1 Introduction

Cotton is a cornerstone of the global textile industry, providing essential raw materials for a wide range of products. Its economic and agricultural significance cannot be overstated, as it supports millions of livelihoods worldwide and contributes substantially to the economies of many countries (Rajput et al., 2023; Wang et al., 2024). Among the various factors that influence cotton yield and fiber quality, boll size and boll weight are particularly critical. These traits directly impact the overall productivity and quality of the cotton produced, making them key targets for improvement in cotton breeding programs (Zhang et al., 2016; Feng et al., 2022; Shi et al., 2024).

Improving boll weight and size in cotton presents several challenges. The genetic basis of these traits is complex, involving multiple quantitative trait loci (QTLs) and candidate genes that interact with environmental factors. Additionally, the stability of these traits across different environments adds another layer of complexity, as breeders must ensure that improvements are consistent and reliable under varying conditions (Zhang et al., 2016; Feng et al., 2022). The identification and manipulation of specific genes associated with boll weight and size require advanced genetic tools and techniques, such as genome-wide association studies (GWAS) and map-based cloning (Ahmed et al., 2020; Shaheen et al., 2021).

Genetic improvement plays a pivotal role in enhancing boll size and weight in cotton. Through the use of modern breeding techniques, such as marker-assisted selection (MAS) and genetic mapping, researchers can identify and select for desirable traits with greater precision (Zhang et al., 2016; Feng et al., 2022; Wang et al., 2024). For instance, the identification of stable QTLs and candidate genes associated with boll weight has provided valuable insights into the genetic mechanisms underlying these traits (Shi et al., 2024). Moreover, the integration of advanced genetic tools, such as next-generation sequencing and specific locus amplified fragment sequencing (SLAF-seq), has facilitated the construction of high-density genetic maps, further aiding in the identification of key genetic determinants (Zhang et al., 2016).

This study will explore successful strategies, genetic mechanisms, and breeding techniques for improving cotton boll weight and size. It will analyze case studies on the pathways for improving these traits, providing a comprehensive overview of the current research status and its practical applications in cotton breeding. This study aims to contribute to the genetic improvement of cotton yield and quality, supporting the development of the global textile industry and promoting agricultural sustainability.

2 Genetic Basis of Cotton Boll Weight and Size

2.1 Quantitative genetic traits of boll weight

The inheritance of boll weight and size in cotton is controlled by a polygenic model, where multiple genes contribute to the traits. These quantitative traits exhibit moderate to high heritability, indicating that genetic factors play a significant role in their variation. Morphological traits such as boll diameter, seed size, and boll number are closely associated with boll weight and size, and their genetic control often overlaps. Studies suggest that these traits are influenced by additive genetic effects, with some contribution from non-additive effects such as epistasis and dominance (Xia et al., 2014).

Genetic mapping has also revealed strong correlations between boll weight and other yield components, such as lint percentage and seed index. These correlations provide insights into the genetic architecture of yield traits, highlighting the interconnectedness of traits in breeding programs aimed at enhancing overall productivity (Zhu et al., 2021).

2.2 Key quantitative trait loci (QTLs)

Numerous QTLs associated with boll weight and size have been identified in cotton through genome-wide association studies (GWAS) and linkage mapping. For example, stable QTLs on chromosomes A08 and D13 have been identified as contributing significantly to boll weight across multiple environments, explaining substantial phenotypic variation (Feng et al., 2022). These stable QTLs are considered highly valuable for marker-assisted selection in cotton breeding programs.

Case studies have demonstrated the importance of consistent QTL performance across environments. For example, QTL hotspots on chromosome 16 for boll size and seed index have been identified, with certain QTLs explaining over 30% of the phenotypic variance (Li et al., 2002). These findings provide a robust foundation for developing high-yielding cotton varieties adapted to diverse environmental conditions.

2.3 Candidate genes and molecular pathways

Key genes regulating cell division, cell expansion, and fiber development have been implicated in the determination of boll weight and size. For instance, genes such as GhBRH1_A12 have been identified as critical regulators of early boll development through their roles in brassinosteroid-mediated pathways. Mutations in these genes result in significant changes in boll size and weight (Ahmed et al., 2020).

Hormonal pathways involving gibberellins and auxins also play crucial roles in boll development. Gibberellins are known to promote cell elongation, while auxins regulate cell division and differentiation, collectively contributing to boll growth. Recent transcriptomic analyses have revealed candidate genes within QTL regions that are significantly expressed during critical stages of boll development, offering targets for genetic improvement (Su et al., 2020).

3 Traditional Approaches to Improving Boll Weight and Size

3.1 Phenotypic selection

Phenotypic selection involves field trials and multi-generational selection to improve large-boll varieties. This method relies on selecting plants with desirable traits and breeding them over several generations to enhance these traits.

