Figure 1. Paramutation of the maize r1 gene. The series of crosses shown involves three alleles of r1, the R-r complex (paramutable allele) the R-mb complex (paramutagenic allele) and a recessive tester allele, r. R-r confers solid purple pigmentation to the aleurone layer of the seeds; R-mb confers a marbled pattern of pigmentation to the aleurone; the r tester allele confers no colour to aleurone. Paramutation is observed following test crossing of the R-r/R-mb heterozygote. Half of the kernels on the test cross ear carry the R-mb allele, which, when transmitted through the female, confers a marbled pattern of pigmentation in aleurone, but when transmitted through the male confers extremely infrequent sectors rendering the aleurones mostly colourless. The R-r allele transmitted from the R-r/R-mb heterozygote is referred to as being ‘paramutant’ and is designated as R-r¢ to indicate this new epigenetic state. Male transmission of R-r¢ results in a pale mottled pattern of pigmentation in the aleurone, while female transmission of R-r¢ results in a normal solid colour pattern of aleurone pigmentation. From Walker (1998).

Figure 2. Cosuppression of the petunia. Chalcone synthase (CHS) gene. CHS regulates floral pigmentation. Overexpression of a chimaeric petunia CHS gene in a wild-type plant (back) induces coexpression effects in some sectors. The white cells in these sectors have lost the blue pigment due to the mutual inactivation of the transferred and the endogenous CHS copy. From Napoli et al. (1990).

Figure 3. Environmental effects can induce epigenetic changes. The left picture shows isogenic petunia transformants that have been transformed with a recombinant maize A1 construct. A1 activity leads to the accumulation of a red pigment in the flowers. The right picture shows the same field at a later stage after a period of intense heat and sunlight. Many new flowers that have emerged from the plants display a white floral phenotype indicative for the inactivation of the A1 transgene, which is accompanied by DNA methylation and a condensed chromatin structure of the A1 transgene. From Meyer et al. (1992).

Figure 4. An illustration of some key enzymes and their involvement in gene silencing pathways in Arabidopsis thaliana. See Note for details.