21.06.2018: Roman VOLKOV (Czernowitz): Inheritance and rearrangement of 5S and 45S rDNA in young and ancient allopolyploids, HS 31.11, Institut für Biologie, Bereich für Pflanzenwissenschaften, Schubertstraße 51, 17:00 Uhr.
The realization that a large number of plant species evolved through circles of hybridization and/or polyploidization has attracted intensive studies on mechanisms of polyploidy genome evolution. Tandemly arranged repeated ribosomal DNA (rDNA) units are present in genomes of all eukaryotes representing an attractive model for investigations of molecular evolution of repeated sequences, especially in allopolyploids. In the first generation hybrids, the 45S rDNA loci inherited from both parents often remain structurally intact while enduring differential transcriptional silencing. In ancient allopolyploid species, a more complicated picture is usually observed with uniparental inheritance and/or structural rearrangements of parental 45S rDNA. In 0.2 Myr old natural allotetraploid Nicotiana tabacum all parental 45S rDNA loci were detected on chromosomes of ancestor diploids, N. sylvestris and N. tomentosiformis. However, the 45S rDNA repeats specific for N. sylvestris were almost completely eliminated and replaced by rearranged repeats of N. tomentosiformis. In contrast, both parental 5S rDNA variants remained conserved in N. tabacum. In 15 Myr old allohexaploid Atropa belladonna, 45S rDNA endured interchromosomal conversion resulting in homogenization of rDNA repeats, whereas 5S rDNA sequences of one parent were either eliminated from the genome, or converted into pseudogenes and then lost. These molecular events led to reduction of the 5S rDNA loci number on A. bellabonna chromosomes while the 45S loci were preserved