Animal Molecular Breeding, 2025, Vol.15, No.2, 60-71 http://animalscipublisher.com/index.php/amb 68 such as DNMT1 and DNMT3A in the early embryo (1 to 10 days) and enhance the activity of DNA methyltransferase. This alteration of enzyme activity means that embryonic DNA methylation may increase, especially in certain gene promoter regions. Accordingly, these altered gene expression programs may affect the phenotype after incubation. For example, duck chicks in the high-temperature treatment group tend to be smaller when they emerge from the shell, with a decrease in growth potential, and may have better heat tolerance. This is because the methylation changes induced by embryos inhibit some growth-promoting genes and activate some stress-responsive genes (Massimino et al., 2021). Temperature also affects the epigenetics of feather development. Conversely, appropriate temperature reduction (cold stimulation) is considered beneficial in some cases: studies have shown that transient cryogenic treatment during the embryo phase can reduce metabolic rates and fat deposition in avian offspring, which may involve epigenetic reprogramming of metabolic regulatory genes, such as increased demethylation of the promoter of PGC-1α gene leads to upregulation of its expression and promotes lipid utilization (Gajendran and Veeramani, 2022). 7.1.2 The epigenetic effect of humidity and light intervention on embryonic development In addition to temperature, hatching humidity and light conditions are also important components of the embryonic environment and may also affect duck embryo development through epigenetic mechanisms. Humidity mainly affects the weight loss rate and air chamber size of the embryo egg, thereby affecting the respiration and metabolism of the embryo. Under low humidity environments, embryos will be in mild hypoxia and dehydrated states, which may activate the stress response genes of the embryo. The promoters of some hypoxia-responsive genes (such as HIF1A) may undergo demethylation under hypoxic conditions, allowing stronger expression to aid embryo adaptation (Dunislawska et al., 2022). Although this improves hatching or stress resistance, it may also change the development process and affect feather growth. Light is another external signal. Traditional hatching is mostly conducted in the dark, but studies have shown that fertilized eggs do not respond to light during hatching. Especially in the late incubation stage, light can pass through the eggshell to a certain extent to affect the embryo's circadian rhythm and pineal gland development. If a regular light-dark cycle stimulation is given in hatching, the biological clock genes of duck embryos may be "programmed". The promoters of these genes (such as Clock, BMAL1) and related clock-controlled gene regulatory elements may undergo epigenetic changes, resulting in different behavioral and metabolic patterns of ducklings after birth. 7.1.3 Programming effect of nutritional supplementation in embryonic stage on the later representative obsogenetics Duck embryos rely entirely on nutrients inside the breeding eggs for development. The yolk and protein provided by the mother not only contain a large amount of protein, lipids and vitamins, but also contains some cofactors required for epigenetic regulation (such as methyl donors). Therefore, the nutritional composition of female duck feed directly affects the epigenetic status of the embryo. For example, levels of folic acid, choline, etc. in maternal diets will affect the basal level of embryonic DNA methylation (Nie et al., 2019). In addition to direct nutrient replenishment of embryos through the mother is also a hot topic in research. Injecting specific nutrients (such as sugars, amino acids, vitamins) into eggs during the mid-embryo period can improve the growth and intestinal development of ducklings after hatching (Bodin et al., 2019). The mechanism behind it can be partially attributed to epigenetics. Intraocular arginine supplementation may activate the embryonic mTOR pathway, thereby promoting muscle protein synthesis by changing the histone acetylation status of downstream metabolic genes. Although this type of epigenetic regulation is indirect, it cannot be ignored. The application of some plant extracts and functional additives in the embryonic stage may also bring about epigenetic effects. It should be emphasized that the epigenetic programming role of nutritional supplementation in embryonic stage is "double-edged". Excessive nutrition may lead to early development of embryonic adipose tissue, and offspring have "metabolic memory" and prefer fat deposition; insufficient nutrition may lead to slow growth of offspring through epigenetic epigenetics. Therefore, nutritional regulation should be carried out based on specific goals. For example, to cultivate lean meat ducks, yolk lipid utilization can be moderately limited during the embryo, which promotes increased metabolic rate in offspring - this can be achieved by giving mild stress or metabolic stimulation in the embryo, but should be controlled within a safe range to avoid negative effects.
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