ObjectiveCandidate genes related to the regulation of litter size traits in English Springer Spaniel breeding bitches are explored, and the genetic mechanism underlying fertility in this breed is investigated, in order to provide reference molecular markers for genomic selection of high fecundity. MethodsWhole genome resequencing was performed on English Springer Spaniel breeding bitches that had given birth to at least 3 litters, and the bitches were divided into a high-litter-size group and a low-litter-size group according to the average litter size. Selection signal analysis was used to obtain the intersection of fixation index (F_st) and nucleotide diversity (Pi) signals as highly selected regions, and candidate genes were screened based on gene annotation and functional enrichment analysis. ResultsThe average litter size in the high-litter-size group (7.41±1.27) was significantly higher than that in the low-litter-size group (3.82±1.20) (P＜0.05), and the total number of live offspring in the high-litter-size group (7.06±1.10) was extremely significantly higher than that in the low-litter-size group (3.67±1.11) (P＜0.01). A total of 3 155 706 SNPs were detected in the two groups, 63.09% of which were located in intergenic regions, 33.96% in intronic regions, and 0.38％, 0.57％, and 0.09％ in exonic regions, 3' untranslated regions (UTRs), and 5'UTRs, respectively. Among the SNPs in exonic regions, 5 256 were nonsynonymous variants, accounting for 43.55%. A total of 1 752 differential genes were identified after annotation screening. Gene ontology （GO） and Kyoto Encyclopedia of Genes and Genomes （KEGG) enrichment analyses identified 13 candidate genes that may affect reproductive performance and litter size traits, including WDR35, SMAD7, RPGR, RERGL, PGRMC2, LOC482182, GIMD1, COX7B2, COX16, BMPR2, BMP6, BICD1, and SLC9C1. Their functions mainly involve reproductive hormone regulation, embryonic development, GTPase activation, and oocyte apoptosis. ConclusionEnglish Springer Spaniel breeding bitches have undergone significant artificial selection for litter size traits. These 13 candidate genes play key roles in oocyte maturation and regulation during early pregnancy, providing a new molecular basis for elucidating the genetic mechanism of canine reproductive traits.