Dynamic nucleolar activity in wheat × Aegilops hybrids: evidence of C-genome dominance

NOR loci of C-subgenome are dominant in wheat × Aegilops interspecific hybrids, which may have evolutionary implications for wheat group genome dynamics and evolution. After interspecific hybridisation, some genes are often expressed from only one of the progenitor species, shaping subsequent allopolyploid genome evolution processes. A well-known example is nucleolar dominance, i.e. the formation of cell nucleoli from chromosomes of only one parental species. We studied nucleolar organizing regions (NORs) in diploid Aegilops markgrafii (syn: Ae. caudata; CC), Ae. umbellulata (UU), allotetraploids Aegilops cylindrica (C^cC^cD^cD^c) and Ae. triuncialis (C^tC^tU^tU^t), synthetic interspecific F₁ hybrids between these two allotetraploids and bread wheat (Triticum aestivum, AABBDD) and in F₃ generation hybrids with genome composition AABBDDC^tC^tU^tU^t using silver staining and fluorescence in situ hybridization (FISH). In Ae. markgrafii (CC), NORs of both 1C and 5C or only 5C chromosome pairs were active in different individual cells, while only NORs on 1U chromosomes were active in Ae. umbellulata (UU). Although all 35S rDNA loci of the C^t subgenome (located on 1C^t and 5C^t) were active in Ae. triuncialis, only one pair (occupying either 1C^c or 5C^c) was active in Ae. cylindrica, depending on the genotype studied. These C-genome expression patterns were transmitted to the F₁ and F₃ generations. Wheat chromosome NOR activity was variable in Ae. triuncialis × T. aestivum F₁ seeds, but silenced by the F₃ generation. No effect of maternal or paternal cross direction was observed. These results indicate that C-subgenome NOR loci are dominant in wheat × Aegilops interspecific hybrids, which may have evolutionary implications for wheat group genome dynamics and allopolyploid evolution.