Epigenetic Marks and Jumping Genes Decide Which Parental Genome Rules Mustard

In the cramped control rooms of plant cells, two copies of the genome often compete for the microphone. When a plant species is born from the hybridization of two distinct ancestors followed by whole-genome doubling, the result is an allopolyploid carrying both parental genomes side by side. For decades, biologists have observed that in such plants the two genomes do not speak with equal volume. One set of genes, the dominant subgenome, is consistently transcribed at higher levels than its counterpart, and that bias can shape metabolism, stress tolerance, environmental adaptation and the agronomic traits that make crops valuable. A new study of the oilseed and vegetable crop Brassica juncea, better known as brown mustard or Indian mustard, now reveals in unusual detail how epigenetic chemical marks and invading transposable elements combine to decide which subgenome calls the shots.

Brassica juncea is an allotetraploid, meaning its genome is the fused product of two diploid progenitor species. Its cells therefore contain an A-derived subgenome, denoted Aj, and a B-derived subgenome, denoted Bj, each of which carries many genes that originally came from a common ancestor. In a paper published in BMC Plant Biology, Shici Zhu, Guoqing Li and Jianbo Wang of Wuhan University and Guizhou Normal University set out to answer a deceptively simple question: why does one subgenome in this crop express its genes more strongly than the other? The relationship between epigenetic modifications, transposable elements and subgenome expression dominance had remained poorly understood in B. juncea, and the team assembled genome-wide maps of gene expression, DNA methylation and four histone marks to close that gap.

The first and most fundamental finding was a clear confirmation of expression asymmetry. Across the transcriptome, the Aj subgenome emerged as the dominantly expressed partner, producing higher levels of transcripts than the Bj subgenome under the conditions examined. This in itself echoes patterns reported in other allopolyploids, but the real contribution of the study lies in what follows: the researchers traced that expression imbalance back to systematic differences in the chemical decoration of the two sets of chromosomes and to starkly uneven distributions of transposable elements between them.

To read the epigenetic landscape, the team used whole-genome bisulfite sequencing, a technique that converts unmethylated cytosines to another base and thereby reveals the position of DNA methylation marks across the entire genome at single-nucleotide resolution. They layered on top of this chromatin immunoprecipitation sequencing, or ChIP-seq, for four histone modifications with well-characterized roles in gene regulation: H3K4me3, typically associated with active gene bodies; H3K27ac, an acetylation mark linked to active promoters and enhancers; H3K27me3, a repressive mark laid down by the Polycomb system; and H3K9me3, a hallmark of heterochromatin that silences repetitive DNA. Comparing these maps between the Aj and Bj subgenomes exposed a pronounced asymmetry, with the two subgenomes differing systematically in where and how heavily their DNA carries methyl groups and in the density of the silencing marks that decorate their chromatin.

The most striking epigenetic pattern involved DNA methylation and the repressive H3K9me3 modification. Both were biased toward the Bj subgenome, the very subgenome whose genes are expressed at lower levels. Methylation of cytosines in and around genes is well known to interfere with transcription factor binding and to recruit proteins that compact chromatin, and H3K9me3 plays a complementary role by anchoring heterochromatic, transcriptionally inert regions. In other words, the less-dominant subgenome carries a heavier load of the silencing apparatus, a finding that closely correlates with its reduced expression and helps explain why the Aj subgenome, relatively freed of these marks, dominates the transcriptome.

The story, however, is not a simple one-mark-one-outcome relationship. When the researchers examined how each epigenetic modification relates to gene expression individually and in combination, they found that expression levels are jointly regulated by synergistic and antagonistic interactions among multiple marks. Activating marks such as H3K4me3 and H3K27ac push genes toward high expression, repressive marks such as H3K27me3 and DNA methylation pull in the opposite direction, and the final transcriptional output of any given gene reflects the balance of these forces acting on its chromatin. This layered, combinatorial control means that subgenome dominance cannot be attributed to a single molecular switch; it emerges from the cumulative physics and chemistry of many marks working together or against each other along the chromosomes.

