Next Article in Journal
Machine Learning Analysis of Maize Seedling Traits Under Drought Stress
Previous Article in Journal
Delimitation and Phylogeny in Fritillaria Species (Liliaceae) Endemic to Alps
Previous Article in Special Issue
Comparison of Superovulated Embryo Quality in Simmental Cattle Inseminated with 0 °C-Refrigerated and Liquid Nitrogen-Frozen Semen
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Biology
Volume 14
Issue 7
10.3390/biology14070786
biology-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Review

The Role of TGF-β Signaling Pathway in Determining Small Ruminant Litter Size

by
Ying Han
,
Guiling Cao
,
Wenting Chen
,
Changfa Wang
* and
Muhammad Zahoor Khan
*
College of Agriculture and Biology, Liaocheng University, Liaocheng 252059, China
*
Authors to whom correspondence should be addressed.
Biology 2025, 14(7), 786; https://doi.org/10.3390/biology14070786
Submission received: 20 May 2025 / Revised: 17 June 2025 / Accepted: 27 June 2025 / Published: 29 June 2025
(This article belongs to the Special Issue The Biology of Animal Reproduction)
Browse Figures
Versions Notes

Simple Summary

This review investigates how specific genes control the number of offspring that sheep and goats produce per pregnancy, which is crucial for farm profitability. We examine key members of the transforming growth factor-beta (TGF-β) superfamily, including BMP15, GDF9, BMPR1B, and associated genes, to understand how they influence litter size across different breeds worldwide. An analysis of published literature demonstrates that natural mutations in these genes create a complex communication network between oocytes and the surrounding granulosa cells, ultimately determining follicular development and ovulation rate. When these genes have certain mutations, they alter the ovary’s sensitivity to reproductive hormones, allowing multiple follicles to mature instead of just one or two, resulting in larger litters. Furthermore, the research revealed that having one copy of a beneficial mutation often increases fertility, while having two copies can sometimes cause infertility, showing the delicate balance required for optimal reproduction. These findings provide valuable genetic markers that farmers and breeders can use to select animals with higher reproductive potential, leading to increased livestock productivity and improved economic outcomes. This knowledge offers practical tools for enhancing breeding programs, ultimately contributing to more efficient and sustainable sheep and goat production systems that can better meet global food security needs.

Abstract

The transforming growth factor-beta (TGF-β) superfamily plays a crucial role in regulating female reproductive traits, particularly litter size, in small ruminants, such as sheep and goats. This review comprehensively examines the molecular mechanisms through which TGF-β superfamily members—including bone morphogenetic proteins (BMPs), growth differentiation factor 9 (GDF9), inhibin (INHA and INHB), and associated signaling genes—influence ovarian follicular development, ovulation rate, and ultimately, litter size. We synthesize recent findings on polymorphisms in key genes, such as BMPR1B, BMP15, GDF9, inhibins and SMADs family genes, across diverse sheep and goat breeds worldwide. The manuscript highlights how specific mutations in these genes create an intricate signaling network that modulates granulosa cell proliferation, follicular sensitivity to FSH, and the prevention of dominant follicle selection. These molecular interactions result in increased ovulation rates and larger litter sizes in prolific breeds. The gene dosage effects observed in heterozygous versus homozygous mutation carriers further illuminate the complex nature of these reproductive regulations. This improved the understanding of the genetic basis for prolificacy provides valuable insights for marker-assisted selection strategies aimed at enhancing reproductive efficiency in small ruminant breeding programs, with significant implications for improving livestock productivity and economic outcomes.

1. Introduction

Reproductive efficiency, particularly litter size, represents a critical economic trait in small ruminant production systems worldwide [1,2,3]. The number of offspring produced per pregnancy directly influences overall productivity and profitability in sheep and goat farming operations. Over the past several decades, significant advances in reproductive genetics have illuminated the molecular mechanisms underlying prolificacy in these species, with the transforming growth factor-beta (TGF-β) superfamily emerging as a central player in this complex biological process [4].
The TGF-β superfamily encompasses a diverse group of structurally related but functionally distinct growth factors that regulate numerous physiological processes, including cellular proliferation, differentiation, migration, and apoptosis [5]. Within this superfamily, bone morphogenetic proteins (BMPs), growth differentiation factor 9 (GDF9), inhibins (INHA and INHB), and their associated signaling molecules have been particularly implicated in female reproductive function [2]. These genes establish an elaborate communication network between the oocyte and surrounding somatic cells that ultimately determines follicular development trajectories and ovulation outcomes.
Recent research has identified naturally occurring polymorphisms in TGF-β superfamily genes that significantly affect litter size across various sheep and goat breeds. These genetic variants modify signaling pathways in ways that alter follicular development dynamics, follicle-stimulating hormone (FSH) sensitivity, and the selection mechanisms that typically limit the number of follicles reaching ovulation [6]. Among these, mutations in BMPR1B (also known as FecB or the Booroola gene), BMP15 (FecX), GDF9 (FecG), mothers against decapentaplegic (SMAD) family genes, INHA, and INHB have demonstrated the most pronounced effects on ovulation rate and litter size, with certain combinations of mutations exhibiting additive or even multiplicative influences on prolificacy [7,8,9,10].
This review aims to synthesize current knowledge regarding the roles of TGF-β superfamily members in regulating litter size in small ruminants. We highlighted the key genes of the TGF-β signaling pathway and their polymorphisms association with female reproductive efficiency with specific focus on litter size in sheep and goats. Furthermore, we discussed the molecular crosstalk between oocyte-derived factors and granulosa cells, evaluate how specific genetic variants disrupt normal signaling dynamics, and explore the downstream consequences for follicular development and ovulation. By integrating findings from molecular genetics, reproductive physiology, and breeding studies, we provide a comprehensive framework for understanding the genetic architecture underlying prolificacy in sheep and goats. This knowledge not only advances our fundamental understanding of reproductive biology but also offers practical applications for marker-assisted selection programs aimed at enhancing reproductive efficiency in small ruminant production systems.

2. Methodology for Literature Search

A comprehensive literature search was conducted to investigate the role of TGF-β signaling pathway and its members in reproductive efficiency, with particular emphasis on litter size in sheep and goats. The search strategy employed multiple electronic databases including Google Scholar, Web of Science, X-MOL, and PubMed. Primary keywords included “litter size”, “reproductive efficiency”, “small ruminants”, “sheep”, “goats”, “genetic polymorphisms”, “TGF-β signaling pathway”, along with specific gene identifiers, including “BMPs”, “INHA”, “INHB”, “GDF9”, anti-müllerian hormone (AMH) and “SMADs.”
The temporal scope of the search covered articles published from 2018 through April 2025, though selected seminal studies published as early as 2007 were incorporated to provide essential background context and historical perspective. Inclusion criteria required all articles to be indexed in Science Citation Index (SCI) journals and published in the English language. Study exclusion criteria eliminated non-SCI indexed publications, articles in languages other than English, book chapters, conference abstracts, and unpublished research data.

3. The TGF-β Superfamily Genes Role in Sheep and Goats Litter Size

The association of TGF-β Superfamily members have been well with litter size in sheep and goats [4]. Transforming growth factor-β1 (TGF-β1) is a multifunctional growth factor that is crucial in regulating various physiological processes, including embryonic growth and development [5]. These signaling pathways modulate follicular development, ovulation rates, and ultimately, fecundity in these economically important livestock species. TGIF1, which is also known as TGF-β induced factor homeobox 1, has been found to help change how sensitive FSH-β is to pulses of gonadotropin-releasing hormone (GnRH), which is critical for follicular development and ovulation [1]. This homeodomain protein serves as a transcriptional repressor and modulates TGF-β signaling, providing a molecular link between this pathway and reproductive outcomes.
Multiple studies have identified significant associations between genetic variants in TGF-β superfamily genes and reproductive performance. Consistently, a study reported through association analysis that a synonymous mutation at g.37871539C>T in TGIF1 was highly associated with litter size in small-tailed Han sheep [11]. Furthermore, given the association of the TGIF1 g.37866222C>T polymorphism with litter size in small-tailed Han sheep (p < 0.05), fecundity differences might be caused by the change in amino acid from proline (Pro) to serine (Ser), which has an impact on secondary, tertiary protein structures with concomitant TGIF1 functionality changes [12]. A study reported that four SNPs (g.9414A>G, g.28881A>G, g.28809T>C, g.10429G>A) in TGFβRI and one TGFβRII SNP (g.63940C>T) were significantly associated with litter size in Tibetan sheep [13]. Recent investigations have elucidated specific molecular pathways through which TGF-β family members influence ovarian function. Consistently, they revealed that TGF-β1 mediates the novel-m0297-5p targeting of WNT5A to inhibit granulosa cell proliferation and activity in small-tailed Han sheep, providing insights into the regulatory mechanisms of follicle development [5].
In line studies on genetic basis of high fecundity in Hu sheep found that all experimental animals were homozygous for the BMPRIB (A746G) mutation, with significant differences in BMP/Smad pathway gene expression between high-fecundity and low-fecundity sheep groups [8,10]. Specifically, BMP4, BMPRIB, BMPRII, SMAD4, GDF9, and TGF-βRI mRNAs were more abundant in high-fecundity animals, while BMP15 mRNA was less abundant, suggesting that unidentified genetic factors may influence ovulation rate through this pathway [10]. The accumulated evidence demonstrates that TGF-β superfamily members and their signaling components play integral roles in determining litter size in small ruminants. Genetic variants within these pathways represent promising markers for selective breeding programs aimed at enhancing reproductive performance in sheep and goat populations.

3.1. Association of BMP Family Genes and GDF9 with Litter Size in Goats

Members of the transforming growth factor-beta (TGF-β) superfamily, particularly the bone morphogenetic proteins (BMPs) family genes and growth and GDF9, serve critical functions in ovarian follicular development, cellular differentiation, cumulus expansion, and ovulation regulation [6,14,15]. The BMP15 and GDF9 operate as oocyte-secreted factors that exert significant regulatory control over female reproductive processes, influencing both somatic granulosa cell fate determination and oocyte developmental competence [15,16,17,18]. Recent investigations have demonstrated that GDF9 and BMP receptors (BMPRs) enhance proliferation in both granulosa and theca cell populations, further supporting their role in folliculogenesis [19]. Extensive research has established the fundamental role of BMP family genes in caprine litter size determination. Significant associations have been documented for several key genes. Multiple studies have confirmed BMP4’s involvement in regulating reproductive parameters in goats [15,20]. The association between BMP15 polymorphisms and reproductive traits has been thoroughly investigated across diverse caprine populations [9,21,22,23,24,25]. The BMPR1B gene, also designated as FECB, has demonstrated consistent associations with reproductive efficiency in goats [26,27,28,29,30].
Significant lambing rate differentials have been observed in relation to specific genotypic variations. Individuals exhibiting CC and CT genotypes at the FecB C94T locus demonstrated substantially higher lambing rates compared to TT genotype carriers, with increases of 45.7% and 46.8%, respectively. Additionally, individuals with the CC genotype at the ESR C463T locus exhibited significantly elevated lambing rates relative to both CT and TT genotypes, with increases of 9% and 15%, respectively [27]. Consistently, Wang Y et al. [25] employed polymerase chain reaction–single-strand conformation polymorphism (PCR-SSCP) analysis and DNA sequencing to examine exon 2 of the BMP15 gene in indigenous Chinese goat breeds. Their findings revealed that Funiu white goats possessing the BB genotype exhibited significantly enhanced litter size at birth, averaging 0.91 or 0.82 more offspring compared to AB or AA genotype carriers, respectively. Furthermore, they emphasized that second-parity litter size constitutes a critical indicator of caprine prolificacy, as it demonstrates the capacity for consistent multiple offspring production across consecutive pregnancies—a key parameter of reproductive fitness [25]. Functional polymorphisms in GNRH1 (g.3548A>G and g.3699G>A) and GDF9 (g.4093G>A) demonstrate significant associations with litter size variation, suggesting their utility as molecular markers for marker-assisted selection in goat breeding programs [30]. Consistent with these observations, additional research has identified associations between specific SNPs (g.3905A>C and g.4135G>A) and litter size in Shaanbei white cashmere goats [31]. A substantial body of evidence has consistently demonstrated associations between GDF9 polymorphic variants and litter size across diverse goat breeds [23,32,33,34,35]. The collective evidence strongly supports that BMP family genes and GDF9 exhibit robust associations with caprine litter size, attributable to their fundamental roles in folliculogenesis, ovulatory regulation, and the modulation of follicular responsiveness to gonadotropic signaling pathways. These molecular genetic insights highlight the potential application of these genes as selection markers in precision breeding programs aimed at enhancing reproductive efficiency in goat populations.

