Elimination and rearrangement of parental rDNA in the allotetraploid Nicotiana tabacum

Origin and rearrangement of ribosomal DNA repeats in natural allotetraploid Nicotiana tabacum are described. Comparative sequence analysis of the inte rgenic spacer (IGS) regions of Nicotiana tomentosiformis (the paternal dipl oid progenitor) and Nicotiana sylvestris (the maternal diploid progenitor) showed species-specific molecular features. These markers allowed us to tra ce the molecular evolution of parental rDNA in the allopolyploid genome of N. tabacum; at least the majority of tobacco rDNA repeats originated from N . tomentosiformis, which endured reconstruction of subrepeated regions in t he IGS. We infer that after hybridization of the parental diploid species, rDNA with a longer IGS, donated by N. tomentosiformis, dominated over the r DNA with a shorter IGS from N. sylvestris; the latter was then eliminated f rom the allopolyploid genome. Thus, repeated sequences in allopolyploid gen omes are targets for molecular rearrangement, demonstrating the dynamic nat ure of allopolyploid genomes.