Figure 1. Discovery of nucleolar dominance as the hybridization-induced absence of a secondary constriction at the nucleolus organizer region (NOR) of metaphase chromosomes. Based on the observations of Navashin (1934) and McClintock (1934), haploid chromosome sets of three ‘pure’ species and their hybrids are shown. Each species has a chromosome with an NOR that forms a secondary constriction at metaphase. In hybrids, often the NOR from only one progenitor forms the characteristic secondary constriction. In the example shown, the NOR of species A is dominant in an A–B hybrid, the NOR of species B is dominant in a B–C hybrid and the NOR of species A is dominant in an A–C hybrid. Only actively transcribed NORs form a secondary constriction at metaphase, apparently due to a physical or enzymatic function of the nucleolus interfering with chromosome condensation.

Figure 2. A silent set of ribosomal (rRNA) genes subjected to nucleolar dominance can be derepressed by 5-aza-2¢-deoxycytosine (Aza-dC), an inhibitor of cytosine methylation or by trichostatin A (trichostatin), an inhibitor of histone deacetylation. This is demonstrated using Brassica napus, the allotetraploid hybrid of B. rapa and B. oleracea, in which rRNA genes inherited from B. oleracea are repressed (compare lanes 3 and 7). Brassica napus seeds were germinated on a medium containing no additions (lanes 3 and 7), or on a medium containing 5-aza-2¢-deoxycytosine (lanes 4 and 8), trichostatin A (lanes 5 and 9) or both chemicals (lanes 6 and 10). Plants were harvested after 2 weeks in culture and an equal aliquot of RNA from each treatment was hybridized to B. rapa (lanes 3–6, 12) or B. oleracea-specific (lanes 7–11) probes and subjected to S1 nuclease protection analysis to detect transcripts from the genes inherited from the two progenitors. RNA isolated from B. oleracea and B. rapa (lanes 11 and 12) served as controls. Lanes 1 and 2 show sequencing ladders used as size markers. Note that 5-aza-2¢-deoxycytosine and trichostatin A together are not significantly more effective at derepressing the B. oleracea genes than is either chemical alone, suggesting that DNA methylation and histone deacetylation are partners in the same repression pathway. (Reprinted with permission from Chen and Pikaard, 1997b.)