200 



In the course of an experiment 

 designed to reveal the genie composi- 

 tion of the short arm of chromosome 

 9, a phenomenon of rare occurrence 

 (or recognition) in maize began to 

 appear with remarkably high fre- 

 quencies in the cultures. The terms 

 mutable genes, unstable genes, variega- 

 tion, mosaicism, mutable loci or "posi- 

 tion-effect" have been applied to this 

 phenomenon. Its occurrence in a wide 

 variety of organisms has been recog- 

 nized. The most extensive investiga- 

 tions of this phenomenon have been 

 undertaken in Drosophila 7f7ela?70- 

 gaster} In this organism, the condi- 

 tions associated with the origin of 

 genie instability have been well de- 

 fined. The part played by the hetero- 

 chromatic materials of the chromo- 

 somes, in inducing and controlling the 

 type of variegation and its time and 

 frequency of occurrence, has been es- 

 tablished. It has not been generally 

 recognized that the instability of genie 

 expression in other organisms may be 

 essentially the same as that occurring 

 in Drosophila. 



As stated above, a large number of 

 mutable loci have recently arisen in the 

 maize cultures and are continuing to 

 arise anew. The loci affect variegation 

 for many different kinds of plant char- 

 acters, each locus being concerned 

 with a particular character or occa- 

 sionally several characters. Some of 

 these loci are c, yg-z, 'wx, ao, y, pyd, 

 which are well-investigated units in 

 maize.2 Others involve previously un- 

 known genetic units. The same types 

 of genie instability appearing in the 

 maize cultures have been described in 



1 Lewis, E. B. Advances in Genetics, 3:73- 

 115, 1950. 



2 The symbols refer to genes affecting the 

 parts of the plant as follows: c, aleurone pig- 

 ment; jgo, chlorophyll; wx, composition of 

 starch in pollen and endosperm; a<>, aleurone 

 pigment; y, starch composition of endo- 

 spemi; pyd, chlorophyll. 



MCCLINTOCK 



many other organisms. The behavior 

 of these new mutable loci in maize 

 cannot be considered peculiar to this 

 organism. The author believes that the 

 mechanism underlying the phenome- 

 non of variegation is basically the same 

 in all organisms. The reasons for this 

 conclusion will be made apparent in 

 the discussion. 



The initial appearance of the burst of 

 newly arising mutable loci occurred in 

 the progeny coming from the self-pol- 

 lination of about 450 plants which had 

 each undergone a series of events in 

 their early development where the 

 short arm of chromosome 9 was sub- 

 jected to drastic structural modifica- 

 tions. These events took place during 

 the "chromosome type" of breakage- 

 fusion-bridge cycle. ^ The modifica- 

 tions that this mechanism produces 

 are: one or more duplications of seg- 

 ments of the short arm, deficiencies of 

 one or more segments of various 

 lengths, structural modifications of the 

 heterochromatic knob substance, du- 

 plications of the knobs with or with- 

 out structural modifications, and vari- 

 ous combinations of these several types 

 of modifications. The chromosome 

 complement of over 150 of these plants 

 were examined at pachytene to deter- 

 mine the nature of the structural 

 modifications that had occurred. In 

 addition to the modifications of the 

 short arm of chromosome 9 listed 

 above, some of the plants had other 

 modifications, many of which are par- 

 ticularly significant because they in- 

 volve the substances in the chromo- 

 some that are believed to be respon- 

 sible for the origin and behavior of 

 mutable loci— the heterochromatic 

 knobs and centromeres. Altogether, 48 

 such structural modifications have 

 been analyzed, most particularly in the 

 above-mentioned plants but also in 



3 McClintock, B., Proc. Natl. Acad. Sci., 

 28:458-463, 1942. 



