Genetic and epigenetic changes of rDNA in a synthetic allotetraploid, Aegilops sharonensis × Ae. umbellulata

Abstract

The synthetic allotetraploid Aegilops sharonensis × Ae. umbellulata (genomic formula SshU) was used to study inheritance and expression of 45S rDNA during early stages of allopolyploid formation. Using silver staining, we revealed suppression of the NORs (nucleolar organizing regions) from the Ssh genome in response to polyploidization. Most allopolyploid plants of the S2–S4 generations retained the chromosomal location of 45S rDNA typical for the parental species, except for two S3 plants in which a deletion of the rDNA locus on one of the homologous 6Ssh chromosomes was revealed. In addition, we found a decrease in NOR signal intensity on both 6Ssh chromosomes in a portion of the S3 and S4 allopolyploid plants. As Southern hybridization showed, the allopolyploid plants demonstrated additive inheritance of parental rDNA units together with contraction of copy number of some rDNA families inherited from Ae. sharonensis. Also, we identified a new variant of amplified rDNA unit with MspAI1 restriction sites characteristic of Ae. umbellulata. These genetic alterations in the allopolyploid were associated with comparative hypomethylation of the promoter region within the Ae. umbellulata–derived rDNA units. The fast uniparental elimination of rDNA observed in the synthetic allopolyploid agrees well with patterns observed previously in natural wheat allotetraploids.