3.2. Role of BMP Family Genes and GDF9 in Litter Size in Sheep

Litter size, a critical reproductive trait in sheep breeding, is influenced by genetic factors, such as BMP15, GDF9, and BMPR1B, which regulate ovarian follicle development and ovulation. Mutations in BMP15 (e.g., c.31_33CTTinsdel in Hu sheep) and GDF9 (e.g., S395F/S427R in Chinese breeds) enhance litter size by disrupting granulosa cell function or oocyte maturation [36,37,38], while BMPR1B polymorphisms, like c.746A>G, improve fecundity in Ujimqin and crossbred sheep [39]. Geographically, studies from China dominate, with BMP15 variants (e.g., g.50985975G>A in Mongolian sheep) and BMPR1B mutations (e.g., g.29362047T>C in Hu sheep) being prominent [40,41], though international research confirms these genes’ roles in Turkish Akkaraman (also known as white Karaman) and Australian white sheep [42,43], suggesting conserved mechanisms with breed-specific allele frequencies. Polygenic interactions, such as synergies between BMPR1B, BMP15, and GDF9 in Han and Hu sheep [44] or *BMPR1B-BMP2* in Chinese indigenous breeds [45], highlight the need for multi-locus approaches in breeding. Methodologically, diverse polymorphisms—including SNPs (BMPR1B c.746A>G), indels (g.30058882_30058873GCAGATTAAA in Gobi sheep), and missense mutations (p.Q249R)—disrupt gene function [46], though reference-year inconsistencies, such as those found in Zhang et al. [36], warrant further verification. Marker-assisted selection (MAS) targeting *BMP15/GDF9* in Hu sheep [47] and crossbreeding strategies (e.g., Suffolk × Ujimqin hybrids) can optimize productivity [39]. Future research should address gene–environment interactions, epigenetics, and underrepresented breeds, like Sudanese desert sheep [48], while functional studies are needed to clarify polymorphism mechanisms. In conclusion, BMP15, GDF9, and BMPR1B are pivotal for litter size regulation; validating these markers across breeds and advancing genomic tools will enhance breeding efficacy and sustainable sheep production. In addition, a study investigated the C864T polymorphism in exon-9 of the BMPR-1B gene and its association with litter size across 596 ewes from three breeds (Dorset, Mongolian, and small-tailed Han) [49]. DNA analysis revealed two genotypes (AA and AB), with AA being most frequent. Furthermore, they reported that BMPR-1B-C864T mutation significantly affected litter size traits, with heterozygosity in exon-9 increasing litter size across all studied breeds [49]. To investigate the role of BMPR1B, BMP15, and GDF9 in sheep prolificacy, ovarian tissue in polytocous small-tailed Han (STH) and monotocous Sunite (SNT) ewes showed the highest mRNA levels for all three genes, with BMPR1B and GDF9 expression significantly elevated in STH ovaries and BMP15 markedly reduced in STH pituitary, ovarian, oviduct, and uterine tissues [50]. Furthermore, they suggest BMPR1B and GDF9 promote litter size through ovarian activity in the hypothalamic–pituitary–gonadal axis, while high BMP15 expression correlates with reduced prolificacy [50]. Consistently, a study reported that GG and AA genotypes of GDF9 gene showed reduced prolificacy in Russian sheep breeds (Salsk and Volgograd sheep) [51]. Low polymorphism was observed, with high frequencies of GG and AA homozygous genotypes, yet heterozygous AG genotypes at both sites were linked to maximal fertility [51]. Another study reported that GDF9 and BMPR1B were linked to litter size in Egyptian Rahmani and Ossimi sheep rams [52]. Furthermore, it was revealed that GDF9 and BMP15 were exclusively expressed in oocytes, with reduced levels in poor-quality oocytes, underscoring their role in ovulation. In addition, BMPR1B and BMP6 transcripts were detected in oocytes, granulosa, cumulus cells, and corpora lutea, suggesting BMP6 may regulate follicular maturation and luteolysis via BMPR1B [52]. A T755C mutation in BMP15 (L252P substitution) was linked to litter size in Iranian sheep, with heterozygous (CT) ewes producing 0.24–0.30 more lambs than CC and TT genotypes [53]. Furthermore, triplet-birth and sterile ewes exclusively carried the CT genotype, suggesting dual impacts on fertility and prolificacy Iranian Afshari, Ghezel, and Shal breeds [53]. The summary of determinant genes (BMPs and GDF9) and their polymorphisms associated with litter size in sheep is provided in Table 1.

3.3. Effect of Inhibins, SMAD Family Genes, and AMH on Litter Size in Goats and Sheep

The transforming growth factor-beta (TGF-β) signaling pathway encompasses several key gene families that play crucial roles in reproductive physiology, cellular differentiation, and tissue growth. These include the AMH, inhibins, and SMAD family genes (Table 2), which have emerged as targets of interest for genetic improvement in breeding programs aimed at enhancing reproductive output in small ruminants [75,76].

3.3.1. The AMH and Reproductive Performance

The AMH, a member of the TGF-β superfamily, has demonstrated significant associations with reproductive traits in goats. Genetic polymorphism studies have revealed critical associations between specific AMH gene variants and reproductive performance in goats. The SNP-g.89172108A>C polymorphism within the AMH gene has been particularly well-characterized, demonstrating significant correlations with litter size in both Dazu black and Chuanzhong black goat breeds [75,77]. Genotype–phenotype analyses have revealed distinct patterns of reproductive performance, with homozygous CC genotypes consistently exhibiting superior litter sizes compared to heterozygous AC genotypes, suggesting this mutation confers a reproductive advantage [77]. Similarly, in Chuanzhong black goats, animals carrying the TT genotype displayed enhanced litter sizes relative to TG and GG genotype carriers [75].
The relationship between AMH and reproductive traits extends beyond genetic polymorphisms to encompass serum hormone concentrations. Multiple studies have established positive correlations between serum AMH levels and various reproductive parameters, including ovarian reserve capacity, antral follicle count, ovarian surface area, ovulation rate, and ultimately, litter size [78,79]. These findings suggest that AMH serves as a reliable predictor of reproductive potential in goats, with higher concentrations indicating greater ovarian activity and fertility. The significance of AMH in small ruminant reproduction is further supported by ovine studies, where elevated AMH concentrations have been associated with increased litter sizes in Romanov sheep [80]. This cross-species consistency reinforces the fundamental role of AMH in regulating reproductive processes across related livestock species.

3.3.2. Role of SMAD Family Genes in Reproductive Regulation

The SMAD family genes, comprising SMAD1, SMAD2, SMAD3, SMAD4, and SMAD5 genes, functions as intracellular signaling mediators within the TGF-β superfamily. These proteins display widespread expression across both developmental and adult tissues, with pronounced roles in normal embryogenesis and reproductive function [8,81,82,83,84,85]. The SMAD family of genes plays a pivotal role in reproductive traits across various livestock species, with mounting evidence demonstrating their fundamental involvement in litter size determination. Genome-wide association studies have established SMAD1 as a significant genetic determinant of litter size in sheep, where high-prolificacy individuals display elevated expression of this gene [86]. The molecular basis for this association stems from SMAD1’s activation of the BMP intracellular signal transduction pathway, which has been causally linked to accelerated ovulation in ewes [8,86,87]. This pathway represents a critical regulatory mechanism, as SMAD proteins encoded by SMAD1 amplify the actions of key reproductive candidate genes through BMP signaling [87]. The functional significance of SMAD signaling extends beyond SMAD1, with the Booroola (FecB) gene demonstrating effects on progesterone levels and the expression of both BMP and SMAD signaling genes within ovine ovaries [87]. At the tissue level, SMAD1 expression has been documented in ovarian tissues, where it contributes to the regulation of follicular growth and ovulation through multiple integrated pathways, including estrogen, TGF-β, retrograde endocannabinoid signaling, and the Hippo pathway [87,88]. Comprehensive expression analysis in Tibetan sheep has revealed widespread tissue distribution of SMAD1, SMAD2, and SMAD3 genes, with particularly elevated expression observed in reproductive tissues, such as the uterus and ovary, as well as in the spleen and lung [89]. The reproductive relevance of SMAD signaling is further supported by evidence from multiple species. In mice, SMAD signaling within granulosa cells has been implicated in regulating metastatic behaviors and other ovarian functions [90]. Similarly, SMAD2 exhibits a significant association with litter size and has been documented in goat ovarian tissue, underscoring its reproductive significance [90]. The functional impact of SMAD gene polymorphisms is evidenced by genotype–phenotype relationships; the CC genotype at the SMAD1 g.10729C>T locus yields significantly higher litter sizes compared to the CT genotype (p < 0.05), while at the SMAD3 g.21447C>T locus, the TT genotype demonstrates superior litter sizes relative to both CC and CT genotypes [7]. In goats, the collective evidence consistently demonstrates associations between the expression and polymorphisms of multiple SMAD family members (SMAD1, SMAD2, SMAD3, and SMAD6) and litter size, revealing a critical genetic component underlying reproductive trait variation [91,92,93,94]. This pattern is reinforced by additional studies identifying key genes, including SMAD2 and AMHR2 as contributors to litter size determination in goats [95], collectively establishing the SMAD family as a central molecular framework governing reproductive success across livestock species.

