Abstract
The expression of rDNA in hybrids inherited from only one progenitor refers to nucleolar dominance. The molecular basis for choosing which genes to silence remains unclear. We report genetic imbalance induced by distant hybridization correlates with formation of rDNA genes (NORs) in the hybrids between Raphanus sativus L. and Brassica alboglabra Bailey. Moreover, increased CCGG methylation of rDNA in F1 hybrids is concomitant with Raphanus-derived rDNA gene silencing and rDNA transcriptional inactivity revealed by nucleolar configuration restriction. Newly formed rDNA gene locus occurred through chromosomal in F1 hybrids via chromosomal imbalance. NORs are gained de novo, lost, and/or transposed in the new genome. Inhibition of methyltransferases leads to changes in nucleolar architecture, implicating a key role of methylation in control of nucleolar dominance and vital nucleolar configuration transition. Our findings suggest that gene imbalance and methylation-related chromatin restructuring is important for rDNA gene silencing that may be crucial for synthesis of specific proteins.
Highlights
Distant hybridization may occur when different genera or different species are crossed
Evidence suggested that epigenetic changes occurred during the formation of F1 hybrids that resulted from distant hybridizations [18] [19] [20] [21], so we examined the methylation of rDNA in this hybrid
After a 72 h treatment, the nucleolus was uniform, and did not have a distinguishable structure (Fig. 8C). These results indicated that rDNA methylation caused heterogeneity in chromatin, while demethylation led to a decrease in heterogeneous chromatin
Summary
Distant hybridization may occur when different genera (intergeneric hybridization) or different species (interspecific hybridization) are crossed. Distant hybridization has been applied to rice, wheat, maize, cotton, soybean, and Brassica systems, its use has resulted in a number of potential cultivars, varieties, as well as some alien substitution lines, additional lines, and translocation lines [3] [4] These genetic stocks or lines are useful in plant breeding. Inhibition of DNA methylation by aza-deoxycytosine (aza-dC) and of histone deacetylation with trichostatin (TSA) in Brassica both prevented nucleolar dominance [11] [12] This gene silencing is a manifestation of rRNA gene dosage control, which is dependent upon the number of active rRNA genes needed by the metabolism of the cell [13]. Our data suggest that chromosomal rearrangements, genetic imbalance, and epigenetic motifs regulate nucleolar structure and chromatin remodeling, and may control rRNA gene expression in intergeneric hybrids
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