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Yolk formation in liver is an important process for egg production in hens. The correlations between egg laying rate decline and liver function changes in Guangxi Ma chickens remain unclear. In this study, a total of 21,750 genes and 76,288 transcripts were identified in the RNA expression profiles isolated from liver tissues of 5 groups of Guangxi Ma chickens divided according to the age and egg laying rate. Numerous differential genes (DEGs) were identified after pairwise comparison among samples, and time series analysis categorization (age-related factors) revealed that down-regulated DEGs with aging were predominantly involved in lipid transportation and metabolic processes in the low egg laying rate groups. Notably, functional enrichment analysis confirmed that DGAT2, LIPG, PNPLA2, LPL, CEL, LIPC, DGKD, AGPAT2, AGPAT1 and AGPAT3 were highlighted as hub genes in glycerolipid metabolism pathway, which may be an essential non-age related factors of egg laying rate by regulating the synthesis of triacylglycerol (TAG) in liver. Finally, we categorized DEGs in Guangxi Ma chickens with different egg laying rate caused by age-related factors and found that DEGs with different expression patterns performing different biological functions. The analysis of DEGs with lower egg laying rate caused by non-age related factors and showed that the transportation of TAG was suppressed. Furthermore, critical genes and pathways involved in the synthesis of TAG in livers were identified, which dynamically regulated the formation of yolk precursors. Our results expanded the knowledge of the molecular mechanisms of the yolk precursor synthesis in chicken livers. The results will be helpful to explore the factors that affect egg laying rate from the perspective of yolk synthesis and provide a theoretical basis for improving the egg production of Guangxi Ma chickens., (Copyright © 2022 Elsevier B.V. All rights reserved.)

During the hierarchical development of follicles in laying hens, a large number of small white follicles (SWFs) undergo atresia caused by apoptosis rather than growing into large white follicles (LWFs), which reduces the number of follicles developing into hierarchical preovulatory follicles and results in declines in egg production. Therefore, figuring out factors responsible for SWFs atresia is necessary to improve egg production of laying hens. Endoplasmic reticulum stress (ERS) is one of the possible causes of cell apoptosis. However, the association between SWFs atresia and ERS in hens is yet to be answered. In this study, cell proliferation and apoptosis, expression levels of proteins and genes related to ERS, and cell ultrastructure were compared to elucidate whether ERS occurred in atretic small white follicles (ASWFs). Subsequently, a tunicamycin-induced ERS model and a serum withdrawal-induced atresia model of SWFs in vitro were established to investigate whether ERS could lead to follicular atresia. The results showed that the cell apoptosis increased significantly while the cell proliferation decreased evidently in ASWFs compared with SWFs. Meanwhile, swollen and vacuolization deformation of ERs was observed in the GCs but not TCs of ASWFs. Results of experiments in vitro demonstrated that treatment with serum withdrawal or tunicamycin inhibited cell proliferation and promoted cell apoptosis in SWFs, and the adverse effects of tunicamycin on SWFs manifested a dose-dependent increase. In comparison with the controls, treatment with serum withdrawal or tunicamycin upregulated expression levels of proteins and genes in ERS pathway. Moreover, the dose of 5 μg/mL tunicamycin seemed to be the optimal concentration for establishing the tunicamycin-induced ERS model. Meanwhile, the results of cell ultrastructure observation showed that ERs in GCs of SWFs of serum withdrawal group and treatment with 5 μg/mL tunicamycin group became swollen, dilatation and vacuolization. In conclusion, the data of this study indicated that ERS is involved in SWF atresia in laying hens, and ERS in GCs might be more severe than that in TCs., Competing Interests: Declaration of competing interest The authors declare that no conflict of interest could be perceived as prejudicing the impartiality of the research reported., (Copyright © 2022 Elsevier Inc. All rights reserved.)

Egg yolk formation and deposition are quite vital for oocyte maturation and egg production in hens. There are great differences in egg production and number of preovulatory follicles among individuals in Guangxi Ma chickens at the same or various ages, however, the causes of this phenomenon remain unclear. This study aimed to elucidate the differences in yolk precursors formation among Guangxi Ma hens at the same or various ages with dissimilar egg laying rate via comparing the synthesis and transportation of yolk precursors, serum estradiol (E 2 ) levels, as well as the antioxidant capacity of the liver and serum from hens of 32-week-old (32W), 50-week-old with higher laying rate (50WH), 50-week-old with lower laying rate (50WL), 72-week-old with higher laying rate (72WH), and 72-week-old with lower laying rate (72WL). The results showed that the contents of triglyceride (TG), total cholesterol (T-CHO), high density lipoprotein cholesterol (HDL-C) and very low density lipoprotein cholesterol (VLDL-C) in the liver, serum and ovarian stroma, serum E 2 levels, expression levels of proteins and genes related to yolk precursors formation, and the antioxidant capacity of liver and serum decreased significantly during aging process. TG, HDL-C and VLDL-C contents in serum and ovarian stroma, as well as HDL-C and VLDL-C levels in liver tissues of hens in 50WL and 72WL were significantly lower compared to hens in 50WH and 72WH, respectively. Meanwhile, expression levels of estrogen receptor ɑ and transcription levels of genes related to lipid transportation in liver tissues, and the antioxidant capacity of livers were remarkably lower in hens in 50WL and 72WL than those in 50WH and 72WH, respectively. However, no significant difference was detected in liver TG levels, serum E 2 levels, genes related to fatty acids synthesis and serum antioxidant capacity between hens in 50WL and 50WH. In conclusion, the age-related decline of egg production in Guangxi Ma chicken maybe related to the age-related decreases in the synthesis and transportation of liver yolk precursor as well as the antioxidant capacity of liver and hens. And the non-age related decline of egg production may mainly attribute to the decreases in yolk precursors transportation ability and hepatic antioxidant capacity., Competing Interests: Declaration of competing interest The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported., (Copyright © 2022 Elsevier Inc. All rights reserved.)