Fish Liver DNA Methylation Reveals Hidden Switches Controlling Egg Development

When a female fish prepares to spawn, her liver becomes an unlikely hero of reproduction. It churns out the yolk proteins, lipids, and energy reserves that will fuel the growing eggs, and the timing of this metabolic handover must be precisely choreographed with the ovary’s developmental stages. A new study of the spotted scat (Scatophagus argus), a commercially important tropical fish farmed across Southeast Asia, suggests that this choreography is guided not only by which genes are switched on, but by a chemical layer of control written directly onto the DNA of liver cells.

The research, published in BMC Genomics by a team at Guangdong Ocean University led by Mei-Zhen Zhang and corresponding author Dong-Neng Jiang, set out to answer a question that had lingered after the group’s earlier work on liver transcriptomes: what governs the expression of the many liver genes tied to ovarian development? Their prime suspect was DNA methylation, an epigenetic mark in which methyl groups are attached to DNA bases, most commonly cytosines in CpG dinucleotides. Methylation in promoter regions is classically associated with gene silencing, while methylation within the body of a gene can have more varied and sometimes stimulatory effects on transcription.

To map this chemical landscape, the researchers turned to whole-genome bisulfite sequencing (WGBS), the gold-standard technique for charting methylation at single-base resolution. Bisulfite treatment converts unmethylated cytosines into uracils while leaving methylated cytosines untouched, so sequencing the treated DNA reveals exactly which cytosines carry the mark. The team applied this method to livers sampled at three stages of ovarian development, designated II, III, and IV, spanning the progression from early maturation toward full vitellogenesis and approaching final oocyte maturation.

The resulting methylome maps delivered a striking pattern: overall CpG methylation in the liver was highest at ovary stage II, at 74.57 percent, and then declined to 70.20 percent at stage III before ticking back up to 71.68 percent at stage IV. In other words, as the ovary advanced from early to mid-development, the liver underwent a substantial global demethylation, a shift consistent with the idea that the organ opens up its transcriptional machinery to ramp up production of egg-supporting molecules.

But global methylation is only half the story. The real power of the study came from integrating the methylome data with RNA sequencing data from the same livers, allowing the team to ask which genes showed both a change in methylation and a corresponding change in expression across developmental transitions. Spearman correlation analysis revealed modest but meaningful associations between expression levels and CpG methylation in both promoter and gene body regions, hinting that methylation contributes to, without wholly dictating, the transcriptional program of the developing liver.

When the researchers overlapped differentially expressed genes with differentially methylated genes, they identified 28 overlapping genes in the stage II versus III comparison, 256 in the II versus IV comparison, and 99 in the III versus IV comparison. The largest set, spanning the full transition from early maturation to late development, contained some of the most compelling candidates, including genes central to the two defining products of the reproductive liver: yolk precursors and egg envelopes.

Among these were the estrogen receptor gene erα, the yolk protein genes vtgb and vtgc, and the egg envelope, or zona pellucida, genes zp4a and zp4b. In the II versus IV and III versus IV comparisons, these vitellogenesis and egg envelope genes showed decreased methylation in their promoter or gene body regions, accompanied by increased expression. The logic is elegant: as estrogen signaling ramps up during ovarian maturation, the liver appears to lift an epigenetic brake on the very genes needed to manufacture yolk proteins and the protective envelopes that surround the egg. The team validated one key finding, methylation of the erα promoter, using bisulfite sequencing PCR (BSP), an independent targeted method, and the results matched the WGBS data, strengthening confidence in the genome-wide map.

The study also traced methylation changes in genes governing the lipid raw materials of the egg. In the II versus IV comparison, hsd17b7, which participates in cholesterol formation, showed decreased promoter methylation together with increased expression, while cyp51, another cholesterol biosynthesis gene, showed decreased gene body methylation alongside increased expression. Cholesterol is the biochemical backbone from which steroid hormones and membrane components are built, so its upregulation fits the demands of a liver preparing to provision thousands of oocytes. In a more nuanced twist, dgat2, a gene involved in triglyceride formation, showed decreased gene body methylation together with decreased expression, illustrating that gene body methylation does not follow a simple one-directional rule and that its relationship with transcription can vary by gene and context.

Why does this matter beyond the spotted scat? Epigenetic regulation of reproduction has been studied extensively in mammals, but fish present a distinctive system: the liver, not the ovary, is the primary factory for yolk precursors, and environmental cues such as temperature and nutrition can leave lasting epigenetic imprints on farmed stocks. Demonstrating that methylation changes in the liver track the ovarian developmental cycle provides a mechanistic bridge between an animal’s environment, its epigenome, and its reproductive output. For aquaculture, that bridge could eventually point to epigenetic markers that predict which broodstock females are primed for spawning, or to interventions that synchronize egg production across a farmed population.

The authors are careful about the limits of the correlation-based analysis. Methylation and expression were measured at matched developmental stages rather than manipulated experimentally, so the study establishes association rather than causation, and the modest strength of the Spearman correlations suggests methylation is one contributor among many, alongside transcription factors, hormones, and chromatin modifications not measured here. Still, the convergence of genome-wide mapping, transcriptome integration, and targeted validation marks this as one of the most complete pictures yet of how a fish liver’s epigenome shifts to support egg development. As sequencing costs fall, similar methylome-transcriptome pairings are likely to follow in other aquaculture species, turning a once-obscure chemical mark into a practical handle on one of farming’s oldest challenges: getting fish to reproduce reliably on schedule.

Subject of Research: Epigenetic regulation of liver gene expression during ovarian development in spotted scat

Article Title: Integration of liver methylome and transcriptome provides new insights into the regulation of ovarian development genes in spotted scat (Scatophagus argus)

Article References: Zhang, M.-Z., Li, Y., Jiao, K.-Z., Huang, Y.-Q., Jiang, M.-Y., Shi, H.-J., Tian, C.-X., Deng, S.-P., Chen, H.-P., & Jiang, D.-N. (2026). Integration of liver methylome and transcriptome provides new insights into the regulation of ovarian development genes in spotted scat (Scatophagus argus). BMC Genomics. https://doi.org/10.1186/s12864-026-13425-z

Image Credits: AI Generated

DOI: 10.1186/s12864-026-13425-z

Keywords: DNA methylation, epigenetics, spotted scat, Scatophagus argus, ovarian development, vitellogenesis, liver transcriptome, whole-genome bisulfite sequencing, aquaculture, gene expression, egg envelope formation, fish reproduction