Regulation of nucleolar dominance through allopolyploidization of hexaploid wheat

We performed expression analyses of nucleolar organizer regions (NORs) to elucidate the status and regulatory mechanism of nucleolar dominance in wheat on the basis of sequence differences in the external transcribed sequence (ETS) region of ribosomal DNAs (rDNAs). In hexaploid wheat, the B-genome ETS subtype was predominantly expressed. In contrast, the D-genome subtype was slightly expressed, and the A-genome subtype was not expressed. D-genome subtype expression increased in lines lacking the B-genome NOR, but A-genome subtype expression was not restored. These results suggested that the regulatory mechanisms of the B genome's dominance for the D and A genomes differed. In synthetic wheat, rDNA units derived from the D genome, which were highly expressed in the diploid parental species, were suppressed to the level of established hexaploid lines, indicating that nucleolar dominance in wheat was established relatively early after polyploidization. Analysis using partial deletion lines of the short arm of the B genome revealed that even after the loss of the NOR, as the degree of deletion of the short arm increased, the recovery of D-genome subtype expression increased. These results suggest that not only does the NOR region on the B genome regulate nucleolar dominance but also that there may be another regulatory region on the short arm.