Abstract: To reveal the molecular mechanism underlining the caudal vertebra formation,the genome-wide selection signal,the related genes of Mongolian sheep tail length traits and the possible mutation sites were detected by comparing the genetic differentiation level of Mongolian sheep population with different numbers of tail vertebrae.Based on whole genome resequencing data,the selection signal of caudal vertebra was screened with the genetic differentiation coefficient(Fst)and genetic diversity ratio(pi ratio)between the two sheep groups.A chromosome region with higher Fst and pi ratio value were selected as a candidate region.A total of 76 selected regions were found ultimately,which belonged to the QTL of tail fat deposition and skeletal development.GO and pathway analysis of the 63 genes contained in these candidate regions found the enrichment of Wnt and FGF signaling pathways related to bone regeneration,and the functional candidate genes such as LRP6 were also found.In summary,we identified the selected regions associated with the number of caudal vertebrae,suggesting that the formation of Mongolian sheep populations with different numbers of caudal vertebrae may be caused by one or more mutations in non-gene coding regions.