402 MUTATION AND PLANT BREEDING 



It may be a kind of genomic heterosis effect. In addition, the dosage 

 of every gene is very unlikely always optimal at the level reached by 

 the addition of whole genomes. For phylogenetical reasons, I am just 

 now studying the degree of diploidization in different 4x wheats, since 

 it is still not settled whether they should be traced to one or more 

 processes of tetraploidization. 



Caldecott: What you have stated is precisely the basis on which the 

 studies outlined here were conducted. 



Grun: There are two aspects on which I have questions to ask. The first 

 concerns the plan described to see whether you can establish perma- 

 nent heterosis in self-pollinated hexaploid cereals. As I understand it, 

 you plan to irradiate plants repeatedly over a number of generations, 

 then cross back onto the original parental lines and attempt to select 

 vigorous lines from the progeny. While you may well increase vigor by 

 this procedure, I wonder whether you would be safe in assuming that 

 such vigorous derivatives are a result of a heterotic interaction between 

 homozygous dominant and homozygous recessive alleles of the same 

 gene on homologous chromosomes. Might not such vigorous deriva- 

 tives result instead from new epistatic interactions resulting from the 

 induced mutations? 



The second question concerns your use of the word "diploidization". 

 I am assuming that you are implying by this a progressive change from 

 autoploidy toward alloploidy. I wonder whether mutations from dom- 

 inant to recessive alleles of polyploid plants, as you have described, 

 would necessarily lead the plant more towards alloploidy. Might not such 

 changes be better termed as simple gene mutations than adding the 

 implications involved in calling them diploidizations? 



Caldecott: In reference to your first question, the answer is yes, and 

 this fact is mentioned in the text. 



Regarding the second question, the term diploidization was chosen 

 simply because after recurrent irradiation one should be able to make 

 crosses that would give a wide range of genes that would segregate in a 

 3:1 ratio. (See text.) 



Dermen: If mutation was induced in the genotype AA AA AA, in a 

 hexaploid wheat, and it was changed successively to Aa Aa Aa, would 

 all three mutations be alike? 



