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

Abstract

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.

Structural features of two major nucleolar organizer regions (NORs), Nor‐B1 and Nor‐B2, and chromosome‐specific rRNA gene expression in wheat

“…In synthetic allotetraploid wheat both sets of parental NORs were expressed just after hybridization, but one parental NOR was immediately silenced after chromosome doubling (Guo and Han, ). C‐subgenome NOR loci were dominant in wheat × Aegilops interspecific hybrids (Mirzaghader et al ., ). In addition, Guo and Han () reported that allopolyploidization was a critical step for NOR silencing that could not be reversed by haploidization in wheat.…”

“…The copy number of rRNA genes in wheat ‘Chinese Spring’ (CS) has been estimated to be 9150; most of these (60%) are located on 6B ( Nor‐B2 ), with approximately 30% on 1B ( Nor‐B1 ) and the remaining 10% on several chromosome arms, as estimated from signal intensities in hybridization studies (Flavell and O'Dell, ). Nucleolar dominances were observed in several interspecific hybrids between species in the tribe Triticeae (Neves et al ., ; Guo and Han, ; Mirzaghader et al ., ). However, the genomic architecture of NORs in wheat and their sequence‐based analysis, including gene expression, remain unexplored because most previous studies on NORs in wheat and related species were based on cytological techniques such as silver staining and fluorescent in situ hybridization (FISH).…”

Chromosome restructuring among hybridizing wild wheats

“…Consistent with a largely stochastic process, species of D‐genome lineage show differential retention and loss of specific NOR loci from progenitors. Reorganization of NOR loci has been commonly reported in interspecific hybrids (Pikaard, ) and recent work highlighted hierarchical nucleolar dominance among current wild wheats (Mirzaghaderi et al , ). Much remains to be understood regarding the molecular underpinnings as well as the evolutionary consequences of such chromosome‐scale reorganization.…”

Microspore culture reveals high fitness of B. napus-like gametes in an interspecific hybrid between Brassica napus and B. oleracea

“…Subgenome dominance is an important phenomenon in allopolyploids, it was also observed in the interspecific hybrids. For example, in the interspecific hybrids (wheat × Aegilops ), C-subgenome nucleolar organizing regions loci are dominant [ 46 ]. In addition, the subgenome dominance occurred instantly following the hybridization [ 47 ].…”