The R-mb:cc allele, characterized by the presence of concentric colored stripes/rings on a colorless aleurone background, was genetically analyzed to determine the paramutagenic property, if any, in crosses with R-r:standard lines. The R-marbled (R-mb) allele, the parental allele of R-mb:cc, was recorded to display a paramutagenic behavior in association with R-r lines (Brink and Weyers, Proc. Natl. Acad. Sci. USA, 1053-1059, 1957), similar to that of its 'sister' pattern allele, R-st. Surprisingly, subsequent workers interested in the phenomenon of paramutation in maize paid less attention to the R-mb allele, in comparison with the R-st allele; consequently, literature on the paramutagenic property of R-mb is almost negligible. Although the phenotype of R-mb:cc is distinctly different from that of R-mb. we were interested in finding out if the derivative retains the paramutagenic property.
Various categories of R-mb:cc -- near colorless, light striped, medium striped and heavy striped -- were crossed with R-r:standard lines and the resultant progeny were selfed/sibbed. For comparison, the same R-r:standard lines were also crossed with the R-mb stock. Results from the F2 and subsequent generation (data not shown) clearly indicated the paramutagenic nature of R-mb:cc. in causing a reduction in the anthocyanin pigmentation intensity of the R/R kernels. To assess the reduction in pigmentation intensity, the resulting ears were assessed by a standard set of R kernels. Observation of paramutation in the progeny kernels aided us in concluding that R-mb:cc, possibly a 'change in state' from R-mb, retains the paramutagenic property despite a distinctly different variegation pattern. Interesting, perhaps, is the occurrence of modified R/R progeny kernels where a few sectors of mottling (with reduced pigmentation) were present in concentric circles of broken lines, resembling the characteristic variegation pattern of R-mb:cc (with solid, concentric circles/stripes originating from the germinal region). Progeny testing of such kernels ruled out the presence of the R-mb:cc allele. Although the reasons behind such occurrences are not clear, speculations can be made about the possible links between DNA methylation, transposable elements and paramutation.
Though the significance of paramutation in higher plants is now increasingly realized, the exact mechanisms behind this puzzling phenomenon continue to elude geneticists. Intensive efforts by maize researchers are needed to provide clues to better understand this interesting phenomenon.
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