464 MUTATION AND PLANT BREEDING 



their breeding structures. If the hazard of mutation has been met by 

 different breeding structures in different ways these adaptations 

 affect the expected efficacy of mutation breeding. 



Mutation and Breeding Structure 



Crossbreds 



By breeding structure I refer to the manner of reproduction 

 of an organism and the consequent organization of its genome. It 

 is the purpose of this section to point out that efficacy of mutation 

 breeding cannot be divorced from the role of mutation in the evolu- 

 tion of breeding structure. That breeding structure is a result of 

 natural selection was an early contribution of genetics and received 

 its first general summary in Darlington's Evolution of Genetic Sys- 

 tems (1 1). This was followed by more explicit association of mating 

 system with the polygenetic organization of the genome as brought 

 out especially in a series of papers by Mather and given a general 

 summary by him (32). The developments in population genetics 

 during the last two decades (13), together with the indicated genomic 

 control of mutation in plants (3, 4, 5, 16, 33, 34, 35, 42, 43), have 

 suggested that the kind and abundance of mutation itself have evolved 

 in conjunction with the evolution of breeding structure. 



The concept of the crossbred population as a system of het- 

 erozygous genotypes maintained in frequencies optimal to the 

 demands of environment has been generally established (8, 13, 23, 

 24, 28, 29, among others). Much work has been done that demon- 

 strates the advantages of deleterious recessives in higher than base 

 mutation-rate frequencies in crossbreds (28). The maintenance 

 of heterozygosity by selection in crossbreds was further supported 

 when Dempster (12) showed that mutation to deleterious recessive 

 was inadequate to account for the variance observed. Recent tests 

 of specific situations in known genetic material show that individual 

 heterozygotes may be at a disadvantage in uniform environment 

 (18, 26) with a consequent return to homozygosity (30). These facts 

 suggest that a price in excellency may be required in a chosen 

 environment in return for the opportunity to change in a chang- 

 ing environment. How generally these conclusions apply is still 

 subject to experimental confirmation since the numerical relation- 