Several studies have demonstrated the effectiveness of phenotypic selection in developing high-boll-weight cotton varieties. For instance, direct selection for boll weight in early segregating generations (F2, F3, and F4) has shown significant improvements in boll weight and other yield traits. The study conducted at Sakha Agricultural Research Station highlighted that direct selection for boll weight could increase itself and other related traits such as seed index, lint index, and fiber length (Hibbiny, 2020). Similarly, another study on the cotton cross Giza 80 x Giza 85 showed that direct selection from F2 to F4 generations resulted in significant genetic gains for boll weight and other yield components (Ahmed and Haridy, 2022).

3.2 Hybrid breeding

Hybrid breeding exploits hybrid vigor (heterosis) to enhance boll weight traits. This approach involves selecting superior parental lines and crossing them to produce hybrids with improved traits. Hybrid breeding has been effective in improving boll weight by utilizing the genetic diversity of parental lines. For example, a study on interspecific hybrids of Gossypium hirsutum and Gossypium barbadense demonstrated significant genetic variability and heritability for boll weight, indicating the potential for hybrid breeding to enhance this trait (Abasianyanga and Balu, 2017). Additionally, the use of recombinant inbred lines (RILs) from interspecific crosses has shown moderate heritability for boll weight, leading to a predicted selection response of 6.7%~24.0% under a selection intensity of 10% (Liu et al., 2011).

3.3 Backcrossing and multi-trait breeding

Backcrossing and multi-trait breeding strategies combine boll weight improvement with resistance traits, aiming to develop varieties with multiple desirable characteristics. Multi-trait breeding strategies have been employed to improve boll weight along with other yield and resistance traits. For instance, a study on the genetic variability and correlation analysis of various yield traits in cotton found significant correlations between boll weight and other yield components, suggesting that selection for boll weight could simultaneously improve other traits (Abbas et al., 2015). Another study using a high-density genetic map identified stable quantitative trait loci (QTLs) for boll weight across multiple environments, providing valuable information for marker-assisted selection (MAS) breeding (Zhang et al., 2016). These strategies enable the development of cotton varieties with enhanced boll weight and improved resistance to environmental stresses.

4 Applications of Modern Genetic Technologies in Boll Weight and Size Improvement

4.1 Marker-assisted selection (MAS)

Marker-Assisted Selection (MAS) has revolutionized cotton breeding by enabling the rapid screening of boll-weight-related genes using molecular markers such as SSR (Simple Sequence Repeat) and SNP (Single Nucleotide Polymorphism). These markers facilitate the identification and selection of desirable traits at the DNA level, significantly accelerating the breeding process.

SSR and SNP markers are extensively used in MAS to identify quantitative trait loci (QTLs) associated with boll weight. For instance, a study identified 19 significant markers and 225 candidate genes related to boll weight using SNPs and InDels in a global collection of cotton accessions (Feng et al., 2022). Similarly, another research utilized 359 SSR markers to detect QTLs associated with boll weight and other yield components in Upland cotton (Qin et al., 2015). These markers help in pinpointing the genetic regions responsible for boll weight, thereby facilitating targeted breeding efforts.

MAS has been instrumental in the development of superior cotton cultivars. For example, by targeting SSR markers associated with fiber quality and boll weight, the "Ravnaq" series of Upland cotton cultivars was developed using MAS (Figure 1). The results showed significant improvements in fiber quality in the "Ravnaq" cultivar, providing a valuable reference for breeding high-value cotton varieties (Darmanov et al., 2022). This approach not only shortened the breeding cycle but also ensured the precise transfer of desirable traits from donor to recipient genotypes. The efficiency of MAS in improving quantitative traits has been well-documented, showing substantial increases in selection efficiency when combined with traditional breeding methods (Lande and Thompson, 1990).

Figure 1 New ICT based fertility management model in private dairy farm India as well as abroad

Genomic prediction models leverage high-density marker data to predict the performance of breeding lines, thereby enabling the selection of superior genotypes at an early stage. For instance, a study on Upland cotton used SNP linkage mapping and QTL analysis to identify stable QTLs for boll weight, which can be incorporated into genomic prediction models to improve breeding efficiency (Li et al., 2016). The integration of genomic data with phenotypic data allows for more accurate selection, reducing the time and resources required for developing high-yielding varieties.

