SMAD proteins are a family of intracellular signalling proteins that are involved in transmitting signals from the cell surface to the nucleus of the cell, where they regulate the expression of target genes involved in a wide range of cellular processes [1]. SMAD proteins are a family of intracellular proteins that play a crucial role in signal transduction in the transforming growth factor beta (TGF-β) signalling pathway, which is critically important in many physiological and pathological processes, including embryonic development, wound healing, tissue repair, and cancer progression [2]. When a TGF-beta ligand binds to its receptor on the cell surface, it can trigger a signalling cascade which leads to the activation of SMAD proteins [3]. Furthermore, SMAD2 is a receptor-regulated SMAD (R-SMAD) protein activated by TGF-beta family ligands, such as TGF-β and activin. Smad2 mainly participates in the proliferation of osteoblasts as a downstream signal of TGF-β [4]. The TGF- β/SMAD signaling pathway regulates many aspects of female reproduction. In mouse preimplantation embryos, there is a Smad-dependent self-regulatory loop, indicating the role of the TGF-β signalling pathway. Prior to implantation, TGF-β signalling is active in mouse embryos [5]. SMAD2 immunolabeling was used to identify transcription factors and TGF- β signalling mediators, and it was discovered that small follicle granulosa cells have a distinct pattern of expression. SMAD2 was found in the nuclei of primordial follicles but was mostly absent in early developing follicles [6]. Smads are also required for embryonic development. Besides, ovarian cell proliferation and differentiation rely on the SMAD2 protein [7].

Genome-wide selective scanning analysis of chromosome 24 in adult goats revealed that SMAD2 was candidate gene for high fertility [8]. Our group recently discovered that SMAD2 indels are associated with litter size [7] and growth traits [9] in Shaanbei white cashmere goats. In addition, SMAD2 gene promotes the effect of follistatin on blastocyst growth during early embryonic development [10]. SMAD2 has associated within total number born, and number born alive piglets [11]. In a prior investigation, it was discovered that SMAD2 mRNA expression was different in all tissues of the goat, including the ovaries [7]. Interestingly, SMAD2 is also involved in male reproduction. Reproductive disorders caused by abnormal SMAD expression include infertility, immotile sperm, and failure of germ cell differentiation [[12], [13], [14]]. SMAD2 is a protein found in the fetal testis that influences male germ cell development, which is dormant until shortly after birth, when it initiates the process of spermatogenesis, which eventually gives rise to mature spermatozoa in adults [15].

Genomic DNA is a key part of the development of phenotypic traits because it carries genetic information. Genomic variation is the primary driver of phenotypic diversity [[16], [17], [18]]. The newly discovered type of genomic structural variation known as copy number variations (CNVs) manifests as deletions or insertions of large-scale fragments of genomic DNA, with sizes ranging from 50 bp to 1 Mb [19,20]. Compared with SNP, CNV has wider coverage and is easier to detect. In livestock, such as pigs and cattle, it has been shown that their phenotypic traits are affected by CNVs. CNV has potential applications in animal health, evolutionary biology, and selective breeding. For examples, CNVs of SMAD2 are associated with skin cancer [21] and breast cancer [22]. In addition, different CNVs, e.g. in the PPP3CA gene, have been found to affect goat litter size [23].

Shaanbei white cashmere (SBWC) goats are a native Chinese breed with high fertility and continuous estrus throughout the year. It is worth to exploring the role that important genes play in determining litter size and sperm characteristics of SBWC goats. However, there are no reports on the distribution of SMAD2 gene CNV and its genetic effects on female and male fertility in SBWC goats. This is the first study to investigate the relationship between SMAD2 gene CNVs and litter size and semen quality traits of SBWC goats.