CALDECOTT AND NORTH: RADIO-SENSITIVITY OF SEEDS 397 



genotype have just been achieved. They will be examined cytologi- 

 cally in the spring of 1961 and it is to be hoped that some of the F x 's 

 will show the kind of reduced pairing that is common in two of the 

 asynaptic mutants that have arisen from the material. 



Use of Interspecific and Intergeneric Chromosome Interchanges 

 With few exceptions, cereal breeders are limited in their 

 improvement programs to using the phenotypes they can obtain by 

 recombining the genes that exist in the species with which they are 

 working. The reason for this is simply that when wide crosses can be 

 effected, there is frequently little homology between chromosomes 

 from the two parents and correspondingly little recombination takes 

 place. The consequence is that there is often either divergence toward 

 parental types or the progenies from such crosses contain additions 

 of whole chromosomes to the basic complement. The lines in which 

 whole chromosomes have been added to a basic complement have 

 invariably proved too inferior for commercial usage. 



In most instances, what the breeder is seeking from intergeneric 

 and inter-specific crosses is one or a few characters, such as disease or 

 insect resistance, that he can incorporate in a commercial strain 

 without impairment to that strain. Recently ionizing radiations have 

 been effectively used to achieve this where conventional methods 

 failed. In this regard, Sears (23) has reported the successful transloca- 

 tion of a segment of chromosome carrying a gene for leaf rust resist- 

 ance from Aeiglops umbellulata to Triticum vulgare, and Elliott (14) 

 has had comparable success in transposing a piece of chromosome 

 carrying a gene for stem rust resistance from Agropyron elongatum 

 to T. vulvar e. 



Because of these successes, there would appear to be considerable 

 merit to the suererestion that ionizing radiations should be used exten- 

 sively in hybrids involving Triticum, Avena, and their related genera, 

 to obtain recombinant types that cannot be obtained by conventional 

 methods. Using the method outlined, it should be possible to derive 

 lines that contained all the 42 chromosomes of T. vulgare plus one 

 pair from a related genus. Theoretically, where there is no homology 

 between chromosomes involved in an intergeneric cross, it should be 

 possible to derive as many strains containing a different pair of 

 chromosomes from the related genus as in the "n" number of chro- 

 mosomes in that genus. By screening the derived strains, it could be 



