Goat SMAD family member 1 (SMAD1): mRNA expression, genetic variants, and their associations with litter size

Goats are mammals that are considered an agricultural industry in China. The goat industry is critical to economic development. However, China's goat industry is currently in a state of unbalanced supply and marketing, which has become a major issue that needs to be addressed immediately. Shaanbei White Cashmere (SBWC) goats are a popular breed in Western China, characterized by high quality cashmere, good genetic performance, high meat quality, and high environmental resistance [1]. On the other hand, these goats have low fecundity. Litter size in goats is influenced by a variety of factors, including genetics [2]. Therefore, one of the primary selection goals in goat breeding programs is to adjust litter size. Advances in molecular technology can now be used to improve the level of response to selection. Marker-assisted selection (MAS) is a simple and efficient method to identify events that have the least negative impact on the environment [3,4]. The most frequently occurring genetic variants are InDels (insertion/deletions), SNPs (single nucleotide polymorphism), and CNVs (copy number variations). InDel is a common occurrence in the eukaryotic genome and it can be easily and rapidly identified [2] using simple PCR amplification and agarose gel electrophoresis [4]. Candidate gene analysis has been proposed as a method to analyze individual genes associated with economic benefits [5]. In goats, genetic markers associated with reproductive characteristics have been determined using methods such as genome-wide association studies (GWAS), which are typically conducted without predicting the importance of functional characteristics specific to the studied characteristics. SMAD family member 1 (SMAD1) was discovered through GWAS in high and low-prolific sheep, and it was found to be associated with high proliferative sheep litter size [6].

SMAD is an intracellular signalling molecule of the transforming growth factor (TGF- β) superfamily that is ubiquitously expressed during development in almost all adult tissues and is particularly prominent in normal embryonic development [[7], [8], [9]]. As a result, ovary-specific Smad-interacting proteins like transcription factors and co-modulators may play an important role in TGF-superfamily ligand target gene selection [7]. The SMAD family, which includes SMAD1, SMAD2, SMAD3, and SMAD4, contributes significantly to ovarian cell stimulation [7,10,11]. In addition, the SMAD1 gene allows stimulation of an intracellular BMP signal transduction molecule that mostly helps ewes ovulate more quickly [6]. SMAD1 encodes the SMAD protein involved in BMP signaling, which greatly enhances the actions of candidate genes related to reproduction. Preliminary researches have shown that the SMAD1 gene is expressed in ovary which the regulation of follicle growth and ovulation in livestock through the estrogen, TGF- β, retrograde endocannabinoid, and Hippo signaling pathways [6,12]. For instance, SMAD1 was found to be associated with highly prolific Hu sheep, followed by changes in the estrogen element [13]. The SMAD1 or SMAD2 signalling of mouse granulosa cells may be involved in the metastatic spread of granulosa cells or the control of other ovarian processes [14]. It has been shown in previous studies that the SMAD2 gene belongs to the SMAD family has a strong association with litter size and was also found to be expressed in ovary tissue [4]. Therefore, it is possible to use SMAD1 genetic variants to enhance litter size and improve the breeding quality of reproductive traits in goats.

However, the correlation between the roles of SMAD1 expression levels, InDel and litter size in Chinese domestic goats is poorly understood. Therefore, the purpose of this study was to determine the SMAD1 mRNA expressions and polymorphisms in SBWC goats and to reveal their possible associations with litter size, which could provide valuable molecular information regarding the usage of SMAD1 in goat breeding programs.

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