Genome-wide association study based on gene set enrichment method (pathway analysis) associated with litter size at birth in Zandi sheep

Reproductive traits (especially litter size at birth) is one of the most important economic traits in sheep breeding systems. The present study aimed to conduct a genome wide association studies based on Gene-set enrichment analysis for identifying the genomic region, candidate genes and biological pathways associated with Litter size in Zandi sheep breed. 

Prolific ewes with at least one twining record and others with only singleton records were genotyped using the medium-density Illumina Ovine SNP50 array. The gene set analysis consists basically in three different steps: the assignment of SNPs to genes, the assignment of genes to functional categories, and finally the association analysis between each functional category and the phenotype of interest. Genome-wide association study for litter size evaluated using Plink software method case-control. Using the biomaRt2 R package the SNP were assigned to genes. Subsequently, gene enrichment analysis was performed with the goseq R package and bioinformatics analysis was implemented to identify the biological pathways performed in GO, KEEG, DAVID and PANTHER databases. 

We identified different sets of candidate genes related to litter size: AFP, FOXO3, ESR1, ESR2, RBP4, AURKA, DLG1 and ZGLP1 in Zandi sheep. According to pathway analysis, 11 pathways were associated with the litter size trait. Among biological pathways, the Oocyte differentiation, Ovulation from ovarian follicle, Estrogen signaling pathway and Positive regulation of peptide hormone secretion pathways have significant association with ovulation rate and ovarian steroidogenesis traits. 

Considering, this study supported previous results from GWAS of  litter size, also revealed additional regions in the sheep genome associated with these economically important traits, presented here should be contribute to a better understanding of the genetic control of litter size in sheep and using these findings can accelerate the genetic progress in sheep breeding programs.