396 MUTATION AND PLANT BREEDING 



Duplications arising in the second-mentioned manner, viz., by 

 joining of sister chromatids that were broken at dissimilar points, 

 are exceedingly common, as based on cytological observations. How- 

 ever, they maye be extremely difficult to detect phenotypically and 

 would appear to have little practical value except in cases where 

 additive gene action prevailed in the production of a particular trait, 

 such as disease resistance, pigmentation, etc. 



Production of fertile tetraploids 



Since the discovery that colchicine was an effective polyploidiz- 

 ing agent, plant breeders have doubled the chromosome complement 

 of numerous species with which they work. In only a few instances 

 have the induced polyploids been useful economically. One immedi- 

 ate reason for this is that meiosis in an autopolyploid is disturbed by 

 multivalent formation which, along with some undetermined physio- 

 logical imbalance, apparently results in sterility and aneuploid types 

 arising in the progenies. If these problems could be overcome, ade- 

 quate tests of polyploids, originating from common genetic back- 

 grounds, could be undertaken and the genetic diversity within a 

 species that was available to the breeder could be increased 

 accordingly. 



One possible method of overcoming multivalent associations 

 would be to reorganize structurally the chromosomes of a species to 

 the point that very little or no homology remained between the 

 structurally modified lines and the progenitor. At this stage Fi's 

 between the modified lines and the progenitor could be doubled to 

 produce a fertile tetraploid that would be similar to artificially and 

 naturally occurring amphidiploids. 



At attempt to produce such structural modification experimen- 

 tally in barley is underway, using ionizing radiations to induce trans- 

 locations, inversions, duplications, and deficiencies. For the past six 

 generations, the procedure has been to use irradiation to induce at 

 least one interchange per generation in each of four lines originally 

 selected from four different varieties. Incidental to the addition of 

 the interchanges, but equally effective in inducing structural differ- 

 entiation, are the addition of the other anomalies mentioned. 



Hybrids between some of the material, in which structural differ- 

 entiation of the chromosomes is being attempted, and the parental 



