International Journal of Aquaculture, 2025, Vol.15, No.4, 197-207 http://www.aquapublisher.com/index.php/ija 200 have been identified in oysters, which are widely involved in development, growth and stress response processes. For example, oyster miR-31, miR-8, etc. play a regulatory role in early shell development and larval metamorphosis; while miR-10, miR-1984, etc. have been reported to have significant changes in expression in immune and antioxidant reactions (Bonin et al., 2019). miRNAs perform functions by negatively regulating target gene expression. When oysters are subjected to environmental stress, the expression of many miRNAs is up-regulated or down-regulated, thereby changing the expression level of their target genes in response to stress. For example, some studies have found that Pacific oysters are significantly upregulated under high temperature stress, where miR-1 and miR-307 regulate the expression of antioxidant genes by acting on the Nrf2-Keap1 pathway and increase the resistance of oysters to oxidative stress. Another study on eastern oysters showed that after 30 days of exposure to an acidification environment with high pCO₂, some miRNA expression in oysters was altered, which may be related to the regulation of gene expression noise. In addition, in viral infection scenarios, small RNAs are also involved in host antiviral responses. When oysters are infected with the herpes virus OsHV-1, more than ten miRNAs will be differentially expressed, some of which can target immune pathway genes, and it is speculated that they regulate host antiviral responses during viral infection (Nahand et al., 2019). 3.3.2 lncRNA regulates growth and stress resistance related signaling pathways Long-chain non-coding RNA is an RNA molecule with a length of more than 200 nt. It has no protein encoding function, but can affect gene expression through various mechanisms. In recent years, a large number of lncRNAs have been identified in oysters and many of them are found to be differentially expressed under different tissues, developmental stages or stress conditions. lncRNA can play a regulatory role at various levels of appearance, transcription and post-transcription, including acting as a molecular scaffold, bait or guide protein complex. In oyster growth, some lncRNAs have been shown to be associated with shell formation and energy metabolism. For example, the LncMPEG1 obtained by cloning in pearl oysters is a lncRNA that is specifically highly expressed in outer membrane tissue. Functional analysis showed that LncMPEG1 was the highest during the critical period of larval shell development. After exogenous interference reduced its expression, the crystal morphology of the oyster shell prism and nacres occurred abnormally, suggesting that the lncRNA regulates the biomineralization of the shell by affecting the secretion function of the outer membrane. Several lncRNAs have been reported to be significantly up-regulated or down-regulated during oyster gonad development, suggesting that they play a regulatory role in reproductive gonad maturation and gamete production. In terms of stress resistance, the review by Sun and Feng et al. pointed out that the differentially expressed lncRNA of bivalve shellfish (including oysters, abalone and mussels) is associated with multiple immune-related pathways, such as TLR signaling, lectin pathway, etc. Research by Lu et al. (2021) also found that abalone has a large number of lncRNA changes under bacterial and viral stimulation, which may participate in host defense responses by affecting classic immune pathways such asNF-κB. 3.3.3 The role of piRNA in genomic stability and stress adaptation PIWI interactive RNA (piRNA) is a small RNA with a length of 24~30 nt that binds to PIWI subfamily proteins. It was first discovered in the germline and can mediate transposon silencing and genomic defense. There are also a large number of piRNA sequences in the oyster genome. In the methylation group analysis of hermaphrodites (such as the European flat oyster), there is evidence that gender switching involves extensive epigenetic reprogramming, where piRNA and PIWI pathways may play a role. Recent studies have found that piRNA is not only limited to germ cells, but is also expressed in somatic cells and exerts regulatory functions. Several piRNAs were identified in shellfish such as pearl oysters, and their expression changes under stress conditions such as hypoxia and high salt. It is speculated that the piRNA pathway may contribute to stress signal regulation and metabolic adaptation. A study of pearl oysters reported 69 highly expressed piRNA sequences, and analysis showed that these piRNAs targeted mRNAs of some metabolic and stress-related genes, suggesting that piRNAs may mediate rapid responses to environmental changes through mRNA silencing. In addition, studies compared the piRNA patterns of diploid and triploid Pacific oysters and found that the abundance and specificity of piRNA in triploid oysters were altered, some of which targeted spermatogenesis-related genes, which may be related to triploid infertility.
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