300 MUTATION AND PLANT BREEDING 



pointed out the parallelism between mutant types found in sorghum 

 and corn, i.e., endosperm color, waxy endosperm, sugary endosperm, 

 etc. 



Added to the genes of the cultivated species as possible sources 

 of variability are the vast array of genes in related species. Classical 

 examples of transfer from a related wild species of genes not known 

 to be available in the cultivated species are found in wheat, potatoes, 

 tobacco, and other crops. Modern advances in embryo culture, poly- 

 ploidy, use of bridging species, alien chromosome and gene substitu- 

 tion, and other techniques are expanding the range of species from 

 which desired genes can be obtained (6). 



The third class of variation, as outlined above, consists of the 

 spontaneous mutations that occur in the breeding cultures. These 

 probably occur more frequently than was once thought. An early 

 recognition of the frequency of spontaneous mutations was by East. 

 From studies of maternal diploids of Nicotiana rustica, obtained in 

 attempts at interspecific hybridization, he found that each progeny 

 row was astonishingly alike, more so than "any ordinary inbred 

 population that I have ever examined". When these lines were con- 

 tinued by self-fertilization, they were, within three or four genera- 

 tions, as variable as ordinary inbred populations (5). Despite their 

 frequency, however, spontaneous mutations do not occur often 

 enough in the breeding cultures to provide the amount and kinds of 

 variation required in plant breeding programs. 



The papers of Doctors Nybom and Gaul have dealt in consider- 

 able detail with the potentialities of using induced mutations. Doctor 

 Nybom has emphasized particularly the use of induced mutations in 

 improving varieties of fruits, bulb flowers, and other perennial orna- 

 mentals that are vegetatively propagated. In these crops, in the 

 absence of a sexual cycle, the only genetic variation that is found in 

 a variety must arise from mutation. When a sexual cycle is used to 

 provide for variation, the extreme heterozygosity of the parent variety 

 or varieties results in populations within which the favorable gene 

 recombinations can rarely be found in the size of sample with which 

 the breeder can work. Furthermore, the long life cycle of many of 

 these species limits the number of generations which can be grown. 

 It is not surprising, therefore, that in these crops so many of the new 

 varieties are mutant types from older varieties. Obviously, in such a 



