Paramutation in maize

Paramutation is a heritable change in gene expression induced by allele int eractions. This review summarizes key experiments on three maize loci, whic h undergo paramutation. Similarities and differences between the phenomenol ogy at the three loci are described. In spite of many differences with resp ect to the stability of the reduced expression states at each locus or whet her paramutation correlates with DNA methylation and repeated sequences wit hin the loci, recent experiments are consistent with a common mechanism und erlying paramutation at all three loci. Most strikingly, trans-acting mutan ts have been isolated that prevent paramutation at all three loci and lead to the activation of silenced Mutator transposable elements. Models consist ent with the hypothesis that paramutation involves heritable changes in chr omatin structure are presented. Several potential roles for paramutation ar e discussed. These include localizing recombination to low-copy sequences w ithin the genome, establishing and maintaining chromatin domain boundaries, and providing a mechanism for plants to transmit an environmentally influe nced expression state to progeny.