Polymorphism, expression and structure analysis of key genes in the ovarian steroidogenesis pathway in sheep (Ovis aries).

Background: Litter size is an important factor that significantly affects the development of the sheep industry. Our previous TMT proteomics analysis found that three key proteins in the ovarian steroidogenesis pathway, STAR, HSD3B1, and CYP11A1, may affect the litter size trait of Small Tail Han sheep.

Objective: The purpose of this study was to better understand the relationship between polymorphisms of these three genes and litter size.

Material And Method: Sequenom MassARRAY detected genetic variance of the three genes in 768 sheep. Real-time qPCR of the three genes was used to compare their expression in monotocous and polytocous sheep in relevant tissues. Finally, bioinformatics analysis predicted the protein sequences of the different SNP variants.

Result: Association analysis showed that there was a significant difference in litter size among the genotypes at two loci of the CYP11A1 gene (p < 0.05), but no significant difference was observed in litter size among all genotypes at all loci of the STAR and HSD3B1 genes (p > 0.05). However, STAR expression was significantly different in polytocous and monotocous sheep in the pituitary (p < 0.01). Tissue-specific expression in the ovary was observed for HSD3B1 (p < 0.05), but its expression was not different between polytocous and monotocous sheep. Bioinformatics analysis showed that the g.33217408C > T mutation of CYP11A1 resulted in major changes to the secondary and tertiary structures. In contrast, gene polymorphisms in STAR and HSD3B1 had minimal impacts on their protein structures.

Discussion: This may explain why the CYP11A1 variant impacted litter size while the others did not. The single nucleotide polymorphism of the CYP11A1 gene would serve as a good molecular marker when breeding to increase litter size in sheep. Our study provides a basis for further revealing the function of the ovarian steroidogenesis pathway in sheep reproduction and sheep breeding.