International Journal of Aquaculture, 2025, Vol.15, No.4, 197-207 http://www.aquapublisher.com/index.php/ija 202 Figure 1 (1) Hypermethylation of histone lysine residues in presence of 10 µM Methylstat. Global H3K4 methylation increase in presence of methylstat, but only H3K4me1 and H3K4me3 are significant. Global H3K9 methylation increase in presence of methylstat, but only H3K9me2 is significant. H3K27me and H3K27me2 increase in presence of methylstat. Asterisk in the legend indicates significant variation of the indicated mark (One-way ANOVA; p < 0.05 was considered significant (<0.05 (*), <0.001 (**), <0.0001 (***)). (2) Abnormal development. Different phenotypes observed in control condition and under methylstat treatment at 6 h after fertilization (a) and 24 h after fertilization (b) (Adopted from Fellous et al., 2019) 5 The Role of Epigenetic Mechanism in Stress Resistance of Oysters 5.1 Dynamic changes in DNA methylation patterns under environmental stress Environmental stress can cause reprogramming of the methylation state of the genome of oysters, thereby affecting the expression of genes related to stress resistance and the tolerance of oysters. Studies have shown that different stress types and stress durations will lead to dynamic adjustment of the methylation level of the whole genome of oysters. Taking dry dew (air exposure) stress as an example, Wang et al. put oysters in air for different times to measure the DNA methylation level of their cuff muscles and gill tissues. It was found that the overall methylation level gradually increased between 0.5 and 7 days of stress and reached its peak on day 7, which was about 10% to 15% higher than the control without dry dew; but when the dry dew continues to day 11, the methylation level drops again and approaches the initial state (Wang et al., 2021). This shows that the oyster genome has undergone a process of partial demethylation first with overall hypermethylation response and then partial demethylation as stress continues. It is speculated that the increase in early methylation may quickly turn off non-essential genes and concentrate resources on stress resistance, while the later demethylation may activate some chronic stress response genes to help survive for a long time (Li et al., 2024). 5.2 Histone modification regulates rapid response of genes related to stress resistance When oysters are challenged by environmental challenges, histone modifications can turn on or off relevant genes by rapidly changing chromatin status, thereby achieving a rapid response to stress. Under hypoxia stress, organisms often use inhibitory markers such as H3K27me3 to activate hypoxia-resistant genes such as erythropoietin and heme oxygenase. In addition, histone modification changes will also be triggered when temperature changes are severely altered to regulate the activity of heat shock protein genes: high temperatures
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