Candidate genes for litter size in Xinjiang sheep identified by Specific Locus Amplified Fragment (SLAF) sequencing

The aim of this study was to investigate the selection signatures at a genome-wide level in 'Pishan' sheep using Specific Locus Amplified Fragment (SLAF)-seq. Blood samples from 126 ewes were sequenced using SLAF tags, and the ovarian tissues from 8 ewes (Bashbay sheep, a single litter size group (SG group); 'Pishan' sheep, double litter size group (DG group)) were collected to detect expression levels by quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR). Selection signature analysis was performed using global fixation index (Fst) and nucleotide diversity (π) ratio. A total of 1,192,168 high-quality SLAFs were identified. Notably, 2380 candidate regions under selection using two approaches were identified. A total of 2069 genes were identified, which were involved in dopaminergic synapses, thyroid hormone synthesis, ovarian steroidogenesis and thyroid hormone signalling pathways. Furthermore, Growth Differentiation Factor 9 (GDF9), Period Circadian Regulator 2 (PER2), Thyroid Stimulating Hormone Receptor (TSHR), and Nuclear Receptor Coactivator 1 (NCOA1) reside within these regions and pathways. The expression levels of GDF9 and PER2 genes in sheep tissue of the DG group were significantly higher than those in the SG group. These genes are interesting candidates for litter size and provide a starting point for further identification of conservation strategies for 'Pishan' sheep.