XVI International Botanical Congess

In many plant and animal inter-species hybrids only one parental set of ribosomal RNA (rRNA) genes is expressed regardless of which species was the maternal or paternal parent. In Arabidopsis and Brassica we've shown that nucleolar dominance is controlled at the level of transcription. Models suggesting inactivation of a species-specific transcription factor or differences in transcription factor binding are not supported by direct tests. Transient expression results suggest that repression is restricted to chromosomal rRNA genes. These silenced genes are de-repressed by chemical inhibitors of cytosine methylation or histone deacetylation. Evidence suggests that cytosine methylation does not block transcription directly but acts upstream of histone deacetylation in the repression pathway.