Animal Molecular Breeding, 2025, Vol.15, No.2, 60-71 http://animalscipublisher.com/index.php/amb 63 inducing exercise can cause increased levels of histone acetylation at certain sites in muscle cells, activate metabolism and growth-related genes. Chromatin status is also affected by the environment and metabolites, such as short-chain fatty acids (metabolism from intestinal flora) that inhibit histone deacetylase, leading to an increase in histone acetylation levels, thereby altering host tissue gene expression (Stoll et al., 2018). 3.3 Functions of non-coding RNA In addition to the regulation of DNA and chromatin levels, the epigenetic mechanism of RNA levels cannot be ignored. Among them, small molecule non-coding RNA and long-chain non-coding RNA are research hotspots. MiRNA is a small class of about 20 to 24 nucleotides in length. It does not encode proteins and mainly mediates mRNA degradation or blocks translation through complementary binding to the 3’ non-translational region of mRNA (Dunislawska et al., 2022). Each miRNA can regulate the expression of multiple target genes, so miRNAs are involved in the fine regulation of the developmental process. In avians, many miRNAs have been found to be associated with muscle proliferation and differentiation, fat metabolism, and feather development. Studies have confirmed in chicken embryo muscle that overexpression of miR-27b can downregulate the MSTN gene, significantly accelerating myogenocyte proliferation and delaying differentiation (Zhang et al., 2021). In ducks, although specific miRNA function is less studied, it has been reported that miRNA expression profiles in muscle tissues of different developmental stages of duck embryos have been significantly changed, and some miRNAs may act as "promoting factors" or "inhibitor factors" to affect muscle growth. On the other hand, lncRNAs usually have a length of more than 200 nt and do not encode proteins, but can function in a variety of ways, including acting as competitive endogenous RNA sponges to bind miRNA, acting as transcriptional co-regulators, inducing three-dimensional conformational changes in chromatin, etc. lncRNA can also affect muscle and fat development. There have been studies that have identified hundreds of differentially expressed lncRNAs in primitive germ cells in early chicken embryos, which may regulate the development of gonads and germ cells. 4 Epigenetic Regulation in Duck Growth and Development 4.1 Epigenetic regulation of key growth genes The growth rate and body size of animals are determined by a series of growth axis hormones and growth factor genes, such as growth hormone gene (GH), growth hormone receptor gene (GHR), islet-like growth factor 1 gene (IGF1), etc. The expression of these key genes is not only controlled by regulatory elements on the DNA sequence, but also by epigenetic mechanisms. In ducks, researchers found that differences in growth performance between different breeds and individuals are often accompanied by epigenetic differences in these genes (Xu et al., 2022). A comparison of genome-wide methylation of Shaoxing duck-protection population and breeding population found that 35 differential methylated genes are closely related to production traits such as meat and egg production, including key genes involved in growth and metabolism. This suggests that epigenetic variations may accumulate and fix during long-term breeding, resulting in line differences. In addition to the genetic background, the methylation dynamics of the promoters of endogenous genes in different growth periods will also affect gene expression. IGF1 is a key factor promoting growth. Studies have observed that the methylation level of the IGF1 gene promoter in duck liver changes during the transition stage from the end of embryonic development to the post-hull outbreak, which has an impact on endocrine regulation of post-natal growth. Under the influence of external factors, the epigenetic status of the growth gene will also change. 4.2 The epigenetic mechanism of muscle growth and fat deposition Skeletal muscle and adipose tissue are important economic traits that affect the production performance of meat ducks, and the growth and development of both are also deeply regulated by epigenetics. In terms of muscle growth, dynamic changes in DNA methylation are highly correlated with the muscle development stage. The study combined genome-wide methylation sequencing and transcriptome analysis to screen out differential methylated regions (DMRs) and genes during skeletal muscle development in ducks during embryonic period, and found that they are enriched in key signaling pathways for muscle formation. This shows that during the process of duck embryonic muscle cells turning from proliferation to differentiation, specific genome sites undergo methylation status changes, which in turn regulates downstream gene networks and affects the formation and
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