NUCLEOLAR DOMINANCE IN THE ALLOTETRAPLOID, Arabidopsis suecica: PROBING THE MECHANISMS THAT SILENCE A. thaliana NORS ON A MULTI-MEGABASE SCALE.

Biology Department, Washington University, Campus Box 1137, One Brookings Drive, St. Louis, MO 63130 USA

Nucleolar dominance describes the formation of nucleoli on chromosomes inherited from one, but not the other, progenitor in an interspecific hybrid, an epigenetic phenomenon resulting from the expression of only one parental set of rRNA genes. The phenomenon occurs in both the plant and animal kingdoms and is characterized by a lack of maternal or paternal effect. We are exploring the genetic mechanisms responsible for nucleolar dominance using allotetraploid hybrids within the genera Arabidopsis and Brassica. Uniparental rRNA gene silencing appears to be enforced through changes in chromatin structure involving cytosine methylation and histone deacetylation. However, the genetic targets of these repression mechanisms remain unknown. Likewise, the mechanisms that discriminate parental sets of rRNA genes and establish nucleolar dominance are unclear. Experiments using Arabidopsis suecica, an allotetraploid derived from A. thaliana and Cardaminopsis arenosa, reveal a non-random direction but stochastic onset to nucleolar dominance in newly-formed (synthetic) hybrids. Interestingly, the direction of dominance can be reversed in an appropriate backcross. Dominance switching and numerous biochemical experiments argue against the prevailing hypothesis that dominant genes compete best for a limiting, or species-specific transcription factor. We speculate that rRNA genes might be coordinately regulated by mechanisms acting on the NORs, the chromosomal loci encoding the rRNA genes, rather than at the level of individual rRNA genes.