Nucleolar dominance: uniparental gene silencing on a multi-megabase scale in genetic hybrids

Nucleolar dominance is a phenomenon in hybrids or allopolyploids in which n ucleoli form on chromosomes inherited from only one of the two parents. The molecular basis for nucleolar dominance is the transcription by RNA polyme rase I of only one parental set of ribosomal RNA genes (rRNA genes). These rRNA genes are clustered by the hundreds, or thousands, of copies, often sp anning tens of millions of basepairs of chromosomal DNA at loci known as nu cleolus organizer regions (NORs). Enforcement of nucleolar dominance appear s to be accomplished by selectively silencing one set of rRNA genes via che mical modifications of chromatin. However, the mechanisms responsible for i nitially discriminating among the parental sets of rRNA genes and establish ing nucleolar dominance remain unclear. Possibilities include mechanisms th at act on each rRNA gene or mechanisms that affect whole NORs or even large r chromosomal domains. This review provides a historical perspective of nuc leolar dominance research, explores the most popular hypotheses and their s hortcomings, and offers some speculations concerning alternative hypotheses to be considered.