Incubation temperature affects yolk utilization through changes in expression of yolk sac tissue functional genes

The yolk sac tissue (YST) is a multifunctional metabolic organ supporting chicken embryonic development. This study examined whether incubation temperatures (ITs) affect YST functions. For this purpose, 300 eggs were assigned to 3 groups and incubated at control IT of 37.8°C, at 1.5°C below, 36.3°C (cold IT), and at 1.5°C above, 39.3°C (hot IT). For each group, 6 embryos' whole body mass and residual yolk (RSY) weights were recorded during incubation, and YST was sampled for both histology and gene expression analysis. YST functionality during incubation was examined by regression analysis, comparing changes in expression patterns of genes involved in lipid uptake and metabolism (LRP2, ApoA1), oligopeptides uptake (PepT1), gluconeogenesis (FBP1), glycogenesis (GYS2), and thyroid hormones regulation (TTR, DIO1, DIO2). Results show that hot and cold ITs affected YST gene expression and yolk utilization. PepT1 expression decreased towards hatch, in both hot and cold ITs, while in the Control IT, it reached a plateau. ApoA1 and DIO2 expression showed a moderate linear fit compared to polynomial fit in the control. GYS2 expression had no change along incubation, while in the control IT, it showed a polynomial fit. Expression of LRP2, FBP1, and DIO1 genes was affected by either cold or hot IT's. TTR expression patterns were similar in all IT groups. The variations in gene expression patterns observed in the 3 ITs can explain the changes in yolk utilization, an important parameter for hatchling quality. While the control IT showed optimal utilization, with an RSY value of 11.12% at the day of hatch, the cold and hot IT groups exhibited lower utilization with an RSY value of 18.18 and 29.99%, respectively. These findings are the first to show that ITs change the expression of key YST genes, leading to variations in yolk utilization by the embryo.