutants decreased with increasing doses. On the other hand, the 

 albo-viridis and xantha mutations from the dry seeds increased with 

 increasing radiation dose. The mutation spectrum in the heat-shock 

 experiments more closely resembled the spectrum obtained from 

 seeds exposed to lower doses (24). There is some indication that 

 these alterations in the proportion of mutation types are associated 

 with the selective influence of induced chromosome aberrations. 

 A previous report (10) has suggested that the proportion of viridis 

 mutants was smaller and that of the xantha type larger at higher 

 radiation doses. This alteration appeared to be associated with spike 

 sterility which, in turn, may be a result of chromosome aberra- 

 tions. 



After treatments of barley seeds with several chemical muta- 

 gens, considerable variation in the proportion of M 2 chlorophyll- 

 deficient seedling mutants has been observed (11, 15, 41). In our 

 studies, it has been found that the gamma-radiation spectrum of 

 induced-mutation types differs from that of diethyl sulfate (Table 3). 

 This difference might be related to the fact that very few chromo- 

 some aberrations are produced by the chemical. In the plant from 

 irradiated seed, aberrations would influence the selection and recov- 

 ery of induced mutations. On the other hand, it is possible that in the 

 DNA molecule some of these chemicals can bring about a specific 

 change which is expressed as a specific type of mutation. Stronger 

 evidence that there is a differential sensitivity of individual genes 



