1292 BIOLOGICAL EFFECTS OF RADIATION 



chromosome together with the attachment of the fragments to one 

 another by their broken ends offers adequate explanation of the origin 

 of induced chromosomal reorganization in maize. In general, cytogenetic 

 investigations of effects of high-frequency radiation have tended to empha- 

 size the role of quantitative or extragenic alterations as a cause of what 

 is interpreted as gene mutation. Certain general considerations in this 

 connection have been discussed elsewhere (Goodspeed, 14). Serebrovsky 

 (39) indicates that the type of chromosomal alteration referred to above 

 may be the principal method of origin of gene mutation and Stadler (44) 

 holds that extragenic changes, and chiefly nonlethal deficiencies, are 

 responsible for the majority of induced mutations in plants. Stadler 

 further suggests that reverse mutations, the occurrence of which is taken 

 to be the strongest argument for intragenic origin of mutations, may 

 prove capable of a mechanical interpretation. 



The question as to the fate of the fragments initially produced by 

 chromosomal disruption after treatment (cf. Fig. 2) is important. 

 In general, chromosome fragments do not occur in progenies from 

 tissues in the cells of which fragmentation was observed, owing either 

 to the fact that the treatment is lethal for these cells or to the fact that 

 the fragments involved are eliminated because they do not possess 

 insertion regions or homologs. Their survival is favored if they contain 

 genie material essential for viability which is not present elsewhere in 

 the chromosome complement. In any case, it is clear that chromosome 

 fragments do persist and become a permanent feature of the chromosome 

 garnitures of plants in generations far removed from the one immediately 

 resulting from irradiation. For example, in Datura (Blakeslee, Bergner, 

 and Avery, 7) a true-breeding compensating type possesses 13 pairs of 

 chromosomes, one of which is a fragment pair. 



Evidence as to survival and significance of chromosome fragments 

 in plants is also furnished by certain types which have been derived by 

 X-raying of a single sex cell of N. Tabacum (Goodspeed and Avery, 20). 

 The Xi plant in which these lines originated possessed fragment chro- 

 mosomes. In later generations 14 distinct derivative types have been 

 obtained, 7 of which are true breeding. Of the latter types, some 

 possess a fragment pair and have maintained such a chromosome con- 

 stitution for five generations. In certain of the nonestablished types 

 a greater number of fragments occur but are not consistently maintained, 

 owing to the fact that they contain duplicated chromosomal material. 

 Similar behavior is characteristic of the fragment type "Loafoid" in 

 Datura (Blakeslee, Bergner, and Avery, 7). 



It is obvious that deficiency may be a product of induced chromosomal 

 fragmentation, occurring wherever a fragment lacks an insertion region 

 and does not produce lethality if lost. Such deficiencies are reflected 

 in distinct morphological types when they become established in the 



