International Journal of Aquaculture, 2025, Vol.15, No.4, 197-207 http://www.aquapublisher.com/index.php/ija 203 will lead to an increase in the acetylation level of H3K9, the promoter of the HSP gene, thereby enhancing the expression of these protective molecules. Under continuous low temperature stimulation, the overall acetylation level of histone H3 in oyster cells will decrease, which will lead to a general reduction in non-essential gene expression, causing the body to enter a low metabolic pattern to survive the cold (Yang et al., 2023). After bacterial stimulation of oyster blood cells, their histone H3K4me2 levels increased significantly, corresponding to the upregulated expression of a variety of immune genes. This may be because the demethylase LSD1 is inhibited under immune stimulation and the active methyl marks on histones accumulate, thereby accelerating the generation of bactericidal effector molecules. 5.3 Non-coding RNA-mediated regulation of immune and antioxidant mechanisms Non-coding RNA is also deeply involved in the immune and anti-resistance regulatory network of oysters, especially in pathogenic responses and oxidative stress. miRNA plays a key role in the oyster immune response. Many oyster miRNA target genes are important components of the immune pathway, such as signal transduction molecules or inflammatory mediators. By regulating these targets, miRNA carefully controls the strength and timing of the immune response. A typical example is oyster miR-223, which is known in humans to regulate granulocyte development and inflammatory response; when oysters are stimulated by bacteria, miR-223 expression is upregulated, which may promote the generation of bactericidal cells and enzymes by inhibiting factors that negatively regulate the immune response (Zhang et al., 2022). Secondly, lncRNA also plays a role in immune cell differentiation and disease resistance. Studies have found in shellfish such as abalone that pathogenic stimulation can cause hundreds of lncRNA expression changes, some of which are adjacent to or co-expressed with immune genes, suggesting that they may participate in the immune response by affecting the chromatin state of immune genes or acting as ceRNA to regulate immune-related miRNAs. In addition, long-chain non-coding RNA is also involved in the antioxidant regulation of oysters. Cells are usually activated when under oxidative stress to induce antioxidant enzyme expression. There is evidence that some lncRNAs can directly bind to Nrf2 protein, affecting their nuclear translocation efficiency; or complement the promoter region of Nrf2 downstream genes to form a three-strand structure of RNA-DNA, thereby changing the local chromatin state (Kundu and Basu, 2021). 6 Interaction Between Environmental Factors and Epigenetic Regulation 6.1 The influence of environmental factors such as temperature, salinity, pH on epigenetic markers Environmental factors can affect the oyster phenotype by changing epigenetic markers, which has gradually been recognized in recent years. In terms of temperature factors, the epigenetic status of oysters is significantly different under different temperature conditions. Oyster seedlings cultured at high temperature showed a tendency to decrease genome-wide methylation levels and improve transcriptional activity, while increased methylation at certain sites was observed under low temperature conditions, accompanied by decreased expression of growth-related genes (Wang et al., 2020). The effects of salinity changes on the epigenetics of oysters are also worthy of attention. There may be systematic differences in the methylation map of the nearshore oyster population at the southeast coastal estuaries and the oyster population in the offshore high-salt environment. Experts compared DNA methylation in four geographical populations of Crassostrea virginica and found that although the genetic differences between populations mainly originate from DNA sequences, population-specific differences in DNA methylation were more significant than genetic variation (Suárez-Ulloa et al., 2019). Under experimental conditions, sudden salinity often causes changes in histone marking and miRNA profile of oyster stress response genes. For example, the promoter H3 acetylation level of some stress protein genes in oyster tissue under low salt stress is increased, which facilitates rapid expression of these genes to restore cellular osmotic pressure balance. In terms of pH factors, seawater acidification also has a subtle influence on the epigenetics of oysters. 6.2 Epigenetic memory: the mechanism of stress resistance formation across generations Epigenetic memory refers to the fact that the epigenetic state of a organism changes and passes it to offspring after experiencing a certain environment, so that offspring also has certain adaptability when they are not exposed to
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