3.3.3. Inhibins’ Role in Follicular Development and Litter Size

The reproductive physiology of animals is significantly influenced by inhibins, glycoproteins predominantly synthesized by ovarian granulosa cells that serve as negative feedback regulators of FSH secretion in the anterior pituitary [96,97]. This regulatory mechanism forms a critical component of the hypothalamic–pituitary–gonadal axis, orchestrating follicular development and ovulation events [98]. The INHA gene specifically encodes a protein that functions as a biomarker for fully developed ovarian follicles, while simultaneously mediating FSH secretion and ovulation frequency [2,98,99]. The modulatory effect of inhibin on FSH concentrations directly impacts ovulation rates and follicular recruitment patterns [99], phenomena that have been established as key determinants of litter size in ruminant species [98]. Consistently, molecular investigations have demonstrated that genetic polymorphisms within inhibin genes correlate with reproductive phenotypes, particularly enhanced litter size outcomes in high-fertility goat populations [100,101,102,103,104,105,106]. The significance of these genetic variations is exemplified by findings at the g.28317663A>C locus of the INHA gene, where individuals carrying the AC genotype exhibited significantly larger litter sizes compared to those with the AA genotype [101]. This amino acid-altering SNP appears to influence protein functionality through structural modifications, establishing a direct mechanistic link between genetic variation and reproductive output. Similarly, Isa et al. documented that the CT genotype at the g.3234C>T locus in the INHA gene was associated with superior litter size performance relative to the CC genotype in Kalahari red and Nigerian goats [103].
The molecular framework governing these reproductive traits extends beyond inhibin genes alone, encompassing multiple components of the TGF-β signaling cascade. Comprehensive genomic analyses have identified SMAD1 and INHB as additional contributors to litter size variation, with these genes influencing hormone secretion patterns (FSH and LH), placental development, embryonic viability, folliculogenesis, ovulation dynamics, and preovulatory follicle maturation in diverse sheep breeds [86]. The regulatory complexity of this system is further highlighted by the recent identification of miR-134-3p as a post-transcriptional regulator of INHBA in ovine granulosa cells [107]. This microRNA exhibits an inverse relationship with follicular development, decreasing in expression as follicular diameter increases. Functionally, miR-134-3p overexpression inhibits granulosa cell proliferation while promoting apoptosis, effects that are reversed upon microRNA knockdown. The mechanistic pathway involves the modulation of cell cycle progression and the suppression of the PI3K/AKT/mTOR signaling cascade, while knockdown activates this pathway [107]. The direct targeting of INHBA by miR-134-3p has been confirmed through co-transfection studies, demonstrating that this microRNA regulates ovine granulosa cell function via the TGF-β/PI3K/AKT/mTOR pathway [107]. This study provides interesting findings; however, future research investigating polymorphisms regulated by miR-134-3p and their effects on reproductive performance would be valuable.
Additional evidence supporting the clinical relevance of inhibin genetic variants derives from studies in thin-tailed sheep, where the g.236311367G>A polymorphism in INHA was associated with significantly enhanced litter size in GA genotype carriers compared to individuals with homozygous AA or GG genotypes [108]. These findings collectively establish the TGF-β signaling pathway genes, particularly AMH, SMAD family members, and inhibins, as fundamental regulators of reproductive traits in small ruminants (Figure 1). The consistent association between specific polymorphisms and enhanced litter size performance provides a foundation for genetic marker development in selection programs targeting reproductive efficiency improvements in goat and sheep production systems. Understanding the molecular mechanisms underlying these genetic associations will enable more precise and targeted approaches to genetic improvement strategies for small ruminant production systems.
Table 2. AMH, Inhibins, and SMADs family genes’ association with litter size in goats and sheep.
Table 2. AMH, Inhibins, and SMADs family genes’ association with litter size in goats and sheep.
GeneSNP/VariantEffectBreedCountryReferences
SMAD1g.10729C>TCC genotype produced significantly higher litter sizes than CT genotypeTibetan sheepChina[7]
SMAD3g.21447C>TTT genotype showed higher litter sizes than CC and CT genotypesTibetan sheepChina[7]
SMAD1rs406357666Higher expression in high-prolificacy sheepHu sheepChina[10]
AMHg.89172108T>GTT genotype exhibited higher litter size compared to TG and GG genotypesChuanzhong black goatsChina[75]
AMHg.89172108A>CCC genotype associated with higher litter size compared to AC genotypeDazu black goatsChina[77]
INHBrs412280524rs429836421Associated with variation in litter sizeIcelandic and Finn sheepChina[86]
SMAD1 Activates BMP signaling pathway associated with accelerated ovulationMalpura sheepIndia[87]
SMAD2 and SMAD1 Associated with litter sizeShaanbei white cashmereChina [91,92,93]
INHAg.28317663A>CAC genotype associated with significantly higher litter size than AA genotypeHainan black goatsChina[101]
INHAg.3234C>TCT genotype associated with significantly larger litter size compared to CC genotypeWest African dwarf goatsNigeria[103]
INHA Target of miR-134-3p regulation affecting follicular development through TGF- TGF-β/PI3K/AKT/mTOR pathway in GCsSheep granulosa cells (GCs) China[107]
INHAg.236311367G>AGA genotype had significantly higher litter size than AA or GG genotypeThin-tailed sheepIndonesia[108]

4. Discussion

The molecular crosstalk between BMPR1B, BMP15, and GDF9 orchestrates the ovulation rate and litter size in sheep and goats through an intricate signaling network. In this network, oocyte-derived BMP15 and GDF9 form heterodimers that bind to BMPR1B receptors on granulosa cells, activating SMAD-dependent pathways that regulate FSH sensitivity, steroidogenesis, and cell proliferation (Figure 2) [109,110,111,112]. Mutations in these genes (such as the FecB mutation in BMPR1B, various FecX mutations in BMP15, and FecG mutations in GDF9) disrupt this signaling balance by either enhancing receptor sensitivity (BMPR1B mutations) or altering ligand bioavailability and function (BMP15/GDF9 mutations). These disruptions result in reduced negative feedback on FSH, accelerated follicular development, increased the recruitment of secondary follicles, and prevented dominant follicle selection. Ultimately, these changes lead to higher ovulation rates and larger litter sizes and birth weight [113]. The phenotypic expression is dependent on gene dosage effects where heterozygous mutations often increase fertility, while homozygous mutations can sometimes cause infertility due to impaired follicular development.
The regulation of litter size in small ruminants, such as sheep and goats, involves an intricate molecular crosstalk centered around the TGF-β superfamily signaling network. This reproductive control system begins with the oocyte, which serves as the command center for follicular development through its secretion of two critical growth factors—BMP15 and GDF9. These oocyte-derived proteins form both homodimers and heterodimers, with the heterodimeric complex known as “cumulin” exhibiting significantly higher bioactivity. BMP15 primarily signals through a receptor complex consisting of BMPR2 (type II receptor) and ALK6/BMPR1B (type I receptor), while GDF9 utilizes TGFβR2 and ALK5 receptors [114,115,116]. When these ligands bind their respective receptors on granulosa cells surrounding the oocyte, they activate parallel but interconnected intracellular signaling cascades. BMP15 binding triggers phosphorylation of receptor-regulated SMAD 1, 5, and 8, whereas GDF9 activates SMAD 2 and 3 [109]. These phosphorylated R-SMADs then form complexes with the common mediator SMAD4, which enables nuclear translocation and the subsequent regulation of target gene expression [117]. This bidirectional communication between oocyte and granulosa cells establishes a local regulatory environment that controls multiple aspects of follicular development, including granulosa cell proliferation, the prevention of premature luteinization, cumulus expansion, and steroidogenesis. The system is further regulated by inhibitory SMADs (SMAD6 and SMAD7), which provide negative feedback by interfering with R-SMAD phosphorylation or competing for SMAD4 binding, thus maintaining signaling homeostasis and preventing excessive pathway activation. This follicular signaling network integrates with the broader hypothalamic–pituitary–gonadal axis through several intermediaries. Granulosa cells produce inhibins (INHA/INHB), which act as endocrine signals to suppress FSH secretion from the pituitary, creating a negative feedback loop that normally limits follicular development [100,118,119]. They also produce AMH, which inhibits primordial follicle recruitment and reduces follicular sensitivity to FSH, adding another regulatory layer for follicle selection [120].
FSH released from the pituitary binds to receptors on granulosa cells, stimulating follicular growth and estradiol production, with the sensitivity to FSH being modulated by BMP15/GDF9 signaling. The remarkable impact of naturally occurring mutations in these pathway components illuminates their functional roles in determining litter size. Various BMP15 mutations (such as FecXI, FecXH, FecXB, FecXG) reduce BMP15 bioavailability or create partially functional proteins, demonstrating a precise dose-dependence where heterozygotes show increased ovulation while homozygotes are often infertile. The BMPR1B c.746A>G mutation, also known as the FecB (Fecundity Booroola) mutation, demonstrates a classic example of gene dosage effects where heterozygous carriers benefit from increased fertility while homozygous carriers can experience fertility problems or sterility. The BMPR1B c.746A>G mutation is a missense mutation that causes an amino acid substitution from glutamine to arginine at position 249 (Q249R) in the BMPR1B protein [39,68]. This mutation was originally identified in Booroola Merino sheep and has since been found in several sheep breeds including Hu, Ujimqin, and small-tailed Han sheep [68]. In heterozygous animals (carrying one mutant and one normal copy), the FecB mutation in BMPR1B (Q249R) increases receptor sensitivity to ligands, enhancing SMAD1/5/8 phosphorylation at lower ligand concentrations and making granulosa cells responsive to FSH at smaller follicle sizes [121,122]. The altered proteins significantly enhance the sensitivity of ovarian granulosa cells to follicle-stimulating hormone, consequently resulting in increased follicular development [123]. Furthermore, this mutation demonstrates an additive effect on both ovulation rate and litter size [124]. Specifically, heterozygous animals typically exhibit four to five ovulations, which represents a substantial increase compared to the two or fewer ovulations observed in wild-type animals [123]. Mechanistically, this partial reduction in normal BMPR1B function appears to attenuate the inhibitory signals that ordinarily limit ovulation, thereby permitting a greater number of follicles to reach maturation and subsequently ovulate. The reproductive pathology observed in homozygous animals primarily results from the presence of two copies of the mutated gene, which consequently leads to the severe disruption of normal reproductive physiology. Specifically, ewes homozygous for mutations in BMPR1B demonstrate an inability to produce sufficient quantities of biologically active protein necessary for stimulating granulosa cell proliferation, thereby preventing normal follicular development [125]. Furthermore, BMPR1B deficiency significantly impairs the proliferation of cumulus granulosa cells while simultaneously reducing aromatase content, ultimately resulting in irregular estrous cycles [70]. Although homozygous mutants characteristically exhibit more than five ovulations [123], this apparent hyperovulation paradoxically proves detrimental to reproductive success. Rather than enhancing fertility, excessive ovulation leads to the production of poor-quality oocytes and subsequent reproductive failure. The underlying mechanism involves BMPR1B’s integral role within the TGF-β signaling pathway, where it functions cooperatively with downstream effectors, such as SMAD4 [126]. Importantly, the mutation affects this critical pathway in a dose-dependent manner, creating a biphasic response; heterozygous mutations partially reduce inhibitory signaling, thereby achieving optimal ovulation enhancement, whereas homozygous mutations severely disrupt essential signaling pathways required for normal follicle development and oocyte maturation. This dose-dependent mechanism provides a molecular explanation for the paradoxical observation that some Hu sheep carrying homozygous FecB alleles continue to produce single offspring [127]. The severe disruption of normal reproductive signaling cascades in homozygous animals can ultimately result in subfertility or complete sterility, demonstrating that optimal reproductive performance requires a delicate balance of signaling pathway activity rather than complete pathway inhibition. Similarly, GDF9 mutations like FecGH alter protein structure or processing, modifying SMAD2/3 signaling dynamics and affecting cumulus expansion and follicular maturation [109]. These mutations do not simply eliminate function but subtly alter the signaling dynamics throughout the entire regulatory network.
The molecular consequences of these pathway alterations lead to increased litter size through several interconnected mechanisms, as follows mutations typically decrease the amount of FSH required for follicular development, allowing more follicles to continue development under normal FSH conditions; changes in granulosa cell differentiation affect inhibin production, leading to higher FSH secretion; normal dominance mechanisms are disrupted, allowing multiple follicles to continue development instead of undergoing atresia; altered SMAD signaling modifies steroid enzyme expression, changing estradiol/progesterone ratios; and anti-apoptotic signaling in granulosa cells may be enhanced, allowing more follicles to reach ovulatory stages. The entire system functions as an integrated network where oocytes communicate with granulosa cells via BMP15/GDF9, granulosa cells signal to the pituitary via inhibins, the pituitary regulates follicular development via FSH, and SMAD proteins integrate these signals intracellularly. Mutations at different points create distinct but related phenotypes, with some combinations showing additive or even multiplicative effects on ovulation rate. This sophisticated molecular dialogue ultimately determines the number of follicles that reach ovulation and, consequently, the litter size in sheep and goats, making it a paradigmatic example of how genetic variations in signaling networks translate into quantifiable reproductive traits with significant economic importance in livestock production.

