VARIEGATED PHENOTYPE AND DEVELOPMENTAL METHYLATION CHANGES OF A MAIZEALLELE ORIGINATING FROM EPIMUTATION

Two instances of genetic transmission of spontaneous epimutation of th e maize P-rr gene were identified. Transmission gave rise to two simil ar, moderately stable alleles, designated P-pr-1 and P-pr-2, that exhi bited Mendelian behavior. Both isolates of P-pr conditioned a variable and variegated phenotype, unlike the uniform pigmentation conditioned by P-rr. Extensive genomic analysis failed to reveal insertions, dele tions or restriction site polymorphisms between the new allele and its progenitor. However, methylation of the P gene was increased in P-pr relative to P-rr, and was greatly reduced (though not lost) in a rever tant to uniform pigmentation. Variability in pigmentation conditioned by P-pr correlated with variability in transcript levels of the P gene , and both correlated inversely with variability in its methylation. P art of the variability in methylation could be accounted for by a deve lopmental decrease in methylation in all tissues of plants carrying P- pr. We hypothesize that the variegated phenotype results from a genera l epigenetic pathway which causes a progressive decrease in methylatio n and increase in expression potential of the P gene as a function of cell divisions in each meristem of the plant. This renders all tissues chimeric for a functional gene; chimerism is visualized as variegatio n only in pericarp due to the tissue specificity of P gene expression. Therefore, this allele that originates from epimutation may exemplify an epigenetic mechanism for variegation in maize.