Probst and colleagues used RNA fluorescent in situ hybridization to examine pericentric satellite expression in early-cleavage-stage embryos, as previous studies had indicated that the organization of satellite repeats is dynamic at this stage in development. At the two-cell stage, they observed a specific peak in major satellite transcription from the zygotic genome followed by a rapid downregulation. This coincided with the organization of pericentric regions into clusters called chromocentres, suggesting a link between the expression dynamics of satellite repeats and chromocentre formation.
The transcription of major satellites can occur from both strands of the DNA, so by using highly specific locked nucleic acid (LNA) oligonucleotide probes, the authors examined whether satellite expression was subject to strand-specific regulation in pre-implantation embryos. They found temporally and spatially restricted strand-specific regulation of these satellite transcripts: forward transcripts accumulated during the S phase of the cell cycle and localized to both pericentric domains and the cytoplasm, whereas reverse transcripts were produced during the G2 phase and accumulated in discrete nuclear foci at the pericentric domains. Moreover, the satellites, particularly the forward strand DNA, were predominantly expressed from the paternal genome and therefore strongly reduced in parthenotes. The authors suggest that this parental bias might reflect the asymmetry in histone marks between maternal and paternal pericentric domains, such that higher levels of satellite transcription are favoured by the lack of somatic heterochromatin marks at paternal pericentric regions.
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