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 (CCDD) and Ae. triuncialis (CCUU), synthetic interspecific F hybrids between these two allotetraploids and bread wheat (Triticum aestivum, AABBDD) and in F generation hybrids with genome composition AABBDDCCUU 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 subgenome (located on 1C and 5C) were active in Ae. triuncialis, only one pair (occupying either 1C or 5C) was active in Ae. cylindrica, depending on the genotype studied. These C-genome expression patterns were transmitted to the F and F generations. Wheat chromosome NOR activity was variable in Ae. triuncialis × T. aestivum F seeds, but silenced by the F 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.