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The hypothalamic-pituitary-adrenal (HPA) axis and hypothalamic-pituitary-gonadal (HPG) axis plays a central role in mediating physiological responses related to the reproductive system under any stressful condition. However, the molecular mechanism underlying the effects of stress on physiology still needs to be elucidated. This study demonstrated that increasing the stocking density from 4 to 8 birds/m2 during the laying period decreased the egg production rate of laying ducks by 13.04 − 63.55% and feed intake by 7.40 − 23.44%. Transcriptome analysis between high- and low-feeding-density laying ducks revealed 469, 509, 428 and 210 differentially expressed genes (DEGs) in the hypothalamus, pituitary, ovary and follicular membrane, respectively. Gene ontology (GO) and KEGG enrichment analysis showed that the DEGs in the hypothalamus and pituitary were primarily enriched in the biostimulation and dopamine secretion pathways. The major enrichment pathways in the ovarian and follicular membranes involved lipid metabolism, negative regulation of inflammatory response, and steroid hormone biosynthesis. Among the DEGs in the HPG system, POMC and GnRH1 were identified, which may be manifesting their crucial roles in regulating the stress response and reproduction. Our data showed that a high stocking density as environmental stress negatively affects the reproductive performance in ducks through transcriptional changes in the HPG axis.Highlights Raising the stocking density from 4 to 8 birds/m2 decreased the egg production rate and feed intake in laying ducks. The transcriptome indicated that stocking density affects the stress response of laying ducks through the hypothalamus-pituitary. The stress signals are subsequently transmitted to affect gene expression related to the reproduction process in laying ducks’ ovarian and follicular tissues. The hypothalamic expression of POMC and GnRH1 may play a central role in integrating stress signals and the reproductive processes of laying ducks.

The eggshell colour is related to the biological functions of birds, and the colour of poultry eggshells can affect consumers' choices. This study explored the difference in the metabolite composition of duck eggshells to screen the key substances that affect the eggshell colours. The green and white duck eggshells were selected for non-targeted metabolomics analysis. We screened 402 and 512 differentially expressed metabolites in the negative and positive ion modes, respectively. Among them, 40 differentially expressed metabolites were annotated with specific names and related functions, of which the expression levels of 8 metabolites showed extremely significant differences. They were 2-heptanone, 12-hydroxydodecanoic acid, D-fructose, dodecanedioic acid, L-leucine, methyl jasmonate, palmitoleic acid, and styrene oxide. Additionally, the annotated differentially expressed metabolites were enriched in 33 metabolic pathways, including aminoacyl-tRNA biosynthesis, amino acid metabolism, galactose metabolism, etc. The results showed that the expression of metabolites between green and white eggshells differed in ducks; among metabolites with extremely significant differences in expression, the expression level of L-leucine in green eggshells was higher than that in ducks white eggshells. Therefore, we speculated that the increased expression of L-leucine promoted the response of related metabolic pathways, enhanced the expression of antioxidants, and changed the eggshell colours.Highlights The metabolites of green and white shells were different. 40 DEMs were annotated with specific names and related functions. The annotated DEMs were enriched in 33 metabolic pathways.

Eggs are a major animal protein source for humans. Egg formation is a complex process involved in coordinating various tissues and organs, while blood is necessary for gonad differentiation and egg formation. In this study, non-targeted metabolomics technology was employed to determine the serum metabolites at the time point of 70 days (before laying period, T0), 130 days (pre-laying period, T1), 300 days (peak laying period, T2), and 340 days (post laying period, T3) in female ducks, to provide comprehensive metabolic profiling in reflecting the overall temporal changes. The results showed significant differences in the metabolites of different comparison groups (T0 vs. T1, T2 vs. T3, and T0 & T1 vs. T2& T3), supporting the main physiological responses in ducks are differential among laying periods. L-Aspartic Acid, Biotin, and Thyroxine can be considered the potential biomarkers related to sexual maturity. L-Lysine, Lumichrome, Thiamine, Riboflavin, and Pyridoxine may be biomarkers related to egg formation. Sorbitol, Isorhamnetin, Naringenin, and Hydroxykynurenine were the biomarkers associated with the decline of egg production. In addition, the Kyoto Encyclopaedia of Genes and Genomes (KEGG) pathway analysis showed that sexual maturity was mainly involved in the biosynthesis of secondary plant metabolites, Tricarboxylic Acid Cycle, Mineral absorption, pentose phosphate pathway, and Oxocarboxylic acid metabolism. Egg formation disturbed amino acid metabolism, vitamin metabolism, and carbohydrate metabolism. The neuroactive ligand-receptor interaction pathway may affect egg production. These findings can detect the different physiological statuses in female ducks and provide a theoretical basis for increasing egg production, and ensuring sustainable farming.Highlights The main physiological responses in ducks are differential among laying periods. 3, 6, and 4 biomarkers related to sexual maturity, the formation of the egg, and the decline of egg production, respectively. Sexual maturity was mainly involved in the biosynthesis of secondary plant metabolites, Tricarboxylic Acid Cycle, Mineral absorption, pentose phosphate pathway, and Oxocarboxylic acid metabolism, and egg formation disturbed amino acid metabolism, vitamin metabolism, and carbohydrate metabolism. while the neuroactive ligand-receptor interaction pathway may affect egg production.

In this study, we investigated the role of liver X receptor (LXR) in regulating cholesterol concentration in goose primary hepatocytes. Intracellular and extracellular cholesterol concentration, mRNA expression levels of genes related to phosphatidylinositol 3- kinase (PI3K) pathway, sterol regulatory element- binding protein 2 (SREBP-2), and 3- hydroxy-3-methylglutaryl-CoA reductase (HMGR) were measured in primary goose hepatocytes. We found that the intracellular cholesterol concentration showed an uptrend manner when the TO901317 concentration was under 1 μmol/L; compared with 1 μmol/L TO901317, 10 μmol/L TO901317 had an inhibiting effect (PSREBP-2 and HMGR gene expression is similar with that of intracellular cholesterol concentration. These data suggested that LXR activation may stimulate the cholesterol biosynthesis by activating expression of SREBP-2 and HMGR. Interestingly, the mRNA levels of genes related with PI3K pathway increased in the presence of TO901317, which suggested that LXR activation could promote PI3K signalling activity.