Ribosomal DNA in plant hybrids: Inheritance, rearrangement, expression

Ribosomal RNA genes (rDNA) represent a useful tool to study reticulate evolution. In allopolyploid plant species biparental or uniparental inheritance of 35S rDNA was demonstrated. Uniparental inheritance; as a result of differential elimination of one of the parental rDNA, can be accompanied by structural rearrangements of rDNA ofthe other parent and by formation of new rDNA variants. Particularly, homo‐genization of rDNA in allopolyploids appears to be an example of fast concerted evolution of repeated sequences. At the functional level, interaction between parental 35S rDNA loci in hybrids/allopolyploids leads to differential transcription/silencing, known as nucleolar dominance (ND). Depending on the combination of parental species, ND may be strong and stable, or weak, unstable and even reversible. Differential transcription of parental rDNA is regulated via cytosine methylation of DNA, histone modifications, and chromatin remodelling factors, but mechanisms providing discrimination of parental rDNA remain poorly understood. Probably, interplay between several factors such as local structural features of rDNA (especially subrepeated elements in the intergenic spacer region), unlinked loci, and the chro‐mosomal/genomic context determine ND. The ND and rDNA rearrangements reflect the dynamic nature and evolutionary plasticity of the genome in allopolyploids.