5. Conclusions

This comprehensive review highlights the crucial role of the TGF-β superfamily in regulating litter size in small ruminants. The complex molecularinteration between BMP15 and GDF9, their receptors (primarily BMPR1B), and downstream signaling mediators (SMADs) forms a sophisticated regulatory network that controls multiple aspects of follicular development and ovulation. Natural mutations in these pathway components alter the dynamics of signaling, ultimately increasing the number of follicles that reach ovulatory maturity, thereby enhancing litter size in sheep and goats. The evidence presented demonstrates that mutations in BMPR1B, BMP15, and GDF9 exert their effects through several interconnected mechanisms, as follows: reducing the FSH threshold required for follicular development, disrupting follicular dominance patterns, altering steroidogenic profiles, and potentially enhancing granulosa cell survival. These changes collectively promote the development of multiple follicles under hormonal conditions that would typically support only one or two dominant follicles in non-prolific breeds. The observed gene dosage effects—where heterozygous mutations often increase ovulation while homozygous mutations sometimes cause infertility—further underscore the delicate balance required in these signaling pathways for optimal reproductive function. Additionally, the expanding body of research on INHA, INHB, AMH, and SMADs reveals additional layers of regulation that modulate the primary BMP/GDF9 signaling axis. The identification of specific SNPs in these genes associated with increased litter size across different breeds provides valuable molecular markers for selection programs. Consistent findings across geographically and genetically distinct sheep and goat populations reinforce the fundamental importance of these pathways in mammalian reproductive biology.
Looking ahead, several research directions merit further exploration. First, the functional consequences of newly identified polymorphisms require biochemical and cellular characterization to elucidate their precise effects on protein function and signaling dynamics. Second, investigating potential epistatic interactions between multiple mutations could uncover synergistic effects that might be exploited in breeding programs. Third, the role of epigenetic regulation in modulating the expression of these key genes is an emerging area that may provide additional insights into reproductive variability. From a practical standpoint, the molecular markers identified in this review present substantial opportunities for implementing marker-assisted selection strategies within small ruminant breeding programs. However, successful implementation requires careful consideration of breed-specific genetic effects and potential antagonistic relationships with other economically significant traits.
Further validation of these candidate genes across diverse sheep and goat breeds is strongly recommended, as the current literature suggests that research in this field exhibits considerable geographic and breed-specific limitations. This validation gap represents a critical barrier to the broader application of these molecular markers in global small ruminant improvement programs. Comprehensive multi-breed studies would enhance the robustness and transferability of these findings, ultimately facilitating the more effective implementation of genomic selection strategies across varied production systems and genetic backgrounds. Integrating these genetic insights with optimal nutritional and management practices will likely lead to the most substantial improvements in reproductive performance. The TGF-β superfamily stands as a master regulator of female litter size in small ruminants, with specific genetic variants providing the molecular foundation for prolificacy. Understanding these intricate signaling networks not only advances reproductive biology but also provides practical tools for improving livestock productivity and sustainability in global agricultural systems.

Author Contributions

Y.H., W.C., M.Z.K., G.C. and C.W.: conceptualization, data curation, writing—original draft, writing—review and editing; M.Z.K. and C.W.: visualization, and supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Key R&D Program of China (grant numbers 2022YFD1600103; 2023YFD1302004), the National Natural Science Foundation of China (grant no. 32202669), the Shandong Province Modern Agricultural Technology System Donkey Industrial Innovation Team (grant no. SDAIT-27), Livestock and Poultry Breeding Industry Project of the Ministry of Agriculture and Rural Affairs (grant number 19211162), Shandong Province Agricultural Major Technology Collaborative Promotion Plan (SDNYXTTG-2024-13), Liaocheng University Scientific Research Fund (grant no. 318052026), and Liaocheng Municipal Bureau of Science and Technology, High-Talented Foreign Expert Introduction Program (GDWZ202401).

