390 



MUTATION AND PLANT BREEDING 



phenotypes following administration of a single dose of radiation to 

 an organism. This is because ionizing radiations induce chromosomal 

 aberrations as well as mutations. Such aberrations are often respon- 

 sible for cell death and it is apparently a simple matter of chance 

 whether or not either a mutational event, a chromosomal anomaly, 

 or both, are induced in a cell following the absorption of photons or 

 ionizing particles. Accordingly, as the dose to which the population 

 is subjected is increased, the number of cells in the population which 

 contain both aberrations and mutations also increases. Because only 

 one or a very few aberrations can result in cell death, it is apparent 

 that the probability is very low of inducing mutations at more than 

 a few loci before a lethal aberration is induced. 



From these considerations, it should be apparent that similar 

 complications prevail when attempts are made to obtain mutations 

 for quantitative traits. As many of the most important characters with 

 which the breeder works are under polygenic control, the desirability 

 of using ionizing radiations in other than a "one-shot" approach in 

 breeding programs should be obvious. 



Mutation From a Dose Administered to a Sequence of Generations 

 In diploids 



It seems obvious that a method that would increase the relative 

 frequency of mutations to lethal or semi-lethal aberrations in a pop- 

 ulation, before it is subjected to the expensive process of screening 

 for economic mutations, would be of distinct value to the geneticist. 

 At this time there appears to be only one simple way in which this 

 can be achieved. That is to irradiate successive seed generations of 

 the material under test, always selecting for re-irradiation seed from 

 those spikes or panicles which do not evidence structural chromo- 

 somal anomalies. This is a particularly simple procedure in diploid 

 species, such as barley, because during the development of the plant 

 from irradiated seed all cells which are cytologically deficient in their 

 chromosome complement are incapable of competing with more 

 normal cells. The end result is that, effectively, only reciprocal inter- 

 changes and inversions are detected in the analysis of sporocytes aris- 

 ing from irradiated diploid seed. Reciprocal interchanges invariably 

 induce 25 to 50 per cent sterility. Thus, when selecting seed for 

 re-irradiation one can select it only from fertile heads. Because muta- 



