Activation of the imprinted polycomb group Fie1 gene in maize endosperm requires demethylation of the maternal allele

Imprinting refers to the epigenetic regulation of gene expression that is dependent upon gene inheritance from the maternal or paternal parent. Previously, we have identified two maize homologs of the single Arabidopsis Polycomb Group gene FIE. Here, we report on the expression pattern of these genes in individual gametes before and after fertilization, and on the role of DNA methylation in determining the maternal expression of the Fie1 gene. We found that Fie1 is neither expressed in the sperm, egg cell nor central cell before fertilization. Activation of the Fie1 maternal allele occurs around two days after pollination (DAP) in the primary endosperm and peaks at 10-11 DAP coinciding with endosperm transition from mitotic division to endoreduplication. In contrast, Fie2 is expressed in the egg cell and more intensively in the central cell similar to Arabidopsis FIE, which strongly supports the hypothesis that it functions as a repressor of endosperm development before fertilization. Using MSREPCR and bisulfite sequencing, we could show that the methylated inactive state is the default status of Fie1 in most tissues. In the endosperm the paternal Fie1 allele remains methylated and silent, but the maternal allele appears hypornethylated and active, explaining monoallefic expression of Fie1 in the endosperm. Taking together, these data demonstrate that the regulation of Fie1 imprinting in maize is different from Arabidopsis and that Fie1 is likely to have acquired important novel functions for endosperm development.