NILAN AND KONZAK: MUTATION INDUCTION EFFICIENCY 451 



to (a) the chimeral nature of mutated sectors; (b) a rapid loss of 

 induced cytogenetic changes during subsequent mitoses and, in 

 sexual crops, during meiosis of the M, plant; and (3) overgrowth 

 of mutated cells by nonmutated cells (8, 33). 



Greater efficiency might be obtained by irradiating a plant when 

 half or all of its genes are located in an individual cell. This imme- 

 diately suggests the application of mutagens to pollen or zygotes. 

 However, pollen, zygotes, and embryos have been used for inducing 

 mutations in very few physical or chemical mutagen experiments. 

 Conger (8) has discussed in some detail a possible improved method 

 for inducing mutation by using pollen instead of seeds, and some 

 results have been obtained. 



Much more work needs to be done on this technique, since 

 its efficiency does not approach modern seed irradiation technics. 

 From exploratory studies, Konzak (21) has noted that pollens of 

 diploid maize and barley suffered such pronounced damage from 

 radiation that high seedling mutation rates were not obtained. In 

 one study (22), barley pollen was exposed to doses of X-rays, thermal 

 neutrons, and ultraviolet radiation. Records were kept of the num- 

 ber of seeds set, number of aborted and shriveled seeds produced, the 

 number and condition of the plants grown from the Mi seed, and the 

 number of second generation progenies showing mutations. M x plants 

 carrying chromosome translocations or inversions and the frequencies 

 of semi-sterility in M 2 and M 3 progenies were also recorded. Although 

 the numbers studied, totalling approximately 1,200 seeds from 

 irradiated pollen, were not large enough for firm conclusions, a dis- 

 appointingly low number of mutations was recovered for the effort 

 expended. Moreover, the ratio of mutations to chromosome trans- 

 locations induced by X-rays and neutrons in pollen was considerably 

 less than observed in seed. Several chromosome translocations were 

 induced by the ultraviolet radiation treatments, but the frequency 

 of mutations observed also was relatively low compared with that 

 obtained from X-ray treatment of seeds. 



A renewal of extensive work with pollen would seem to be war- 

 ranted, however, if for no other reason than to take advantage of the 

 fact that the mathematical analysis of induced mutation rates can 

 be more critically estimated. Also, with irradiated pollen, an Mi 

 plant carrying a mutation is almost always completely heterozygous 