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Abuzahra, M.; Wijayanti, D.; Effendi, M.H.; Mustofa, I.; Munyaneza, J.P.; Moses, I.B. Improved Litter Size in Thin-Tailed Indonesian Sheep Through Analysis of TGIF1 Gene Polymorphisms. Vet. Med. Int. 2025, 2025, 7778088. [Google Scholar] [CrossRef] [PubMed]
  2. Chen, W.; Han, Y.; Chen, Y.; Liu, X.; Liang, H.; Wang, C.; Khan, M.Z. Potential Candidate Genes Associated with Litter Size in Goats: A Review. Animals 2025, 15, 82. [Google Scholar] [CrossRef]
  3. Tao, L.; He, X.; Wang, X.; Di, R.; Chu, M. Litter size of sheep (Ovis aries): Inbreeding depression and homozygous regions. Genes 2021, 12, 109. [Google Scholar] [CrossRef]
  4. Wang, W.; La, Y.; Zhou, X.; Zhang, X.; Li, F.; Liu, B. The genetic polymorphisms of TGFβ superfamily genes are associated with litter size in a Chinese indigenous sheep breed (Hu sheep). Anim. Reprod. Sci. 2018, 189, 19–29. [Google Scholar] [CrossRef]
  5. Ren, S.; Liu, Y.; Guo, Y.; Zhao, Z.; Cui, J.; Li, M.; Wang, J. TGF-β1 Mediates Novel-m0297-5p Targeting WNT5A to Participate in the Proliferation of Ovarian Granulosa Cells in Small-Tailed Han Sheep. Int. J. Mol. Sci. 2025, 26, 1961. [Google Scholar] [CrossRef] [PubMed]
  6. Fountas, S.; Petinaki, E.; Bolaris, S.; Kargakou, M.; Dafopoulos, S.; Zikopoulos, A.; Moustakli, E.; Sotiriou, S.; Dafopoulos, K. The Roles of GDF-9, BMP-15, BMP-4, and EMMPRIN in Folliculogenesis and In Vitro Fertilization. J. Clin. Med. 2024, 13, 3775. [Google Scholar] [CrossRef]
  7. Li, M.; He, N.; Sun, R.; Deng, Y.; Wen, X.; Zhang, J. Expression and polymorphisms of SMAD1, SMAD2 and SMAD3 genes and their association with litter size in Tibetan sheep (Ovis aries). Genes 2022, 13, 2307. [Google Scholar] [CrossRef] [PubMed]
  8. Kumar, S.; Rajput, P.K.; Bahire, S.V.; Jyotsana, B.; Kumar, V.; Kumar, D. Differential expression of BMP/SMAD signaling and ovarian-associated genes in the granulosa cells of FecB introgressed GMM sheep. Syst. Biol. Reprod. Med. 2020, 66, 185–201. [Google Scholar] [CrossRef]
  9. Ahlawat, S.; Sharma, R.; Roy, M.; Tantia, M.S.; Prakash, V. Association Analysis of Novel SNPs in BMPR1B, BMP15 and GDF9 Genes with Reproductive Traits in Black Bengal Goats. Small Rumin. Res. 2015, 132, 92–98. [Google Scholar] [CrossRef]
  10. Xu, Y.; Li, E.; Han, Y.; Chen, L.; Xie, Z. Differential expression of mRNAs encoding BMP/Smad pathway molecules in antral follicles of high-and low-fecundity Hu sheep. Anim. Reprod. Sci. 2010, 120, 47–55. [Google Scholar] [CrossRef]
  11. Zhang, Z.; He, X.; Liu, Q.; Tang, J.; Di, R.; Chu, M. TGIF1 and SF1 polymorphisms are associated with litter size in Small Tail Han sheep. Reprod. Domest. Anim. 2020, 55, 1145–1153. [Google Scholar] [CrossRef] [PubMed]
  12. Chen, S.; Tao, L.; He, X.; Di, R.; Wang, X.; Chu, M. Single-nucleotide polymorphisms in FLT3, NLRP5, and TGIF1 are associated with litter size in Small-tailed Han sheep. Arch. Anim. Breed. 2021, 64, 475–486. [Google Scholar] [CrossRef] [PubMed]
  13. Zhang, J.; Li, M.; Sun, R.; He, N.; Wen, X.; Han, X.; Luo, Z. Molecular characterization and expression of TGFβRI and TGFβRII and its association with litter size in Tibetan sheep. Vet. Med. Sci. 2023, 9, 934–944. [Google Scholar] [CrossRef]
  14. Belli, M.; Shimasaki, S. Molecular Aspects and Clinical Relevance of GDF9 and BMP15 in Ovarian Function. Vitam. Horm. 2018, 107, 317–348. [Google Scholar] [PubMed]
  15. Sharma, R.; Ahlawat, S.; Maitra, A.; Roy, M.; Mandakmale, S.; Tantia, M.S. Polymorphism of BMP4 Gene in Indian Goat Breeds Differing in Prolificacy. Gene. 2013, 532, 140–145. [Google Scholar] [CrossRef]
  16. Christoforou, E.R.; Pitman, J.L. Intrafollicular Growth Differentiation Factor 9: Bone Morphogenetic Protein 15 Ratio Determines Litter Size in Mammals. Biol. Reprod. 2019, 100, 1333–1343. [Google Scholar] [CrossRef]
  17. Paulini, F.; Melo, E.O. The Role of Oocyte-Secreted Factors GDF9 and BMP15 in Follicular Development and Oogenesis. Reprod. Domest. Anim. 2011, 46, 354–361. [Google Scholar] [CrossRef]
  18. Spicer, L.J.; Aad, P.Y.; Allen, D.T.; Mazerbourg, S.; Payne, A.H.; Hsueh, A.J. Growth Differentiation Factor 9 (GDF9) Stimulates Proliferation and Inhibits Steroidogenesis by Bovine Theca Cells: Influence of Follicle Size on Responses to GDF9. Biol. Reprod. 2008, 78, 243–253. [Google Scholar] [CrossRef]
  19. Kumar, A.; Nanda, R.; Samad, H.A.; Maurya, V.P.; Singh, G.; Chouhan, V.S. Transcriptional Profiling of GDF9 and ITS Signaling Receptors in Goat Granulosa and Theca Cells. Indian J. Small Rumin. 2024, 30, 41–45. [Google Scholar]
  20. Chu, M.X.; Lu, L.; Feng, T.; Di, R.; Cao, G.L.; Wang, P.Q.; Fang, L.; Ma, Y.H.; Li, K. Polymorphism of Bone Morphogenetic Protein 4 Gene and Its Relationship with Litter Size of Jining Grey Goats. Mol. Biol. Rep. 2011, 38, 4315–4320. [Google Scholar] [CrossRef]
  21. Abuzahra, M.; Abu Eid, L.; Effendi, M.H.; Mustofa, I.; Lamid, M.; Rehman, S. Polymorphism Studies and Candidate Genes Associated with Litter Size Traits in Indonesian Goats: A Systematic Review. F1000Research 2023, 12, 61. [Google Scholar] [CrossRef]
  22. Zergani, E.; Rashidi, A.; Rostamzadeh, J.; Razmkabir, M.; Tetens, J. Meta-Analysis of Association between c.963A>G Single-Nucleotide Polymorphism on BMP15 Gene and Litter Size in Goats. Arch. Anim. Breed. 2022, 65, 309–318. [Google Scholar] [CrossRef]
  23. Das, A.; Shaha, M.; Gupta, M.D.; Dutta, A.; Miazi, O.F. Polymorphism of Fecundity Genes (BMP15 and GDF9) and Their Association with Litter Size in Bangladeshi Prolific Black Bengal Goat. Trop. Anim. Health Prod. 2021, 53, 230. [Google Scholar] [CrossRef] [PubMed]
  24. Liandris, E.; Kominakis, A.; Andreadou, M.; Kapeoldassi, K.; Chadio, S.; Tsiligianni, T.; Gazouli, M.; Ikonomopoulos, I. Associations between Single Nucleotide Polymorphisms of GDF9 and BMP15 Genes and Litter Size in Two Dairy Sheep Breeds of Greece. Small Rumin. Res. 2012, 107, 16–21. [Google Scholar] [CrossRef]
  25. Wang, Y.; Li, Y.; Zhang, N.; Wang, Z.; Bai, J. Polymorphism of Exon 2 of BMP15 Gene and Its Relationship with Litter Size of Two Chinese Goats. Asian-Australas. J. Anim. Sci. 2011, 24, 905–911. [Google Scholar] [CrossRef]
  26. Liang, C.; Han, M.; Zhou, Z.; Liu, Y.; He, X.; Jiang, Y.; Ouyang, Y.; Hong, Q.; Chu, M. Hypothalamic Transcriptome Analysis Reveals the Crucial MicroRNAs and mRNAs Affecting Litter Size in Goats. Front. Vet. Sci. 2021, 8, 747100. [Google Scholar] [CrossRef] [PubMed]
  27. Yue, C.; Bai, W.L.; Zheng, Y.Y.; Hui, T.Y.; Sun, J.M.; Guo, D.; Guo, S.L.; Wang, Z.Y. Correlation Analysis of Candidate Gene SNP for High-Yield in Liaoning Cashmere Goats with Litter Size and Cashmere Performance. Anim. Biotechnol. 2021, 32, 43–50. [Google Scholar] [CrossRef] [PubMed]
  28. Shokrollahi, B.; Morammazi, S. Polymorphism of GDF9 and BMPR1B Genes and Their Association with Litter Size in Markhoz Goats. Reprod. Domest. Anim. 2018, 53, 971–978. [Google Scholar] [CrossRef]
  29. Ahlawat, S.; Sharma, R.; Roy, M.; Mandakmale, S.; Prakash, V.; Tantia, M.S. Genotyping of Novel SNPs in BMPR1B, BMP15, and GDF9 Genes for Association with Prolificacy in Seven Indian Goat Breeds. Anim. Biotechnol. 2016, 27, 199–207. [Google Scholar] [CrossRef]
  30. An, X.P.; Hou, J.X.; Zhao, H.B.; Li, G.; Bai, L.; Peng, J.Y.; Yan, M.Q.; Song, Y.X.; Wang, J.G.; Cao, B.Y. Polymorphism Identification in Goat GNRH1 and GDF9 Genes and Their Association Analysis with Litter Size. Anim. Genet. 2013, 44, 234–238. [Google Scholar] [CrossRef]
  31. Wang, X.; Yang, Q.; Wang, K.; Yan, H.; Pan, C.; Chen, H.; Liu, J.; Zhu, H.; Qu, L.; Lan, X. Two Strongly Linked Single Nucleotide Polymorphisms (Q320P and V397I) in GDF9 Gene Are Associated with Litter Size in Cashmere Goats. Theriogenology 2019, 125, 115–121. [Google Scholar] [CrossRef] [PubMed]
  32. Shaha, M.; Miah, G.; Lima, A.; Miazi, O.F.; Gupta, M.D.; Das, A. Identification of Polymorphisms in GDF9 and BMP15 Genes in Jamunapari and Crossbred Goats in Bangladesh. Trop. Anim. Health Prod. 2022, 54, 350. [Google Scholar] [CrossRef] [PubMed]
  33. Mahmoudi, P.; Rashidi, A.; Rostamzadeh, J.; Razmkabir, M. Association between c.1189G>A Single Nucleotide Polymorphism of GDF9 Gene and Litter Size in Goats: A Meta-Analysis. Anim. Reprod. Sci. 2019, 209, 106140. [Google Scholar] [CrossRef] [PubMed]
  34. Arefnejad, B.; Mehdizadeh, Y.; Javanmard, A.; Zamiri, M.J.; Niazi, A. Novel Single Nucleotide Polymorphisms (SNPs) in Two Oogenesis-Specific Genes (BMP15, GDF9) and Their Association with Litter Size in Markhoz Goat (Iranian Angora) Iran. J. Appl. Anim. Sci. 2018, 8, 91–99. [Google Scholar]
  35. Chu, M.X.; Wu, Z.H.; Feng, T.; Cao, G.L.; Fang, L.; Di, R.; Huang, D.W.; Li, X.W.; Li, N. Polymorphism of GDF9 Gene and Its Association with Litter Size in Goats. Vet. Res. Commun. 2011, 35, 329–336. [Google Scholar] [CrossRef]
  36. Zhang, Y.; Wang, H.; Li, T.; Zhang, N.; Chen, J.; Yang, H.; Peng, S.; Ma, R.; Wang, D.; Liu, Q.; et al. Association of BMP15 and GDF9 Gene Polymorphisms with Litter Size in Hu Sheep. Genes 2025, 16, 168. [Google Scholar] [CrossRef]
  37. Chen, Y.; Shan, X.; Jiang, H.; Sun, L.; Guo, Z. Regulation of litter size in sheep (Ovis aries) by the GDF9 and BMP15 genes. Ann. Agric. Sci. 2023, 68, 148–158. [Google Scholar] [CrossRef]
