Study on the Function of ID2 Gene in Granulosa Cells of Ovaries of Hetian Sheep and Its Correlation Analysis with Lambing Traits

Reproductive efficiency is a key determinant of sheep productivity, yet Hetian sheep remain limited by relatively low fecundity despite their adaptability to harsh environments. The inhibitor of DNA binding 2 (ID2) gene is known to regulate cell proliferation and differentiation, but its specific role in sheep reproduction is not well understood. This study aimed to characterize the <i>ID2</i> gene in Hetian sheep and to assess its functional association with ovarian granulosa cells and litter size. The coding sequence of ovine <i>ID2</i> was cloned and analyzed using bioinformatics tools. Tissue-specific expression patterns were measured by quantitative PCR at different pubertal stages. A total of 157 ewes were genotyped to identify single-nucleotide polymorphisms (SNPs) and their association with litter size. Functional studies were performed by lentiviral overexpression of ID2 in granulosa cells, with effects evaluated using CCK-8 proliferation assays, ELISA for hormone secretion, and RT-qPCR for related gene expression. <i>ID2</i> was highly expressed in the ovary, particularly during puberty. Four SNPs (g.18202368 A>T, g.18202372 G>A, g.18202431 G>C, g.18202472 G>C) were significantly associated with increased litter size. Overexpression of ID2 promoted granulosa cell proliferation, increased progesterone, decreased estradiol, and altered expression of key genes in the TGF-β/BMP-SMAD signaling pathway. The <i>ID2</i> gene plays a crucial role in ovarian function and reproductive regulation in Hetian sheep. Its polymorphisms and functional impact on granulosa cells suggest that ID2 is a promising candidate gene for marker-assisted selection to improve reproductive efficiency in sheep.