362 MUTATION AND PLANT BREEDING 



As to your second problem, the possibility is certainly extremely remote 

 of being able to induce a very specific type of duplication directly by 

 prophase irradiation. Thus, the success will depend more or less entirely 

 on the efficiency of the available screening technique. For that reason, 

 the method must have a limited, practical value. However, this does not 

 necessarily mean that it must always be preferable to work via trans- 

 location lines. I would definitely prefer more data to be compiled before 

 any clear answer can be given to your question. 



Caldecott: The need for attention being directed to the production 

 of duplications is most important. Today we are relying almost entirely 

 on translocation between the same arms of homologous chromosomes 

 that were broken at dissimilar points. 



What we need is a good method of obtaining cells in which the chro- 

 mosomes are uniformly in the bipartite condition before irradiation. 

 This would enchance sister strand reunions and in instances where the 

 breaks were at dissimilar points should prove very effective. 



Dr. B. H. Beard is studying the problem using the 2-row vs. 6-row 

 gene in barley. His procedure is to irradiate seeds in which the genes 

 are in the heterozygous state and screen in the X 3 for "nonsegregating 

 permanent heterozygotes". 



Auerbach: The two-step process of the production of duplications, 

 which Doctor Caldecott outlined for X-rays, occurs normally at a high 

 frequency after treatment with chemical mutagens whose breakage 

 often is delayed until after the treated chromosome has divided into 

 chromatids. Ford found many small duplications after treatment of 

 Vicia with nitrogen mustard, and Slizynska found many after formalde- 

 hyde treatment on Drosophila. 



Lewis: In cotton many introductions cannot be evaluated in the lati- 

 tude of the United States because of a short-day photoperiod require- 

 ment. Do you think irradiation to break this reaction might be feasible? 



MacKey: The answer to this question greatly depends on the genetic 

 complexity of the photoperiodic response. A complex genetic background 

 greatly impairs the chance to break the short-day reaction by induced 

 mutation, while a simple genetic control would greatly favor a pro- 

 gram along this line. A shift in photoperiodic response can easily be 

 detected in a large material and a simple mutation is likely sometimes 

 to hit just right. The chance that many mutational events should happen 

 simultaneously according to a fixed pattern is, however, practically ////. 



