342 CELL HEREDITY 



be transposed from locus to locus, as judged by the change in the linkage 

 relations of its effects. The other element was called Ac (for activator); 

 its presence was required for expression of Ds activity, and the timing 

 of /)i-induced events could be altered by the number of Ac elements 

 present. Ac could also change its location and induce breaks and muta- 

 tion<- and resembled Ds in these respects. They differed, however, in 

 that Ds activity was strictly dependent on the presence of Ac, but Ac 

 could act autonomously. 



An astonishing diversity of phenomena appeared in this material which 

 can be grouped in terms of (1) chromosome breakage, (2) transposition 

 of controlling elements from one locus to another, (3) changes in gene 

 expression, mimicking mutation, and (4) operational systems of controll- 

 ing elements. The starting material included plants in which cycles of 

 chromosome breakage had been occurring, and it is likely that this con- 

 dition accounts for the origin of much of the variability. In later 

 studies with this material, and in related investigations in other labora- 

 tories, it has become evident that chromosome breakage is not an essen- 

 tial feature of the basic phenomenon. In mutable systems not involving 

 Ds, no breakage has been observed, and even with Ds, mutational and 

 transpositional events occur in the absence of detectable breaks. 



1. The location and the breakage effect of Ds were first recognized by 

 the loss of dominant marker genes lying distal to Ds on the Ds-containing 

 chromosomes, as shown in Figure 11.19. The timing of these breaks 

 was controlled by the number of Ac alleles present. Increasing the Ac 

 dosage resulted in later time of breakage, as shown by the decreased size 

 of the colored sectors found. In the absence of Ac, no D^-induced 

 breaks occurred at all. In addition to showing dramatically the existence 

 of some kind of metabolic control upon Ds exercised through Ac, this 

 system was used to assay the Ac content of unknown material. 



2. Ds was observed to undergo sudden shifts, called "changes of 

 state, in which the Ds-induced events no longer involved breakage 

 primarily, but more frequently (sometimes exclusively) changes in gene 

 action, resembling mutations. Becau.se of the high frequency of Ds- 

 induced events, loci being influenced by Ds appeared mutable. Often the 

 germinal mutations occurring under the influence of Ds were found 

 subsequently to be stable, even in the presence of Ac. In such 

 mutations, the Ds element was thought to have left the locus in ques- 

 tion and, in numerous instances, the Ds activity reappeared at another 

 site. Mc('lintock has interpreted the.sc finding in the most direct man- 

 ner, as the physical transposition of the Ds element from one site to 

 another. 



