Pre-rRNA represses gene transcription in the XY body of male germ cells

Nucleolus is disassembled when the nuclear envelope breaks down during mitosis prophase. However, the dynamics of nucleolus in meiotic prophase remain unclear. Here, by tracking nucleolar components, we show that nucleolus is fragmented even before the breakdown of the nuclear envelope. Instead of dissolving in nucleoplasm or cytoplasm, the nucleolar components are reaggregated and fuse with the XY body during the prophase I of meiosis. The dynamic relocation of the nucleolar components to the XY body is governed by the ATR-dependent signaling pathway. Moreover, the relocated pre-rRNA covers the entire X and Y chromosomes. The treatment with RNA polymerase I inhibitor BMH-21 only disrupts the accumulation of pre-ribosomal RNA (pre-rRNA) but not other nucleolar components on the X and Y chromosomes. Surprisingly, this disruption of pre-rRNA induces the recruitment of RNA polymerase II to the XY body and partially abolishes meiotic sex chromosome inactivation (MSCI). Taken together, this study reveals pre-rRNA as a key epigenetic regulator for MSCI.