STEPHENS: RESUME OF THE SYMPOSIUM 507 



led to the production of improved, well-adapted varieties. If the 

 genetic variation in an irradiated population is essentially of the 

 same kind as that existing in natural populations, it is a little 

 difficult to understand why the mutation breeder should pay so 

 much attention to the production of "the occasional, spectacular, 

 useful variant" (Laughnan) and so little to the polygenic back- 

 ground. It is possible that some of the potentially most useful var- 

 iation is discarded when selection is made on the basis of mutant 

 phenotype. 



The second limitation is common both to mutation breeding 

 and conventional breeding, and it involves certain problems which 

 are not currently by-passed by either system. Whether the initial 

 pool of genetic variation is enriched by mutation induction or by 

 hybridization, the fact remains that in subsequent generations most 

 of it is expressed as recombinational variance. Every new mutant 

 introduced into a population can, theoretically, generate three new 

 genotypes: with n mutants there are 3 n genotypes. A single mutation- 

 al event therefore triples the potential number of different genotypes 

 present in the initial population. This means that a relatively low mu- 

 tation rate, or a relatively small amount of hybridization, can gener- 

 ate a large amount of recombinational variance. In many conven- 

 tionl programs the time-consuming element is not the inclusion in 

 the breeding population of a new character per se, but its stabili- 

 zation and effective selection from a bewildering variety of pos- 

 sible combinations. While it is possible that this time-consuming 

 task may be reduced in certain mutation breeding systems, I do not 

 think that a reduction need follow as a natural consequence and 

 advantage of mutation breeding in general. Gaul has suggested that 

 if a character is available either in a "primitive or non-adapted 

 form" or in a "mutant collection of an adapted variety", it will be 

 much easier to use the latter in a breeding program. Caldecott 

 has pointed out the potential advantages of a recurrent irradiation 

 program, aimed at accumulating favorable mutants in a common 

 breeding pool. These advantages over conventional methods are 

 readily appreciated if we can assume that an adapted variety will 

 remain adapted in respect of its residual genotypic background 

 after mutagenic action. Otherwise the recovery of adapted forms 

 from a segregating population remains a problem — no less for the 



