Differential rRNA genes expression in bread wheat and its inheritance

The expression of the ribosomal RNA (rRNA) genes from rye, located within the nucleolus organizer regions (NORs), is repressed by cytosine methylation in wheat x rye hybrids and in triticale, as consequence of nucleolar dominance. Our previous study revealed that bread wheat cultivars with a maximum number of four Ag-NORs presented high level of rDNA cytosine methylation when compared to others with a maximum of six Ag-NORs. In order to evaluate the inheritance of the Ag-NORs number and NOR methylation patterns, we produced F₁ hybrids between bread wheat cultivars with four Ag-NORs and bread wheat cultivars with six Ag-NORs (in the direct and reciprocal senses). The F₂ progenies of these F₁ hybrids were also evaluated for the NOR number and methylation patterns. Parent bread wheat cultivars with a maximum of four Ag-NORs after treated with 5-azacytidine evidenced a maximum of six Ag-NORs per metaphase cell and a maximum of six nucleoli per interphase nucleus, confirming that the expression of the rRNA genes in bread wheat is related to cytosine methylation. Most of the F₁ hybrids showed a maximum number of four or six Ag-NORs, similarly to that of the female parent suggesting a non-mendelian inheritance, while other hybrids presented four or six Ag-NORs in both senses of the cross. The F₁ NOR methylation patterns showed some fragments common to their parents but also novel fragments suggesting genomic and/or chromosome rearrangements after hybridization. Despite the different NOR patterns among the parents, an invariable NOR pattern was found among the F₁ plants suggesting a tendency to stability, which was also transmitted to the F₂. The F₂ progenies showed plants with a maximum of four, five and/or six Ag-NORs. The ratio of plants with four, five and/or six Ag-NORs per F₂ progeny was variable and did not follow any specific mendelian proportion. These results allowed us to suggest that the inheritance of the number of Ag-NORs by the F₁ and F₂ plants did not follow any mendelian inheritance and were not correlated to NOR methylation patterns in contrast to what was verified for their parents.