Transcriptome Analysis on Single Small Yellow Follicles Reveals That Wnt4 Is Involved in Chicken Follicle Selection

Abstract

Ovarian follicle selection is an important process impacting the laying performance and fecundity of hens, and is regulated by follicle-stimulating hormone (FSH) through binding to its receptor [follicle-stimulating hormone receptor (FSHR)]. In laying hens, the small yellow follicle (6-8 mm in diameter) with the highest expression of FSHR will be recruited into the preovulatory hierarchy during ovarian follicle development. The study of molecular mechanism of chicken follicle selection is helpful for the identification of genes underlying egg-laying traits in chicken and other poultry species. Herein, the transcriptomes of chicken small yellow follicles differing in the mRNA expression of <i>FSHR</i> were compared, and a total of 17,993 genes were identified in 3 pairs of small yellow follicles. The Wnt signaling pathway was significantly enriched in the follicles with the greatest fold change in <i>FSHR</i> expression. In this pathway, the expression level of <i>Wnt4</i> mRNA was significantly upregulated with a log₂(fold change) of 2.12. We further investigated the expression, function, and regulation of <i>Wnt4</i> during chicken follicle selection and found that <i>Wnt4</i> mRNA reached its peak in small yellow follicles; <i>Wnt4</i> stimulated the proliferation of follicular granulosa cells (GCs), increased the expression of <i>StAR</i> and <i>CYP11A1</i> mRNA in prehierarchical and hierarchical follicles, increased the expression of <i>FSHR</i> mRNA, and decreased the expression of anti-Müllerian hormone and <i>OCLN</i> mRNA. Treatment with FSH significantly increased <i>Wnt4</i> expression in GCs. Moreover, <i>Wnt4</i> facilitated the effects of FSH on the production of progesterone (P4) and the mRNA expression of steroidogenic enzyme genes in the GCs of hierarchical follicles, but inhibited the effects of FSH in the GCs of prehierarchical follicles. Collectively, these data suggest that <i>Wnt4</i> plays an important role in chicken follicle selection by stimulating GC proliferation and steroidogenesis. This study provides a theoretical basis for improving the egg-laying performance of chicken and a reference for the elucidation of the molecular mechanism of follicular selection in mammals.