MCCLINTOCK 



now be considered. Until recently, the 

 investigation was not focused on this 

 problem. It is believed, however, that 

 this material is probably heterochro- 

 matin. This statement is based, in part, 

 on the evident homologies in the ex- 

 pression of variegation in maize and 

 Drosophila, but is more convincingly 

 suggested by the results of a prelimi- 

 nary experiment focused on the induc- 

 tion of mutations at the ai locus in 

 maize when the known Dt (Dotted) 

 locus is absent. The action of Dt in 

 chromosome 9 on the ai locus in chro- 

 mosome 3 is very much the same as the 

 action of Ac on the mutable loci it 

 controls.® The similarities are too great 

 to be dismissed as being due to causally 

 unrelated phenomena. The Dt locus 

 activates the ^i locus; mutations to 

 higher Ai alleles occur (A^, colored 

 aleurone; a\, colorless aleurone, reces- 

 sive to Ax). Without Dt in the nucleus, 

 ^1 has been shown to be completely 

 stable. Dt is located in the heterochro- 

 ?}iatic knob terminating the short arm 

 of chromosome 9. The suspicion is im- 

 mediately aroused: Is Dt action caused 

 by some modification of the hetero- 

 chromatic knob in chromosome 9? If 

 so, could this modification be produced 

 anew by subjecting a chromosome 9 

 to the breakage-fusion-bridge cycle? 

 Would the effective alterations of the 

 knob arise directly because of the in- 

 duced changes, or would they be pro- 

 duced secondarily by some other in- 

 duced structural alteration, either 

 within the short arm of chromosome 9 

 or elsewhere, that would upset, in 

 some way, the normal functioning of 

 the knob substance and thus bring 

 about an alteration in its action? This 

 last question is pertinent because some 

 of the structural alterations in Droso- 



«Rhoades, M. M., Genetics 23:377-397, 

 1938. Cold Spring Harbor Symposia Quant. 

 Biol. 9:138-155, 1941; ¥roc. Natl. Acad. Sci. 

 31:91-95, 1945. 



207 



phila appear to affect the functioning 

 of the centrically placed heterochro- 

 matin. For example, some of the Mi- 

 nutes bring about chromosome elimi- 

 nation and "somatic-crossinijover," 

 both of which may well be related to 

 adhesions of specific heterochromatin 

 that occur at certain times in develop- 

 ment. '^ To answer the above questions, 

 plants homozygous for a] and having 

 no Dt locus (designated dt by 

 Rhoades) were crossed by plants sim- 

 ilarly constituted with reference to a^ 

 and dt but carrying a rearrangement of 

 the short arm of chromosome 9 that 

 would introduce a chromosome 9 with 

 a newly broken end into many of the 

 primary endosperm nuclei in the given 

 cross.^ Breakage-fusion-bridge cycles 

 involving such a chromosome 9 with a 

 newlv^ broken end would occur during 

 the development of the kernels. Some 

 of these broken chromosomes 9 would 

 carry a knob, and this knob could then 

 be subjected to modifications as a 

 consequence of the breakage events. 

 If some of these modifications gave rise 

 to the same conditions that were pres- 

 ent at Dt, mutations from a^ to A^ 

 could appear in some of the kernels re- 

 sulting from the cross. A large number 

 of crosses of this type were made. The 

 results were positive with respect to 

 inducing mutations of a-i to A^. A small 

 number of the kernels resulting from 

 these crosses showed mutations of a^ 

 MO Ax. Often, only a single small Ax 

 spot was present on the kernel. Sev- 

 eral of the kernels, however, had a 

 pattern of mutations of ai to Ax that 

 was indistinguishable from that pro- 

 duced by Dt. These kernels could not 

 have arisen by contamination, for 

 stocks with the known Dt locus had 

 never been obtained and thus no plants 

 with this locus could have been pres- 



7 Stern, C, Genetics, 21:625-730, 1936. 



8 McClintock, B., Ibid. 26:234-282, 1941. 



