208 



MCCLINTOCK 



ent in the field. Furthermore, the stock 

 having ai and dt, originally obtained 

 from Rhoades, had been grown for 

 several years. A number of sib crosses 

 were made each year and no muta- 

 tions of ai to Ai were observed in the 

 kernels on these ears. 



The facts (1) that Dt is located in 

 the heterochromatic knob of chromo- 

 some 9, (2) that the effect it produces 

 can be recreated by subjecting chro- 

 mosome 9 to the breakage-fusion- 

 bridge cycle, (3) that Ac appeared in 

 stocks that had undergone this cycle, 

 and (4) that Ac and Dt are alike in 

 their respective actions, all point to 

 heterochromatin as the material com- 

 posing Ac. The burst of new mutable 

 loci which appeared in the self-pol- 

 linated progeny of plants that had been 

 subjected to the chromosome type of 

 breakage-fusion-bridge cycle becomes 

 comprehensible if it is considered that 

 the alterations in the quantity or struc- 

 ture of heterochromatic elements dur- 

 ing this cycle were primarily respon- 

 sible for the initial appearance of these 

 mutable loci. This report has shown 

 that, once such loci arise, other mut- 

 able loci arise through transposition 

 of the inhibiting chromatin substances 

 to other loci which in turn become 

 mutable. 



Why should altered heterochroma- 

 tin be responsible for initiating such a 

 chain of events? To answer this ques- 

 tion, attention must be centered on the 

 action of heterochromatin in the nor- 

 mal nucleus. That it is associated with 

 the exchange of materials between nu- 

 cleus and cytoplasm has been indi- 

 cated.^ Changes in quantity, quality or 

 structural organization of heterochro- 

 matic elements may well alter the kind 

 and/or degree of particular exchanges 

 that occur, and in this way control the 

 chromosome organization and the kind 



and the relative effectiveness of genie 

 action. There can be little question that 

 transpositions of both Ds and Ac oc- 

 cur and that the time of their occur- 

 rence in the development of a tissue is 

 under precise control. This control is 

 determined by the number of Ac loci 

 present and their organization and pos- 

 sibly their position in the chromosome 

 complement. Is this transposition of 

 heterochromatin? Is it a reflection of a 

 process that normally occurs in nuclei? 

 Is it responsible for controlling the 

 rates and types of exchange that occur 

 between nucleus and cytoplasm? Is it 

 usually an orderly mechanism, which 

 is related to the control of the pro- 

 cesses of differentiation? If so, induced 

 disturbances in quantity and organiza- 

 tion of the heterochromatic elements 

 of the chromosome could give rise to 

 a series of alterations reflected both 

 in chromosome structure and behavior 

 and in genie reactions that could 

 markedly alter phenotypic expres- 

 sions.^^ it is well known that the vari- 

 ous knobs and centromeres may co- 

 alesce in the resting nuclei. This co- 

 alescence is also frequently observed 

 both in the somatic and the meiotic 

 prophases. Are the transpositions and 

 the changes in state of Ac products of 

 this coalescence? This is suspected be- 

 cause of the frequent transpositions of 

 Ac from one chromosome to another. 

 It may be considered that these 

 speculations with regard to hetero- 

 chromatin behavior and function have 

 been carried further than the evidence 

 warrants. This may be true; but it 

 cannot be denied that one basic kind 

 of phenomenon appears to underlie the 

 expression of variegation in maize. 



» Vanderlyn, L., Bot. Rev. 15:507-582, 1949. 



10 This report deals only with the origin 

 and behavior of mutable loci arising in these 

 cultures. A number of other heritable 

 changes are also arising. Many are associated 

 with marked alterations in morphological 

 characters. 