  38. Tang, J.; Hu, W.; Di, R.; Liu, Q.; Wang, X.; Zhang, X.; Zhang, J.; Chu, M. Expression analysis of the prolific candidate genes, BMPR1B, BMP15, and GDF9 in Small Tail Han ewes with three fecundity (FecB gene) genotypes. Animals 2018, 8, 166. [Google Scholar] [CrossRef]
  39. Ji, X.; Cao, Z.; Hao, Q.; He, M.; Cang, M.; Yu, H.; Ma, Q.; Li, X.; Bao, S.; Wang, J.; et al. Effects of New Mutations in BMPRIB, GDF9, BMP15, LEPR, and B4GALNT2 Genes on Litter Size in Sheep. Vet. Sci. 2023, 10, 258. [Google Scholar] [CrossRef]
  40. Wang, Y.; Chi, Z.; Jia, S.; Zhao, S.; Cao, G.; Purev, C.; Cang, M.; Yu, H.; Li, X.; Bao, S.; et al. Effects of novel variants in BMP15 gene on litter size in Mongolia and Ujimqin sheep breeds. Theriogenology 2023, 198, 1–11. [Google Scholar] [CrossRef]
  41. Yang, Z.; Yang, X.; Liu, G.; Deng, M.; Sun, B.; Guo, Y.; Liu, D.; Li, Y. Polymorphisms in BMPR-IB gene and their association with litter size trait in Chinese Hu sheep. Anim. Biotechnol. 2022, 33, 250–259. [Google Scholar] [CrossRef] [PubMed]
  42. Kirikçi, K. Investigation of BMP15 and GDF9 gene polymorphisms and their effects on litter size in Anatolian sheep breed Akkaraman. Turk. J. Vet. Anim. Sci. 2023, 47, 248–254. [Google Scholar] [CrossRef]
  43. Akhatayeva, Z.; Cao, C.; Huang, Y.; Zhou, Q.; Zhang, Q.; Guo, Z.; Tan, S.; Yue, X.; Xu, H.; Li, R.; et al. Newly reported 90-bp deletion within the ovine BMPRIB gene: Does it widely distribute, link to the famous FecB (p. Q249R) mutation, and affect litter size? Theriogenology 2022, 189, 222–229. [Google Scholar] [CrossRef] [PubMed]
  44. Wang, J.; Liu, Y.; Guo, S.; Di, R.; Wang, X.; He, X.; Chu, M. Polymorphisms of the BMPR1B, BMP15 and GDF9 fecundity genes in four Chinese sheep breeds. Arch. Anim. Breed. 2024, 67, 51–60. [Google Scholar] [CrossRef]
  45. Cui, W.; Wang, H.; Li, J.; Lv, D.; Xu, J.; Liu, M.; Yin, G. Sheep litter size heredity basis using genome-wide selective analysis. Reprod. Domest. Anim. 2024, 59, e14689. [Google Scholar] [CrossRef]
  46. Bai, Y.; Wang, S.; Wu, K.; Zhang, M.; Alatan, S.; Cang, M.; Cao, G.; Jin, H.; Li, C.; Tong, B. The Impact of Novel BMPR1B Mutations on Litter Size in Short-Tailed Gobi Sheep and Larger-Tailed Ujimqin Sheep. Vet. Sci. 2024, 11, 297. [Google Scholar] [CrossRef]
  47. Li, Y.; Jin, W.; Wang, Y.; Zhang, J.; Meng, C.; Wang, H.; Qian, Y.; Li, Q.; Cao, S. Three complete linkage SNPs of GDF9 gene affect the litter size probably mediated by OCT1 in Hu sheep. DNA Cell Biol. 2020, 39, 563–571. [Google Scholar] [CrossRef] [PubMed]
  48. Abdelgadir, A.Z.; Musa, L.M.; Jawasreh, K.I.; Saleem, A.O.; El-Hag, F.; Ahmed, M.K. G1 point mutation in growth differentiation factor 9 gene affects litter size in Sudanese desert sheep. Vet. World 2021, 14, 104. [Google Scholar] [CrossRef]
  49. Jia, J.; Chen, Q.; Gui, L.; Jin, J.; Li, Y.; Ru, Q.; Hou, S. Association of polymorphisms in bone morphogenetic protein receptor-1B gene exon-9 with litter size in Dorset, Mongolian, and Small Tail Han ewes. Asian-Australas. J. Anim. Sci. 2019, 32, 949. [Google Scholar] [CrossRef]
  50. Tang, J.S.; Hu, W.P.; Di, R.; Wang, X.Y.; Zhang, X.S.; Zhang, J.L.; Liu, Q.Y.; Chu, M.X. Expression analysis of BMPR1B, BMP15, and GDF9 in prolific and non-prolific sheep breeds during the follicular phase. Czech J. Anim. Sci. 2019, 64, 439–447. [Google Scholar] [CrossRef]
  51. Gorlov, I.F.; Kolosov, Y.A.; Shirokova, N.V.; Getmantseva, L.V.; Slozhenkina, M.I.; Mosolova, N.I.; Bakoev, N.F.; Leonova, M.A.; Kolosov, A.Y.; Zlobina, E.Y. GDF9 gene polymorphism and its association with litter size in two Russian sheep breeds. Rendiconti Lincei. Sci. Fis. E Nat. 2018, 29, 61–66. [Google Scholar]
  52. El-Halawany, N.; Kandil, O.M.; Abd-El-Monsif, A.S.; Al-Tohamy, A.F.; El-Sayd, Y.A.; Abdel-Shafy, H.; Abou-Fandoud, E.S.; Abdel-Azeem, S.N.; Abd El-Rahim, A.H.; Abdoon, A.S.; et al. Investigating the effect of GDF9, BMP15, BMP6 and BMPR1B polymorphisms on Egyptian sheep fecundity and their transcripts expression in ovarian cells. Small Rumin. Res. 2018, 165, 34–40. [Google Scholar] [CrossRef]
  53. Amini, H.R.; Ajaki, A.; Farahi, M.; Heidari, M.; Pirali, A.; Forouzanfar, M.; Eghbalsaied, S. The novel T755C mutation in BMP15 is associated with the litter size of Iranian Afshari, Ghezel, and Shal breeds. Arch. Anim. Breed. 2018, 61, 153–160. [Google Scholar] [CrossRef]
  54. Wang, S.; Bai, Y.; Wang, D.; Zhang, M.; Alatan, S.; Cang, M.; Jin, H.; Li, C.; Du, G.; Cao, G.; et al. Variants in BMP15 Gene Affect Promoter Activity and Litter Size in Gobi Short Tail and Ujimqin Sheep. Vet. Sci. 2025, 12, 222. [Google Scholar] [CrossRef]
  55. Cao, C.; Zhou, Q.; Kang, Y.; Akhatayeva, Z.; Liu, P.; Bai, Y.; Li, R.; Jiang, Y.; Zhang, Q.; Lan, X.; et al. A repertoire of single nucleotide polymorphisms (SNPs) of major fecundity BMPR1B gene among 75 sheep breeds worldwide. Theriogenology 2024, 219, 59–64. [Google Scholar] [CrossRef] [PubMed]
  56. Dimitrova, I.; Stancheva, N.; Bozhilova-Sakova, M.; Tzonev, T. Polymorphism in SNP G1 of the GDF9 gene associated with reproductive traits in Bulgarian dairy sheep. Biotechnol. Biotechnol. Equip. 2024, 38, 2399939. [Google Scholar] [CrossRef]
  57. Akhatayeva, Z.; Bi, Y.; He, Y.; Khan, R.; Li, J.; Li, H.; Pan, C.; Lan, X. Survey of the relationship between polymorphisms within the BMPR1B gene and sheep reproductive traits. Anim. Biotechnol. 2023, 34, 718–727. [Google Scholar] [CrossRef]
  58. Li, D.; Zhang, L.; Wang, Y.; Chen, X.; Li, F.; Yang, L.; Cui, J.; Li, R.; Cao, B.; An, X.; et al. FecB mutation and litter size are associated with a 90-base pair deletion in BMPR1B in East Friesian and Hu crossbred sheep. Anim. Biotechnol. 2023, 34, 1314–1323. [Google Scholar] [CrossRef]
  59. Mo, F.; Sun, W.; Zhang, L.; Zhang, X.; La, Y.; Xiao, F.; Jia, J.; Jin, J. Polymorphisms in BMPRIB gene affect litter size in Chinese indigenous sheep breed. Anim. Biotechnol. 2023, 34, 538–545. [Google Scholar] [CrossRef]
  60. Chong, Y.; Jiang, X.; Liu, G. An ancient positively selected BMPRIB missense variant increases litter size of Mongolian sheep populations following latitudinal gradient. Mol. Genet. Genom. 2022, 297, 155–167. [Google Scholar] [CrossRef]
  61. Zhang, Y.; Wang, Y.; Chen, Q.; Song, Y.; Zhang, H.; Jia, J. Evaluation of the BMPR-1B gene functional polymorphisms and their association with litter size in Qinghai Tibetan sheep. Small Rumin. Res. 2022, 216, 106816. [Google Scholar] [CrossRef]
  62. Çelikeloğlu, K.; Tekerli, M.; Erdoğan, M.; Koçak, S.; Hacan, Ö.; Bozkurt, Z. An investigation of the effects of BMPR1B, BMP15, and GDF9 genes on litter size in Ramlıç and Dağlıç sheep. Arch. Anim. Breed. 2021, 64, 223–230. [Google Scholar] [CrossRef]
  63. Di, R.; Wang, F.; Yu, P.; Wang, X.; He, X.; Mwacharo, J.M.; Pan, L.; Chu, M. Detection of novel variations related to litter size in BMP15 gene of luzhong mutton sheep (Ovis aries). Animals 2021, 11, 3528. [Google Scholar] [CrossRef] [PubMed]
  64. Wang, F.; Chu, M.; Pan, L.; Wang, X.; He, X.; Zhang, R.; Tao, L.; La, Y.; Ma, L.; Di, R. Polymorphism detection of GDF9 gene and its association with litter size in Luzhong mutton sheep (Ovis aries). Animals 2021, 11, 571. [Google Scholar] [CrossRef]
  65. Drobik-Czwarno, W.; Martyniuk, E.; Nowak-Życzyńska, Z.; Kaczor, U.; Kucharski, M. Frequency of BMP15 and GDF9 mutations increasing litter size and their phenotypic effects in Olkuska sheep population. Ann. Anim. Sci. 2021, 21, 89–108. [Google Scholar] [CrossRef]
  66. Najafabadi, H.A.; Khansefid, M.; Mahmoud, G.G.; Haruna, I.L.; Zhou, H.; Hickford, J.G. Identification of sequence variation in the oocyte-derived bone morphogenetic protein 15 (BMP15) gene (BMP15) associated with litter size in New Zealand sheep (Ovis aries) breeds. Mol. Biol. Rep. 2021, 48, 6335–6342. [Google Scholar] [CrossRef]
  67. Niu, Z.G.; Qin, J.; Jiang, Y.; Ding, X.D.; Ding, Y.G.; Tang, S.; Shi, H.C. The identification of mutation in BMP15 gene associated with litter size in Xinjiang cele black sheep. Animals 2021, 11, 668. [Google Scholar] [CrossRef] [PubMed]
  68. Gao, Y.; Hao, Q.; Cang, M.; Wang, J.; Yu, H.; Liu, Y.; Zhang, W.; Tong, B. Association between novel variants in BMPR1B gene and litter size in Mongolia and Ujimqin sheep breeds. Reprod. Domest. Anim. 2021, 56, 1562–1571. [Google Scholar] [CrossRef]
  69. Li, H.; Xu, H.; Akhatayeva, Z.; Liu, H.; Lin, C.; Han, X.; Lu, X.; Lan, X.; Zhang, Q.; Pan, C. Novel indel variations of the sheep FecB gene and their effects on litter size. Gene 2021, 767, 145176. [Google Scholar] [CrossRef]
  70. Wen, Y.L.; Guo, X.F.; Ma, L.; Zhang, X.S.; Zhang, J.L.; Zhao, S.G.; Chu, M.X. The expression and mutation of BMPR1B and its association with litter size in small-tail Han sheep (Ovis aries). Arch. Anim. Breed. 2021, 64, 211–221. [Google Scholar] [CrossRef]
  71. Tong, B.; Wang, J.; Cheng, Z.; Liu, J.; Wu, Y.; Li, Y.; Bai, C.; Zhao, S.; Yu, H.; Li, G. Novel variants in GDF9 gene affect promoter activity and litter size in Mongolia sheep. Genes 2020, 11, 375. [Google Scholar] [CrossRef] [PubMed]
  72. Saleh, A.A.; Hammoud, M.H.; Dabour, N.A.; Hafez, E.E.; Sharaby, M.A. BMPR-1B, BMP-15 and GDF-9 genes structure and their relationship with litter size in six sheep breeds reared in Egypt. BMC Res. Notes 2020, 13, 215. [Google Scholar] [CrossRef] [PubMed]