Transposable elements, the mobile DNA sequences often described as genomic parasites, turned out to be a second major axis of asymmetry. The team counted the copies of Class I transposable elements, retrotransposons that move through RNA intermediates, and found that the Bj subgenome harbors roughly three times as many of them as the Aj subgenome. The bias extends beyond copy number. The Bj subgenome carries a higher density of transposable elements inserted within gene bodies, and it hosts a greater number of Class I elements in the vicinity of genes. Because retrotransposon insertions tend to attract DNA methylation and H3K9me3, these insertional biases provide a mechanistic bridge between the element distributions and the epigenetic imbalance: a subgenome littered with more retrotransposons near its genes is also a subgenome more heavily methylated and more thickly marked with heterochromatin.

The picture that emerges is one in which the two parental subgenomes of B. juncea arrived at their current relationship partly through the historical accumulation of transposable elements. One genome, the Bj lineage, appears to have absorbed a substantially larger retrotransposon load, and the cell’s silencing machinery, deployed to keep those elements in check, spilled over into neighboring genes and depressed their expression. The other genome, Aj, with fewer insertions in and around its genes, escaped much of this collateral silencing and now supplies the dominant share of transcripts. The authors note that these insertional biases, together with their association with epigenetic modifications, may underlie the distinct regulatory landscapes that produce subgenome expression dominance, and their findings offer valuable insight into the epigenetic regulatory mechanisms operating in polyploid plants more broadly.

Why does this matter beyond evolutionary theory? Subgenome expression dominance plays a pivotal role in metabolic regulation, stress responses, environmental adaptation and the formation of agronomic traits, so knowing which molecular features mark out the dominant subgenome gives plant breeders and genome editors a map of where the cell’s transcriptional authority resides. Brown mustard is a widely cultivated and economically important oilseed and vegetable crop, and traits such as oil content, pungency and stress tolerance are all ultimately governed by which genes are transcribed and how strongly. If silencing marks and retrotransposon density are the levers that tilt expression between subgenomes, then manipulating those levers, for example by targeting methylation patterns or by using breeding strategies that shuffle the epigenetic load, could become a route to fine-tuning trait expression in allopolyploid crops.

The study also adds an important comparison point for polyploid biology in general. In other allopolyploids, including Brassica napus and various wheats, researchers have documented similar dominance relationships, and transposable element load has repeatedly surfaced as a candidate explanation. The B. juncea data strengthen the case that retrotransposon accumulation is not merely a symptom of genome aging but an active determinant of how two genomes negotiate their roles after merger. The combination of whole-genome bisulfite sequencing and ChIP-seq for multiple histone marks provides an unusually complete portrait of that negotiation in a single crop species, showing asymmetry in DNA methylation, in repressive histone methylation, and in the distribution of the very elements that recruit those marks. As genome assemblies and epigenomic datasets improve for more polyploid crops, the pattern seen here may prove to be a general rule: the genome that wins the expression contest is often the one that carries less ancient clutter and, consequently, less of the silencing apparatus that clutter attracts. For mustard farmers and genome biologists alike, the quiet war between two fused genomes is decided, in large part, by jumping genes and the methyl marks they leave behind.

Subject of Research: Epigenetic and transposable element basis of subgenome expression dominance in allotetraploid Brassica juncea

Article Title: Asymmetric epigenetic modifications and transposable elements contribute to subgenome expression dominance in allotetraploid Brassica juncea

Article References: Zhu, S., Li, G., & Wang, J. (2026). Asymmetric epigenetic modifications and transposable elements contribute to subgenome expression dominance in allotetraploid Brassica juncea. BMC Plant Biology. https://doi.org/10.1186/s12870-026-09981-x

Image Credits: AI Generated

DOI: 10.1186/s12870-026-09981-x

Keywords: Brassica juncea, subgenome expression dominance, DNA methylation, histone modification, transposable elements, epigenetics, allotetraploid, retrotransposons, H3K9me3, gene expression, polyploidy, plant genetics