s823631036@kpu.ac.jp

Many crops are polyploid, and understanding the genetic and genomic changes during polyploidization is important for crop improvement.

rDNA (ribosomal DNAs) encodes rRNAs (ribosomal RNAs), components of the ribosome, and is located in chromosomal regions called NORs (Nucleolar Organizer Regions). The NORs contain hundreds to thousands of copies of rDNA units repeated in tandem. Although the sequences of each rDNA unit are highly similar, their expression is tightly regulated.

Nucleolar Dominance (ND) is the phenomenon that only one of the NORs inherited from both parents is expressed in hybrid offspring. Our project aims to reveal the genomic harmonization that occurred during the wheat’s allopolyploidization based on the analysis of ND.

In a previous report (Handa et al., 2018), we found sequence differences in the ETS region after the 26S rRNA gene on chromosomes 1B, 5D, and 6B, and identified four rDNA subtypes. Expression studies revealed that only B genome-derived subtypes were expressed in the normal hexaploid wheat. However, 5D chromosome-derived subtypes began to be expressed if the NORs on chromosome 1B or 6B were missing.

This time, we investigated the reference sequences of chromosomes 1A and 7D, which have yet to be analyzed in the five chromosomes with NORs in wheat, and we identified two new subtypes of rDNA unit. Together with the previous four subtypes, six were identified, providing a basis to investigate all NORs throughout the wheat genome. As with previous results, only B genome-derived subtypes were expressed in normal hexaploid wheat.

However, in strains lacking NORs on chromosomes 1B or 6B, subtypes derived from 5D and 7D were found to be expressed. In contrast, no expression of chromosome 1A-derived subtype was observed even when the B genome-specific NOR was deleted. These suggest a hierarchical structure in ND in wheat in the order B-genome>D-genome>A-genome. Further studies using strains with partial deletion of chromosome 1B revealed that as the degree of deletion of the short arm of chromosome 1B increased, the degree of recovery of expression of the D genome subtype also increased. This result contradicts the previous hypothesis that only the NOR on the B genome chromosome regulates ND and suggests there may be regulatory regions other than the NOR on the short arm.

Now, to clarify more details of ND in wheat, we are analyzing the changes in ND according to growth stage and wheat evolutionary processes.

This work was supported by MEXT KAKENHI Grant Number JP21K05518.