Figure 1. Paramutation behaviour. Anther phenotypes and corresponding diploid genotypes are represented for two sets of genetic crosses that together illustrate the nonmendelian inheritance pattern typifying paramutation. Pl-Rhoades (Pl-Rh) and Pl¢ alleles control anthocyanin pigment production in the anthers (see text).

Figure 2. Allelic interaction models. Two general models of allelic interactions mediating trans-silencing. Solid horizontal arrows represent gene sequences and grey dots represent chromosome materials unfavourable to gene transcription. Bidirectional arrow indicates a diffusible substance. (a) Allelic interaction involving transfer of chromosome materials via physical pairing of homologous sequences. (b) Allelic interaction involving transfer of a diffusible substance.

Figure 3. Paramutation phenotypes. Visible pigment phenotypes conferred by paramutable and paramutagenic alleles or haplotypes are displayed. (a) B-I versus B¢ leaf sheath phenotypes. (b) Pl-Rh versus Pl¢ anther phenotypes. (c) R-r:std, R-r:std¢, R-st and R-mb kernel phenotypes.

Figure 4. Paramutable versus paramutagenic allele and haplotype structures. Schematics (not to scale) represent genome organizations of the various alleles and haplotypes referred to in the text. Solid black arrows denote transcribed regions. Open boxes represent repetitive regions. Small horizontal triangles represent individual repeats; in the pl1 and p1 alleles these represent 3¢ regions of the respective transcribed regions. Vertical triangles denote the presence of transposable elements. Hash marks indicate large regions of intervening sequences. Several individual r1 units are labeled; P is only expressed in the plant, S1 and S2 regions are expressed in the kernel, Self coloured (Sc) and Self coloured marbled (Scm) are highly expressed in kernels but are interrupted by the presence of two unrelated transposable elements. Somatic excisions of these elements from Sc and Scm during kernel development restore gene function and give rise to the pigment patterns displayed in Figure 3. Remaining unlabelled r1 regions are very weakly expressed in kernels.