Expression and Regulation of FGF9 Gene in Chicken Ovarian Follicles and Its Genetic Effect on Laying Traits in Hens

Objectives: Fibroblast growth factor 9 (FGF9), a crucial member of the FGF family, functions as an intercellular signaling molecule involved in angiogenesis, embryogenesis, and tissue repair. Our previous study demonstrated that FGF9 expression in chicken hierarchical granulosa cells (Post-GCs) is regulated by LSD1 Ser54 phosphorylation and that FGF9 promotes cell proliferation. This study aims to analyze the expression and regulation of the FGF9 gene in chicken ovarian follicles and its genetic effect on laying traits in hens. Methods: Chicken FGF9 mRNA expression patterns were examined by real-time quantitative PCR (RT-qPCR). Detection of single nucleotide polymorphisms (SNPs) was performed using PCR amplification and Sanger sequencing. Transcription activity was compared using dual-luciferase reporter assay. Results: Following follicle selection, chicken FGF9 expression significantly decreased in granulosa cells (p < 0.05) while it increased in theca cells (p < 0.05). Hormonal treatments revealed differential regulation; estradiol and FSH downregulated FGF9 in both pre-hierarchical and hierarchical granulosa cells (p < 0.05), whereas progesterone exhibited opposing effects, suppressing expression in pre-hierarchical granulosa cells (Pre-GCs) but stimulating its expression in Post-GCs (p < 0.05). In theca cells, estradiol consistently inhibited FGF9 expression (p < 0.05), while FSH only affected FGF9 expression in pre-hierarchical follicles. Six SNPs in the promoter region (g.−1965G>A, g.−2177G>A, g.−2289G>A, g.−3669A>G, g.−3770A>G, g.−3906G>A) were identified, five of which (g.−1965G>A, g.−2177G>A, g.−2289G>A, g.−3669A>G, g.−3906G>A) showed significant associations with egg production traits. Notably, alleles A (g.−2289), G (g.−3669), and A (g.−3906) enhanced the transcription activity of chicken FGF9 in Pre-GCs. Conclusions: These findings provide novel insights into the expression pattern and regulatory mechanisms of chicken FGF9 during follicular development and identify some genetic markers for egg-laying traits in chickens.