PARAMUTATION AT THE R LOCUS IN MAIZE PLANTS TRISOMIC FOR CHROMOSOME 10

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Abstract
A comparison was made of the aleurone phenotypes of the Rrrgrg kernels resulting from the following 3 matings in maize: (1) rgrg female X Rrrg male (2) rgrg female X RrRst male (disomic), and (3) rgrg female X Rr/Rst/rg male (trisomic). The male parents were moderately inbred sibs, and the rgrg female parent was a highly uniform inbred strain. The object of the experiment was to test the hypothesis that paramutation of Rr to a weakly pigmenting form, conveniently designated R[image], which was known from previous observations to occur in 100% of the cases in RrRst plants, takes place at the zygotene stage of meiosis, when the Rr and Rst alleles are synapsed. The rg allele present in the Rrrg and Rr/Rst/rg individuals had earlier been shown to be nonparamutagenic. Assuming 2 by 2 pairing at zygotene at any given chromosome level, only the Rr male gametes that carry Rr ex-conjugants from Rr-Rst zygotene pairings should be paramutant, and the rest should be normal, if the hypothesis in question is valid. Not more than 50%, and perhaps as few as 33%, of the Rr gametes formed by the Rr/Rst/rg trisomic plants should be paramutant on this basis. Aside from a few seeds on both groups of testcross ears that could have resulted from pollen contamination, it was found that the Rrrgrg (R[image] rgrg) kernels resulting from the application to rgrg individuals of pollen from the trisomic Rr/Rst/rg plants, as well as from the disomic RrRst controls, were of the paramutant phenotype throughout. The rgrg female X Rrrg male mating yielded the expected 50% of standard Rrrgrg seeds. The results of the experiment, therefore, negate the hypothesis that Rr is changed to the paramutant form, R[image], in RrRst plants, when the Rr and Rst alleles are conjugated at zygotene.

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