INDUCED MUTATIONS IN PLANTS 1277 



chromosomal irregularities. Several seedling mutations were found also 

 in this strain. 



There is no evidence in plants of a direct connection between mutation 

 and chromosomal rearrangement. In Drosophila, on the contrary, almost 

 all translocations are lethal when homozygous, a fact accounted for by 

 the assumption of lethal mutation at the point of breakage of the chromo- 

 some. In some instances viable mutations are found at or near a point of 

 chromosome breakage. In maize, although both translocations and 

 viable mutations are induced by irradiation, the evidence thus far 

 available does not show any tendency of variations of the two types to 

 occur in the same individual more often than would be expected by 

 chance coincidence. Furthermore, the rather scanty evidence available 

 indicates that lethal mutations do not commonly occur at the points of 

 translocation in maize. This may be a consequence of gene duplication 

 connected with the possible polyploid origin in maize (46) and may not be 

 true of nonpolyploid species. 



C. The direct study of specific mutations to determine whether they 

 result from intragenic or extragenic changes involves many difficulties. 

 If the mutation is due to a change in the constitution of the individual 

 gene, it is hopeless with present technique to attempt to demonstrate a 

 morphological or chemical difference in the region of the chromosome 

 involved. The attempt to support this hypothesis, therefore, takes the 

 form of demonstration that the mutant gene behaves in a manner incon- 

 sistent with a mechanical explanation. 



The evidence from reverse mutation is most plausibly interpreted 

 by the assumption that the induced mutations are intragenic. In addi- 

 tion to the evidence of induced reverse mutation in Drosophila, submitted 

 by Patterson and Muller (35) and Timofeeflf-Ressovsky (52), there is 

 similar evidence of induced reverse mutation in plants in the studies of 

 Horlacher and Killough (23). 



On the other hand, if the apparent mutation is due to a nonlethal 

 deficiency or duplication, it may be possible in exceptionally favorable 

 instances to demonstrate cytologically the change in the chromosome. 

 Though this would be impossible in the condensed chromosome (unless 

 the chromosome region involved were astonishingly large), it might be 

 possible in a thin-strand stage of the meiotic prophase, when the modified 

 and the unmodified chromosomes are synapsed. The technique for the 

 study of the maize chromosomes in the prophase of meiosis, developed by 

 McClintock (29), permits the cytological identification of small structural 

 alterations with a precision far beyond that possible in studies of the 

 condensed chromosomes. 



Cytological evidence recently secured shows that viable deficiencies 

 are responsible for some of the apparent mutations induced by X-rays in 

 maize. Typical deficiencies, as previously stated, are eliminated as 

 lethals in the haploid gametophyte generation, while typical mutations 