  73. Amirpour Najafabadi, H.; Khansefid, M.; Mahmoud, G.G.; Zhou, H.; Hickford, J.G. Identification of polymorphisms in the oocyte-derived growth differentiation growth factor 9 (GDF9) gene associated with litter size in New Zealand sheep (Ovis aries) breeds. Reprod. Domest. Anim. 2020, 55, 1585–1591. [Google Scholar] [CrossRef]
  74. Zhang, Z.; Liu, Q.; Di, R.; Hu, W.; Wang, X.; He, X.; Ma, L.; Chu, M. Single nucleotide polymorphisms in BMP2 and BMP7 and the association with litter size in Small Tail Han sheep. Anim. Reprod. Sci. 2019, 204, 183–192. [Google Scholar] [CrossRef]
  75. Guo, C.; Ye, J.; Liu, J.; Li, Z.; Deng, M.; Guo, Y.; Liu, G.; Sun, B.; Li, Y.; Liu, D. Whole-Genome Sequencing Identified Candidate Genes Associated with High and Low Litter Size in Chuanzhong Black Goats. Front. Vet. Sci. 2024, 11, 1420164. [Google Scholar] [CrossRef]
  76. Knight, P.G.; Glister, C. TGF-β Superfamily Members and Ovarian Follicle Development. Reproduction. 2006, 132, 191–206. [Google Scholar] [CrossRef] [PubMed]
  77. Han, Y.G.; Zeng, Y.; Huang, Y.F.; Huang, D.L.; Peng, P.; Na, R.S. A Nonsynonymous SNP within the AMH Gene Is Associated with Litter Size in Dazu Black Goats. Anim. Biotechnol. 2021, 33, 992–996. [Google Scholar] [CrossRef]
  78. Kharayat, N.S.; Parameshwarappa, M.A.; Ramdas, G.A.; Bisht, D.; Gautam, S.; Singh, S.K.; Krishnaswamy, N.; Chand, K.; Rialch, A.; Chandra, P.; et al. Anti-Müllerian Hormone Profile and Its Association with Ovarian Parameters in the Chaugarkha Goat. Small Rumin. Res. 2024, 230, 107165. [Google Scholar] [CrossRef]
  79. Monniaux, D.; Baril, G.; Laine, A.L.; Jarrier, P.; Poulin, N.; Cognié, J.; Fabre, S. Anti-Müllerian Hormone as a Predictive Endocrine Marker for Embryo Production in the Goat. Reproduction. 2011, 142, 845–854. [Google Scholar] [CrossRef]
  80. Turgut, A.O.; Koca, D. Anti-Müllerian Hormone as a Promising Novel Biomarker for Litter Size in Romanov Sheep. Reprod. Domest. Anim. 2024, 59, e14692. [Google Scholar] [CrossRef]
  81. Saneyasu, T.; Honda, K.; Kamisoyama, H. Myostatin Increases Smad2 Phosphorylation and Atrogin-1 Expression in Chick Embryonic Myotubes. J. Poult. Sci. 2019, 56, 224–230. [Google Scholar] [CrossRef]
  82. Villacorte, M.; Delmarcelle, A.-S.; Lernoux, M.; Bouquet, M.; Lemoine, P.; Bolsee, J.; Umans, L.; Lopes, S.C.d.S.; Van Der Smissen, P.; Sasaki, T.; et al. Thyroid Follicle Development Requires Smad1/Smad5- and Endothelial-Dependent Basement Membrane Assembly. Development. 2016, 134, 171. [Google Scholar]
  83. Weiss, A.; Attisano, L. The TGFβ Superfamily Signaling Pathway. WIREs Dev. Biol. 2013, 2, 47–63. [Google Scholar] [CrossRef] [PubMed]
  84. McReynolds, L.J.; Gupta, S.; Figueroa, M.E.; Mullins, M.C.; Evans, T. Smad1 and Smad5 Differentially Regulate Embryonic Hematopoiesis. Blood. 2007, 110, 3881–3890. [Google Scholar] [CrossRef]
  85. Kaivo-Oja, N.; Jeffery, L.A.; Ritvos, O.; Mottershead, D.G. Smad Signaling in the Ovary. Reprod. Biol. Endocrinol. 2006, 4, 21. [Google Scholar] [CrossRef]
  86. Xu, S.-S.; Gao, L.; Xie, X.-L.; Ren, Y.-L.; Shen, Z.-Q.; Wang, F.; Shen, M.; Eyϸórsdóttir, E.; Hallsson, J.H.; Kiseleva, T.; et al. Genome-Wide Association Analyses Highlight the Potential for Different Genetic Mechanisms for Litter Size Among Sheep Breeds. Front. Genet. 2018, 9, 118. [Google Scholar] [CrossRef] [PubMed]
  87. Bahire, S.V.; Rajput, P.K.; Kumar, V.; Kumar, D.; Kataria, M.; Kumar, S. Quantitative Expression of mRNA Encoding BMP/SMAD Signaling Genes in the Ovaries of Booroola Carrier and Non-Carrier GMM Sheep. Reprod. Domest. Anim. 2019, 54, 1375–1383. [Google Scholar] [CrossRef]
  88. Arnold, S.J.; Maretto, S.; Islam, A.; Bikoff, E.K.; Robertson, E.J. Dose-Dependent Smad1, Smad5, and Smad8 Signaling in the Early Mouse Embryo. Dev. Biol. 2006, 296, 104–118. [Google Scholar] [CrossRef]
  89. Sun, R.; Li, M.; He, N.; Wen, X.; Zhang, J. Molecular characterization, expression profiles of SMAD4, SMAD5 and SMAD7 genes and lack of association with litter size in Tibetan sheep. Animals 2022, 12, 2232. [Google Scholar] [CrossRef]
  90. Li, Q.; Pangas, S.A.; Jorgez, C.J.; Graff, J.M.; Weinstein, M.; Matzuk, M.M. Redundant Roles of SMAD2 and SMAD3 in Ovarian Granulosa Cells In Vivo. Mol. Cell. Biol. 2008, 28, 7001–7011. [Google Scholar] [CrossRef]
  91. Wijayanti, D.; Luo, Y.; Bai, Y.; Pan, C.; Qu, L.; Guo, Z.; Lan, X. New Insight into Copy Number Variations of Goat SMAD2 Gene and Their Associations with Litter Size and Semen Quality. Theriogenology 2023, 206, 114–122. [Google Scholar] [CrossRef] [PubMed]
  92. Wijayanti, D.; Zhang, S.; Bai, Y.; Pan, C.; Chen, H.; Qu, L.; Guo, Z.; Lan, X. Investigation on mRNA Expression and Genetic Variation within Goat SMAD2 Gene and Its Association with Litter Size. Anim. Biotechnol. 2023, 34, 2111–2119. [Google Scholar] [CrossRef]
  93. Wijayanti, D.; Zhang, S.; Yang, Y.; Bai, Y.; Akhatayeva, Z.; Pan, C.; Zhu, H.; Qu, L.; Lan, X. Goat SMAD Family Member 1 (SMAD1): mRNA Expression, Genetic Variants, and Their Associations with Litter Size. Theriogenology 2022, 193, 11–19. [Google Scholar] [CrossRef] [PubMed]
  94. Wang, K.; Liu, X.; Qi, T.; Hui, Y.; Yan, H.; Qu, L.; Lan, X.; Pan, C. Whole-Genome Sequencing to Identify Candidate Genes for Litter Size and to Uncover the Variant Function in Goats (Capra hircus). Genomics 2021, 113, 142–150. [Google Scholar] [CrossRef] [PubMed]
  95. Lai, F.N.; Zhai, H.L.; Cheng, M.; Ma, J.Y.; Cheng, S.F.; Ge, W.; Zhang, G.L.; Wang, J.J.; Zhang, R.Q.; Wang, X.; et al. Whole-Genome Scanning for the Litter Size Trait Associated Genes and SNPs under Selection in Dairy Goat (Capra hircus). Sci. Rep. 2016, 6, 38096. [Google Scholar] [CrossRef]
  96. Kempisty, B.; Jackowska, M.; Woźna, M.; Antosik, P.; Piotrowska, H.; Zawierucha, P.; Bukowska, D.; Jaśkowski, J.M.; Nowicki, M.; Brüssow, K.P. Expression and Cellular Distribution of INHA and INHB Before and After In Vitro Cultivation of Porcine Oocytes Isolated from Follicles of Different Size. BioMed Res. Int. 2012, 2012, 742829. [Google Scholar]
  97. Andreone, L.; Velásquez, E.V.; Abramovich, D.; Ambao, V.; Loreti, N.; Croxatto, H.B.; Parborell, F.; Tesone, M.; Campo, S. Regulation of Inhibin/Activin Expression in Rat Early Antral Follicles. Mol. Cell. Endocrinol. 2009, 309, 48–54. [Google Scholar] [CrossRef]
  98. Han, Y.; Jiang, T.; Shi, J.A.; Liu, A.; Liu, L. Role and regulatory mechanism of inhibin in animal reproductive system. Therogenology 2023, 202, 10–20. [Google Scholar] [CrossRef]
  99. Walton, K.L.; Goney, M.P.; Peppas, Z.; Stringer, J.M.; Winship, A.; Hutt, K.; Goodchild, G.; Maskey, S.; Chan, K.L.; Brûlé, E.; et al. Inhibin inactivation in female mice leads to elevated FSH levels, ovarian overstimulation, and pregnancy loss. Endocrinology 2022, 163, bqac025. [Google Scholar] [CrossRef]
  100. Abuzahra, M.; Wijayanti, D.; Effendi, M.H.; Mustofa, I.; Munyaneza, J.P.; Eid, L.A.; Ugbo, E.N. Association of INHA Gene Polymorphisms with Litter Size Trait in Indonesian Thin-Tailed Sheep. Trop. Anim. Sci. J. 2024, 47, 3. [Google Scholar] [CrossRef]
  101. Bian, Z.; Li, K.; Chen, S.; Man, C.; Wang, F.; Li, L. Association between INHA Gene Polymorphisms and Litter Size in Hainan Black Goats. PeerJ. 2023, 11, e15381. [Google Scholar] [CrossRef]
  102. Wang, S.Z.; E, G.X.; Zeng, Y.; Han, Y.G.; Huang, Y.F.; Na, R.S. Three SNPs within Exons of INHA and ACVR2B Genes Are Significantly Associated with Litter Size in Dazu Black Goats. Reprod. Domest. Anim. 2021, 56, 936–941. [Google Scholar] [CrossRef] [PubMed]
  103. Isa, A.M.; Bemji, M.N.; Wheto, M.; Williams, T.J.; Ibeagha-Awemu, E.M. Mutations in Inhibin Alpha Gene and Their Association with Litter Size in Kalahari Red and Nigerian Goats. Livest. Sci. 2017, 203, 106–109. [Google Scholar] [CrossRef]
  104. Liu, Q.Y.; He, Y.Q.; Ge, Y.; Chu, M.X.; Jin, M.; Zhang, Y.J.; Wang, J.Y.; Ma, X.K.; Di, R.; Huang, D.W.; et al. Polymorphism of Inhibin A Gene and Its Relationship with Litter Size in Chinese Indigenous Goat. J. Anim. Plant Sci. 2017, 27, 1488–1495. [Google Scholar]
  105. Zoheir, K.M.; El-Halawany, N.; Harisa, G.I.; Yang, L. Preliminary Investigation on Activin-A as a Candidate Gene Affecting Litter Size in Goats. Pak. J. Zool. 2015, 47, 3. [Google Scholar]
  106. Wu, W.; Hua, G.; Yang, L.; Wen, Q.; Zhang, C.; Zoheir, K.M.; Chen, S. Association Analysis of the INHA Gene with Litter Size in Boer Goats. Small Rumin. Res. 2009, 82, 139–143. [Google Scholar] [CrossRef]
  107. Huang, X.; Bao, Y.; Yang, F.; Li, X.; Wang, F.; Zhang, C. miR-134-3p Regulates Cell Proliferation and Apoptosis by Targeting INHBA via Inhibiting the TGF-β/PI3K/AKT Pathway in Sheep Granulosa Cells. Biology 2024, 14, 24. [Google Scholar] [CrossRef]
  108. Abuzahra, M.; Wijayanti, D.; Effendi, M.H.; Mustofa, I.; Munyaneza, J.P.; Eid, L.A.; Ugbo, E.N. Association of a Synonymous SNP of INHA Gene with Litter Size Trait in Indonesian Thin-Tailed Sheep. Trop. Anim. Sci. J. 2024, 47, 273–279. [Google Scholar]
  109. Tang, Y.; Lu, S.; Wei, J.; Xu, R.; Zhang, H.; Wei, Q.; Han, B.; Gao, Y.; Zhao, X.; Peng, S.; et al. Growth differentiation factor 9 regulates the expression of estrogen receptors via Smad2/3 signaling in goat cumulus cells. Theriogenology 2024, 219, 65–74. [Google Scholar] [CrossRef]
  110. Otsuka, F.; McTavish, K.J.; Shimasaki, S. Integral role of GDF-9 and BMP-15 in ovarian function. Mol. Reprod. Dev. 2011, 78, 9–21. [Google Scholar] [CrossRef]
