470 



< HAITI R 37 



2. How the two kinds of controlling loci 

 arise, and whether their origin is inter- 

 dependent 



3. Whether the two elements of a con- 

 trolling system usually are adjacent or 

 separate 



4. The nucleotidic basis for and the bio- 

 chemical mode of action of, op-R gene- 

 controlling systems. 



Paramutation 



The R locus in chromosome 10 of maize 

 produces anthocyanin pigment in the aleu- 

 rone and certain vegetative parts of the plant. 

 Two alleles — stippled. R', and marbled, 

 R'"' 1 — are aleurone-spotting genes found oc- 

 casionally in strains of corn native to Peru 

 and neighboring countries. Heterozygotes 

 for one of these mutants are phenotypically 

 as expected if the other R allele is also 

 from one of these South American countries. 

 Mutant homozygotes are also as expected. 

 However, when the other R allele in a hetero- 

 zygote with these mutants is native to other 

 countries, exceptional phenotypes result. 

 This suggests that a control system has 

 evolved in populations where R*' or R'" h 

 occur which does not operate properly when 

 these mutants are heterozygous with foreign 

 R alleles. 



The exceptional result from "foreign /?" 

 R*' hybrids is that pigmentation is reduced 

 or suppressed in 100% of the R -containing 

 progeny of a test cross. Moreover, all of 

 the /?-containing descendants continue to 

 show the same pigment suppression — even 

 though R 8t is no longer present. For this 

 reason. R at and /?'"'' are said to be para- 

 mutagenic. They induce foreign R alleles 

 to undergo paramutation to form paramutant 

 alleles. Paramutation can occur somatically. 

 An R factor which becomes a paramutant 

 may itself become weakly paramutagenic to 

 other R alleles. The pigment spotting action 



•See R. A. Brink (1960). 



of R ' and R"' 1 ' is separable from their para- 

 mutagenic ability. The paramutagenic ac- 

 tion of R ' is not depleted by exercising this 

 function repeatedly in R R sl plants; never- 

 theless, no evidence has been found for the 

 occurrence of a cytoplasmic element released 

 by a paramutagenic allele and taken up at 

 the paramutable locus.' 



Paramutational events may occur in or- 

 ganisms other than maize. The conditional 

 segregation-distortion phenomenon observed 

 in Drosophila (pp. 387-388) resembles 

 paramutation. Since it is considered to in- 

 volve a system which controls gene action, 

 paramutation does not imply a modifica- 

 tion of the chemical composition of genetic 

 material. Additional studies should further 

 elucidate the nature of paramutation. 



Superregulatory Mechanisms ' 



A single control system is capable of regu- 

 lating the action of many genes in develop- 

 ment. One of the systems studied — (Spm, 

 Suppressor-mutator) — according to B. Mc- 

 Clintock (1963) ". . . serves as a model 

 of the mode of operation of one type of 

 superregulatory mechanism. Such a system 

 can activate or inactivate particular genes 

 in some cells early in development, and 

 activate or inactivate other genes later in 

 development. It can turn on the action of 

 some genes at the same time that it turns 

 off the action of others. It can adjust the 

 level of activity of a particular gene in dif- 

 ferent parts of an organism. . . . The con- 

 trolling elements of the examined systems 

 may represent foreign, nonessential, epi- 

 somelike components that have been inte- 

 grated into the maize genome; or, on the 

 other hand, they may be true chromosomal 

 components of present-day maize, whatever 

 their evolutionary origins and histories may 

 have been." 



4 See R. A. Brink. J. L. Kermickle, and D. F. 



Brown (1964). 



•See B. McClintock ( 1963). 



