Short Communication: QSOX1 promotes first egg weight in indigenous chickens by supporting isthmus epithelial viability and ovalbumin accumulation.

First egg weight is an economically important trait in indigenous chickens, but its molecular regulation remains unclear. We tested whether a missense variant in quiescin Q6 sulfhydryl oxidase 1 (QSOX1; g.8_5992660 T>C, p.Ser291Gly) was associated with egg traits and whether QSOX1 influenced oviduct epithelial function. Among 450 Huainan Partridge hens, 375 were TT and 75 were TC; the C-allele frequency was 0.08 and the population conformed to Hardy-Weinberg equilibrium (P = 0.16). TC hens had heavier first eggs, greater albumen and thick-albumen weights, and greater eggshell weight than TT hens (P < 0.05). QSOX1 expression was greatest in the isthmus and was higher in TC than TT hens at the mRNA, protein, and immunohistochemical levels (P < 0.05). In primary isthmus epithelial cells, QSOX1 overexpression increased CCK-8 absorbance at 24 h, altered viability -associated transcripts, and reduced apoptosis, whereas QSOX1 knockdown produced the opposite responses. In a transwell co-culture model, QSOX1 overexpression in isthmus cells increased ovalbumin protein accumulation and upregulated COL6A1 and SEC31A expression. These findings support the hypothesis that QSOX1 contributes to first egg weight by maintaining isthmus epithelial viability and favoring egg-white protein accumulation.