  111. Paradis, F.; Novak, S.; Murdoch, G.K.; Dyck, M.K.; Dixon, W.T.; Foxcroft, G.R. Temporal regulation of BMP2, BMP6, BMP15, GDF9, BMPR1A, BMPR1B, BMPR2 and TGFBR1 mRNA expression in the oocyte, granulosa and theca cells of developing preovulatory follicles in the pig. Reproduction 2009, 138, 115. [Google Scholar] [CrossRef] [PubMed]
  112. Orisaka, M.; Orisaka, S.; Jiang, J.Y.; Craig, J.; Wang, Y.; Kotsuji, F.; Tsang, B.K. Growth differentiation factor 9 is antiapoptotic during follicular development from preantral to early antral stage. Mol. Endocrinol. 2006, 20, 2456–2468. [Google Scholar] [CrossRef]
  113. Getmantseva, L.; Bakoev, N.; Shirokova, N.; Kolosova, M.; Bakoev, S.; Kolosov, A.; Usatov, A.; Shevtsova, V.; Yuri, K. Effect of the GDF9 gene on the weight of lambs at birth. Bulg. J. Agric. Sci. 2019, 25, 153–157. [Google Scholar]
  114. de Castro, F.C.; Cruz, M.H.; Leal, C.L.V. Role of growth differentiation factor 9 and bone morphogenetic protein 15 in ovarian function and their importance in mammalian female fertility—A review. Asian-Australas. J. Anim. Sci. 2015, 29, 1065. [Google Scholar] [CrossRef]
  115. Gilchrist, R.B.; Lane, M.; Thompson, J.G. Oocyte-secreted factors: Regulators of cumulus cell function and oocyte quality. Hum. Reprod. Update 2008, 14, 159–177. [Google Scholar] [CrossRef] [PubMed]
  116. Moore, R.K.; Otsuka, F.; Shimasaki, S. Molecular basis of bone morphogenetic protein-15 signaling in granulosa cells. J. Biol. Chem. 2003, 278, 304–310. [Google Scholar] [CrossRef]
  117. Pangas, S.A. Bone morphogenetic protein signaling transcription factor (SMAD) function in granulosa cells. Mol. Cell. Endocrinol. 2012, 356, 40–47. [Google Scholar] [CrossRef]
  118. Lu, C.; Yang, W.; Chen, M.; Liu, T.; Yang, J.; Tan, P.; Li, L.; Hu, X.; Fan, C.; Hu, Z.; et al. Inhibin A inhibits follicle-stimulating hormone (FSH) action by suppressing its receptor expression in cultured rat granulosa cells. Mol. Cell. Endocrinol. 2009, 298, 48–56. [Google Scholar] [CrossRef]
  119. Knight, P.G.; Satchell, L.; Glister, C. Intra-ovarian roles of activins and inhibins. Mol. Cell. Endocrinol. 2012, 359, 53–65. [Google Scholar] [CrossRef]
  120. Yu, X.; Qiao, T.; Hua, L.; Liu, S.; Zhao, X.; Lv, C.; Zhao, X.; Wang, J.; Han, L.; Yang, L.; et al. Synergistic regulatory effect of inhibin and anti-Müllerian hormone on fertility of mice. Front. Vet. Sci. 2021, 8, 747619. [Google Scholar] [CrossRef]
  121. Jiao, Y.; Zhu, Y.; Guo, J.; Jiang, X.; Liu, X.; Chen, Y.; Cong, P.; He, Z. FecB mutation enhances follicle stimulating hormone sensitivity of granulosa cells by up-regulating the SMAD1/5–USF1–FSHR signaling pathway. Int. J. Biol. Macromol. 2024, 280, 135697. [Google Scholar] [CrossRef] [PubMed]
  122. Wang, X.; Guo, X.; He, X.; Liu, Q.; Di, R.; Hu, W.; Cao, X.; Zhang, X.; Zhang, J.; Chu, M. Effects of FecB mutation on estrus, ovulation, and endocrine characteristics in small tail Han sheep. Front. Vet. Sci. 2021, 8, 709737. [Google Scholar] [CrossRef] [PubMed]
  123. Sasi, R.; Kanakkaparambil, R.; Thazhathuveettil, A. Polymorphism of fecundity genes, BMPR1B, BMP15 and GDF9, in tropical goat breeds of Kerala. Gene Rep. 2020, 21, 100944. [Google Scholar] [CrossRef]
  124. Qi, M.Y.; Xu, L.Q.; Zhang, J.N.; Li, M.O.; Lu, M.H.; Yao, Y.C. Effect of the Booroola fecundity (FecB) gene on the reproductive performance of ewes under assisted reproduction. Theriogenology 2020, 142, 246–250. [Google Scholar] [CrossRef]
  125. Juengel, J.L.; Davis, G.H.; McNatty, K.P. Using sheep lines with mutations in single genes to better understand ovarian function. Reproduction 2013, 146, R111–R123. [Google Scholar] [CrossRef]
  126. Guo, X.; Fang, Y.; Liang, R.; Wang, X.; Zhang, J.; Dong, C.; Wang, B.; Liu, Y.; Chu, M.; Zhang, X.; et al. Single-cell RNA-seq reveals the effects of the FecB mutation on the transcriptome profile in ovine cumulus cells. Sci. Rep. 2024, 14, 13087. [Google Scholar] [CrossRef]
  127. Zhong, T.; Hou, D.; Zhao, Q.; Zhan, S.; Wang, L.; Li, L.; Zhang, H.; Zhao, W.; Yang, S.; Niu, L. Comparative whole-genome resequencing to uncover selection signatures linked to litter size in Hu Sheep and five other breeds. BMC Genom. 2024, 25, 480. [Google Scholar] [CrossRef] [PubMed]
Biology 14 00786 g001
Figure 1. Effects of AMH, SMADs, and Inhibins on litter size in sheep and goats.
Figure 1. Effects of AMH, SMADs, and Inhibins on litter size in sheep and goats.
Biology 14 00786 g001
Biology 14 00786 g002
Figure 2. Crosstalk between TGF-β signaling members and their association with litter size in sheep and goats. This figure depicts signaling pathways in ovarian follicles mediated by TGF-β superfamily members. GDF9, BMPs, and inhibins activate distinct SMAD-dependent pathways that regulate fertility genes and control follicular development. Mutations in GDF9, BMPR1B, and BMP15 alter these pathways, affecting ovulation rates and litter size. Inhibins modulate the process by blocking FSH action. These integrated pathways collectively determine follicular growth dynamics and reproductive outcomes. Note: “TFs” stands for transcription factors.
Figure 2. Crosstalk between TGF-β signaling members and their association with litter size in sheep and goats. This figure depicts signaling pathways in ovarian follicles mediated by TGF-β superfamily members. GDF9, BMPs, and inhibins activate distinct SMAD-dependent pathways that regulate fertility genes and control follicular development. Mutations in GDF9, BMPR1B, and BMP15 alter these pathways, affecting ovulation rates and litter size. Inhibins modulate the process by blocking FSH action. These integrated pathways collectively determine follicular growth dynamics and reproductive outcomes. Note: “TFs” stands for transcription factors.
Biology 14 00786 g002
Table 1. Summary of determinant genes (BMPs and GDF9) and their polymorphisms associated with litter size in sheep.
Table 1. Summary of determinant genes (BMPs and GDF9) and their polymorphisms associated with litter size in sheep.
GenesPolymorphismFemale Reproductive Traits BreedsCountryReferences
BMP15 
GDF9		c.31_33CTTLitter sizeHu SheepChina[36]
GDF9
BMP15		S395F and S427RIncrease the ovulation rate and litter sizeSheepChina[37]
BMPR1B
BMP15
GDF9		c.746A>G
c.31_33CTTinsdel
c.994A>G		Litter sizeUjimqin, Dorper × Ujimqin crossbred
Ujimqin
Suffolk × Ujimqin crossbred		China[39]
BMP15g.50985975 G>A and c.755 T>C
g.50988478C>A and g.50987863G>A		Litter sizeMongolia sheep
Ujimqin sheep		China[40]
BMPRIBg.29362047T > C, g.29427689G > ALitter sizeHu sheepChina[41]
BMP15
GDF9		 Litter sizeAkkaramanTurkey[42]
BMPRIBp.Q249RLitter sizeAustralian white, small-tailed Han, Guiqian semi-fine wool sheepChina[43]
BMPR1B, BMP15, GDF9 ProlificacyHan, Hu, Wadi, Tan sheep China[44]
BMPR1B, BMP2 Litter sizeChinese indigenous sheepChina[45]
BMPR1Bc.687G>A g.30058882_30058873GCAGATTAAAIndel Litter sizeGobi short-tailed sheepChina [46]
GDF9 Litter sizeSudanese desert sheepChina[48]
BMPR1BC864TLitter sizeDorset, Mongolian, small-tailed HanChina[49]
BMP15g.54285159_54285161TTA indel
g.54291460G>A, g.54288671C>T, g.54285159_54285161TTA indel		Litter sizeGobi short-tailed sheep
Ujimqin sheep		China[54]
BMPR1Bg.30050773C>TFecundityDuolang sheepChina [55]
GDF9 Litter sizeBulgarian sheepBulgaria[56]
BMPR1Bg.746A>G, g.29362047T>C, g.29427689G>A, g.29382337G>A, g.29382340G>A, g.29380965A>GLitter sizeSheep China[57]
BMPR1Bc.746A > GLitter sizeHu, East Friesian/Hu crossbred sheepChina[58]
BMPRIBrs427897187 G>A
rs403555643 A>G		Litter sizeOula sheepChina[59]
BMPRIB Litter sizeMongolian sheepChina[60]
BMPRIBA746G, T864C, A1354GLitter sizeQinghai Tibetan sheepChina[61]
BMPR1B, BMP15, GDF9 Litter sizeDağlıç sheepChina[62]
BMP15, GDF9 Litter sizeLuzhong mutton sheepChina[63,64]
BMP15, GDF9 Ovulation rate and litter size Olkuska sheepPoland[65]
BMP15c.31_33delLitter sizeFinnish Landrace sheep, Finnish, Landrace  ×  Texel-cross sheep, composite sheepNew Zealand[66]
BMP15p.L252PFecundityCele black SheepChina[67]
BMPR1Bg.29346567C>TLitter sizeMongolia, Ujimqin sheepChina[68]
BMPR1B Litter sizeChinese Australian white sheepChina[69]
BMPR1Bg.29380965A>GLitter sizeSmall-tailed Han sheepChina[70]
GDF9 Litter sizeHu sheep, Mongolian sheepChina[47,71]
BMPR1B, BMP15, GDF9 Litter sizeRahmani, Rahmani × Barki crossEgypt[72]
GDF9c.1111ALitter sizePurebred Finnish Landrace sheep, Finnish Landrace × Texel-cross sheep, composite sheepNew Zealand[73]
BMP2
BMP7		g.48462350C>T
g.58171856C>G and g.58171886A>C 		Litter sizeSmall-tailed Han sheepChina[74]
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Han, Y.; Cao, G.; Chen, W.; Wang, C.; Khan, M.Z. The Role of TGF-β Signaling Pathway in Determining Small Ruminant Litter Size. Biology 2025, 14, 786. https://doi.org/10.3390/biology14070786

AMA Style

Han Y, Cao G, Chen W, Wang C, Khan MZ. The Role of TGF-β Signaling Pathway in Determining Small Ruminant Litter Size. Biology. 2025; 14(7):786. https://doi.org/10.3390/biology14070786

Chicago/Turabian Style

Han, Ying, Guiling Cao, Wenting Chen, Changfa Wang, and Muhammad Zahoor Khan. 2025. "The Role of TGF-β Signaling Pathway in Determining Small Ruminant Litter Size" Biology 14, no. 7: 786. https://doi.org/10.3390/biology14070786

APA Style

Han, Y., Cao, G., Chen, W., Wang, C., & Khan, M. Z. (2025). The Role of TGF-β Signaling Pathway in Determining Small Ruminant Litter Size. Biology, 14(7), 786. https://doi.org/10.3390/biology14070786

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop