PRC2, or Polycomb repressive complex 2, is best known for placing the histone modification H3K27me3 on chromatin. Histones are proteins around which DNA is wrapped, and chemical marks on these proteins help regulate whether genes remain accessible for transcription. H3K27me3 is generally associated with gene repression, although its effects depend on developmental context and the presence of other regulatory signals. During embryogenesis, the genome is extensively reorganized after fertilization. Maternal messenger RNAs and proteins initially control development, but the embryo must soon activate its own genome. This handover, called the maternal-to-zygotic transition, requires precise changes in chromatin structure and gene activity. The new study shows that PRC2 is an important part of this transition rather than a passive regulator acting later in development.

The researchers used the dTAG system to remove PRC2 components rapidly and selectively at defined developmental stages. This approach differs from conventional genetic knockout experiments, which eliminate a gene throughout development and can make it difficult to determine when a protein is actually required. In the dTAG system, a small chemical molecule targets a tagged protein for destruction, allowing investigators to examine the consequences of acute depletion at particular time points. By applying this strategy during preimplantation development, the team could distinguish PRC2 functions in the fertilized egg, early cleavage-stage embryos and the blastocyst from functions that emerge later. The results revealed that the complex has distinct requirements across these stages, indicating that its role changes as embryonic cell states become progressively specialized.

During preimplantation development, the embryo passes through a series of divisions before forming the blastocyst, a structure containing the future embryonic cells, the trophectoderm and the primitive endoderm. The study found that PRC2 contributes to the maternal-to-zygotic transition and to the formation of the epiblast, the cell population that will generate the embryo proper. When PRC2 activity was disrupted at inappropriate times, embryos showed defects in developmental progression and in the establishment of the epiblast. These observations suggest that PRC2 helps prevent premature or misplaced gene-expression programs while allowing the correct developmental network to emerge. Its activity therefore appears to provide temporal control: genes must be silenced at the right moment, but the chromatin landscape must also remain capable of being remodeled as cells acquire new identities.

A major focus of the work was the relationship between H3K27me3 and H3K4me3. H3K4me3 is commonly associated with gene activation and is enriched near promoters of actively transcribed genes. When H3K27me3 and H3K4me3 occur together at the same regulatory region, the chromatin is described as bivalent. Bivalent domains have traditionally been viewed as “poised” regulatory regions, carrying both repressive and activating signals so that developmental genes can respond rapidly when a cell commits to a new fate. However, the precise timing of bivalency formation in early embryos has remained uncertain, partly because embryonic chromatin changes rapidly and because different studies have used different definitions of a bivalent domain.

By systematically tracking both histone modifications, the researchers propose a revised model for when bivalency is established. Rather than appearing as a fully formed state at a single developmental moment, bivalency emerges through a stepwise process. One modification can be detected before the other, and the two marks become combined as development proceeds and cellular identities sharpen. This sequence challenges the idea that bivalent chromatin is installed in one coordinated event. It instead suggests that early embryonic cells build regulatory competence gradually, integrating repressive and active information over time. PRC2 is central to this process because it supplies the H3K27me3 component, while the distribution of H3K4me3 reflects the changing transcriptional and chromatin environment around developmental genes.

The study also identifies an unexpected connection between PRC2 activity outside the embryo proper and the production of primordial germ cells, or PGCs. PGCs are the embryonic precursors of sperm and eggs, and their numbers must be tightly controlled: too few can compromise reproductive potential, while abnormal expansion may disrupt tissue organization and developmental balance. The researchers found that PRC2 regulates appropriate PGC numbers in the epiblast through its control of Esrrb expression in the extraembryonic ectoderm. The extraembryonic ectoderm does not become part of the fetus, but it produces signals that influence neighboring embryonic cells. This places PRC2 within a non-cell-autonomous pathway, in which an epigenetic event in one tissue changes the behavior of cells in another.

The role of Esrrb helps explain how this intercellular effect is transmitted. Esrrb encodes a transcription factor, a protein that binds DNA and regulates suites of genes involved in cell identity and development. According to the study, changes in PRC2-dependent regulation of Esrrb in the extraembryonic ectoderm alter the developmental environment experienced by epiblast cells and consequently affect PGC formation. The finding illustrates that epigenetic regulation does not operate solely within the boundaries of a single cell. A chromatin complex can modify gene expression in one embryonic compartment, which then changes signals or conditions that guide fate decisions elsewhere. This epigenetic-to-transcription-factor-to-cell-communication pathway adds another layer to the already complex regulation of germline specification.

Taken together, the findings expand the biological definition of PRC2 during early embryogenesis. The complex acts cell-autonomously to organize chromatin and support preimplantation development, yet it also has broader developmental consequences through interactions between embryonic and extraembryonic tissues. The researchers’ results emphasize that the same epigenetic regulator can have different effects depending on developmental stage, cellular location and the surrounding transcriptional network. They also show why acute protein-depletion tools such as dTAG can be valuable for developmental biology: timing is often as important as molecular identity. By connecting histone-mark dynamics, transcription-factor control and intercellular signaling, the study offers a framework for understanding how embryos convert rapidly changing epigenetic information into coordinated developmental outcomes.

Subject of Research: Stage-specific functions of PRC2 in preimplantation development, epigenetic bivalency and primordial germ cell formation.

Article Title: Decoding stage-specific functions of PRC2 in early embryogenesis uncovers roles in preimplantation development and primordial germ cell fate.

Article References: Zhou, C., Wang, M., Chen, Z. et al. “Decoding stage-specific functions of PRC2 in early embryogenesis uncovers roles in preimplantation development and primordial germ cell fate.” Nature Cell Biology 28, 1700–1714 (2026). https://doi.org/10.1038/s41556-026-02002-x

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41556-026-02002-x; publication date: August 2026

Keywords: PRC2, embryogenesis, epigenetics, H3K27me3, H3K4me3, bivalency, maternal-to-zygotic transition, epiblast, primordial germ cells, Esrrb, preimplantation development, dTAG system

Epigenetics refers to the complex regulatory modifications atop the DNA sequence that influence gene activity without altering the genetic code itself. Among these, DNA methylation—where methyl groups are chemically added to DNA bases—is one of the most widely studied. These methylation marks serve as crucial regulators, ensuring genes turn on or off at the right moments during development and cell differentiation. Interestingly, it is a universal hallmark in mammals that embryos undergo a thorough genome-wide wipe of DNA methylation shortly after fertilization but before implantation. Despite its ubiquity, the underlying reason why this "wiping" or DNA demethylation exists has puzzled scientists for decades.

Prior hypotheses posited that this demethylation was essential for fundamental developmental milestones—such as activating the embryonic genome or enabling the diverse differentiation of embryonic cells into various specialized tissues. However, studying this was complicated by the synchronous timing of these events in placental mammals (eutherians) like mice and humans, which develop rapidly and overwrite their methylation marks early.

Breaking new ground, the Crick Institute team turned their attention to marsupials—a distinct infraclass of mammals that diverged from eutherian ancestors approximately 160 million years ago. The Australian opossum, in particular, was chosen for its slower, staggered embryonic development, providing a natural biological system to decouple the processes traditionally entangled in placental mammals.

Generating a comprehensive DNA methylation map, the researchers analyzed methylation profiles in opossum eggs, sperm, and embryos to compare with known data on eutherian species. Remarkably, they found that, unlike placental mammals, opossum embryos did not fully erase DNA methylation after fertilization. Instead, methylation levels were retained through the earliest embryonic stages, with demethylation occurring much later and localized predominantly to the trophectoderm—a specialized cell lineage destined to become the placenta in marsupials.

This insight is profound. It indicates that the dramatic, early embryonic wipe of methylation seen in placental mammals is not a universal mammalian feature but rather an evolutionary adaptation associated explicitly with placental development. The evolutionary divergence between marsupials and eutherians appears to be mirrored at the epigenetic level, with the methylation resetting potentially emerging as a key driver in the complex evolution of the placenta.

Interestingly, the research further suggests that selective removal of methylation in the placenta facilitates the activation of transposable elements—so-called “jumping genes.” These genetic elements, once thought to be mere genomic parasites, have increasingly been recognized for their role in regulating host gene expression and contributing to genomic innovation. Their expression within the placenta may drive rapid adaptive evolution of this critical organ, aligning with its remarkable diversity and complexity across mammalian species.

Dr. Bryony Leeke, a former Crick PhD student and co-first author of the study, explained that the unique epigenetic landscape in the placental precursor cells likely enables these transposon-driven regulatory modifications. Meanwhile, senior author James Turner expressed surprise that the universal model derived from placental mammals does not fully apply to marsupials, underscoring how comparative studies in diverse mammals can reshape our understanding of fundamental biology.

The implications of this research extend far beyond developmental biology. By illuminating how epigenetic mechanisms have evolved alongside mammalian reproductive strategies, this work opens avenues for understanding diseases linked to epigenetic dysregulation. The placenta itself is a critical organ for fetal development and maternal health, and uncovering the epigenetic nuances of its formation could inform medical strategies related to pregnancy complications and developmental disorders.

Technologically, the study leveraged state-of-the-art sequencing approaches to generate high-resolution methylation maps, a feat previously unachievable in non-model organisms like marsupials. These detailed epigenetic profiles provide a rich resource for further studies into mammalian evolution and development.

From an evolutionary perspective, the findings reinforce the concept that what are often considered universally conserved biological processes can, in fact, show surprising diversity when studied across a broader taxonomic range. The divergence in embryonic epigenetic reprogramming between marsupials and eutherians illustrates how evolutionary pressures tied to reproductive biology shape molecular mechanisms in distinct lineages.

Moreover, the persistence of methylation marks in early opossum embryos suggests alternative strategies for genome regulation during development, challenging the prevailing dogma that embryonic methylation wiping is indispensable. This compels the scientific community to reconsider models of epigenetic inheritance and reprogramming, with potential repercussions for the fields of developmental biology, epigenomics, and evolutionary genetics.

Dr. Turner emphasized the value of studying “the odd ones out” in nature, such as marsupials, noting that their unique biology offers critical windows into understanding processes otherwise obscured when focusing solely on traditional model organisms. This principle underscores a broader scientific approach: diversity is a key to unlocking hidden mechanisms underpinning complex biological phenomena.

As the researchers continue to explore the functional consequences of epigenetic divergence in mammals, these findings pave the way for future investigations into how the intricate dance of DNA modification and gene expression across species shapes development, adaptation, and ultimately survival. The placenta, as one of the fastest-evolving mammalian organs, stands at the nexus of this evolutionary narrative, spotlighted by the subtle yet profound epigenetic modifications illuminated by this pioneering work.

Subject of Research: DNA methylation dynamics during mammalian embryonic development
Article Title: Divergent DNA methylation dynamics in marsupial and eutherian embryos.
News Publication Date: 14 May 2025
Web References: http://dx.doi.org/10.1038/s41586-025-08992-2
References: Leeke, B.J. and Varsally, W. et al. (2025). Divergent DNA methylation dynamics in marsupial and eutherian embryos. Nature. 10.1038/s41586-025-08992-2.
Keywords: Marsupials, Embryos, Placenta, Developmental biology