Highly paramutated R gene expression shows up in the aleurone as clonal patterns or sectors of cells similar to pattern alleles R-st and R-mb . Sector sizes, based on excision patterns for three different transposable elements, were assigned to specific cell divisions in aleurone development (Levy and Walbot, Science 248:1534-1537, 1990). The number of cells per sector can be related to the number of cell divisions that followed the loss of the TE. When in ontogeny is it decided that TE excision will be early or late? If the size of the clonal sectors can be controlled, then the sizes of the paramutated R sectors can be used as a reporter system to identify when the developmental program of the host can be set for clonal pattern expression in the aleurone. Under paramutagenic conditions, the sizes of aleurone sectors are determined in the second week of seedling development, under constant light and 32 C or a week later under 22 C conditions. Unlike the R-st or R-lst alleles responsible for paramutation, the pigmented sectors of cells in the aleurone lack sharp boundaries. Nevertheless the clonal nature of these sectors is evident upon magnification. The aleurone patterning can be considered to have been host-determined, before tassel primordia were visible, by conditions administered to the seedling when the meristem was ready to switch from the vegetative to the reproductive phase of development.
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