The application of GS in cotton breeding has shown promising results. By using genotyping-by-sequencing (GBS), researchers have been able to discover and genotype SNPs across the cotton genome, facilitating the implementation of GS in breeding programs (He et al., 2014). This approach has led to the development of cotton varieties with enhanced boll weight and other desirable traits. The use of GS in combination with MAS provides a comprehensive strategy for improving boll weight, as it allows for the simultaneous selection of multiple traits, thereby accelerating the breeding process and increasing the likelihood of success (Wu, 2024).

4.3 Gene editing technologies (CRISPR/Cas9)

The CRISPR/Cas9 system has revolutionized genetic engineering by enabling precise and efficient modifications of target genes. This technology has been widely adopted in various crops to enhance traits such as disease resistance, yield, and quality. In the context of cotton, CRISPR/Cas9 offers a powerful tool for the functional validation and precise manipulation of genes associated with boll weight traits.

CRISPR/Cas9 allows for the targeted editing of specific genes to study their functions and impacts on boll weight and size. For instance, the technology has been used to create knockouts and gain-of-function alleles, which are essential for understanding gene roles in crop development (Arora and Narula, 2017; Xu et al., 2020; Zhou et al., 2020). The ability to generate precise modifications, such as single-nucleotide polymorphisms (SNPs) in regulatory elements, further enhances the potential of CRISPR/Cas9 in crop improvement (Li et al., 2022).

A specific application of CRISPR/Cas9 in cotton involves the targeted editing of genes related to fruit development, such as GhExpansin. By manipulating these genes, researchers can study their direct impact on boll size. For example, editing the promoter regions of genes like GhExpansin can lead to variations in gene expression, thereby affecting boll development and size (Gao et al., 2017; Li et al., 2022). This approach has been successfully demonstrated in other crops, such as tomatoes, where editing the KLUH promoter resulted in significant increases in fruit weight (Li et al., 2022).

4.4 Integration of transcriptomics and metabolomics

The integration of transcriptomics and metabolomics provides a comprehensive approach to deciphering the regulatory networks involved in boll weight traits. These technologies enable the identification of key metabolic pathways and regulatory genes that govern boll development.

Transcriptomics involves the study of RNA transcripts to understand gene expression patterns, while metabolomics focuses on the chemical processes involving metabolites. By combining these approaches, researchers can gain insights into the complex interactions between genes and metabolic pathways that influence boll weight and size (Chen et al., 2019; Wang et al., 2021; Wan et al., 2021).

In cotton, the integration of transcriptomics and metabolomics has been used to identify critical genes and pathways involved in boll development. For instance, studies have shown that manipulating the expression of certain genes can lead to changes in metabolic fluxes, thereby affecting boll size and weight (Zhu et al., 2018; Kaur et al., 2020). This holistic approach allows for the identification of potential targets for genetic improvement, providing a roadmap for enhancing boll traits through precise genetic interventions.

5 Case Studies on Boll Weight and Size Improvement

5.1 Development of high-yield varieties

In India, breeding programs have focused on enhancing boll weight to improve overall cotton yield. Studies have shown that hybridization and irradiation techniques significantly contribute to increasing boll weight and other yield components. For instance, progenies from F4M4 generations exhibited higher mean performance and a wider range of values for traits like boll weight and seed cotton yield, indicating the effectiveness of these breeding strategies (Wadeyar and Kajjidoni, 2021). Additionally, genetic studies on diverse genotypes collected across India revealed high heritability and genetic advance for boll weight, suggesting that these traits are governed by additive gene action, making them suitable targets for breeding programs (Rajamani et al., 2015).

5.2 Utilizing exotic germplasm resources

The utilization of exotic germplasm has proven beneficial in enhancing boll weight in commercial cotton varieties. For example, the introduction of wild cotton germplasm into breeding programs has led to the identification of stable elite haplotypes and potential candidate genes associated with boll weight. A global collection study of 290 diverse germplasm resources revealed that the Northwest Inland region and the Yellow River region are the primary concentration areas for resource distribution. Breeding activities significantly increased after 2000, reflecting the rapid development of modern cotton breeding technologies. The introduction of Amerasian resources expanded genetic diversity, providing an important germplasm foundation for future improvements in cotton stress resistance, yield, and quality (Figure 2). The study also identified several key markers and candidate genes critical to boll weight, which can be utilized for molecular marker-assisted selection (MAS) breeding (Feng et al., 2022). Furthermore, the genetic dissection of boll size regulation using map-based cloning has identified key loci and genes, such as qBWT-c12 and GhBRH1_A12, which play a role in boll development and can be utilized to improve commercial varieties (Ahmed et al., 2020).

Figure 2 Map of the 290 cotton accessions (Adopted from Feng et al., 2022) Image Caption: (A) Geographic distribution of the natural population; each accession is represented by a dot; (B) Pie chart of the proportions of diverse cotton-growing areas in 290 accessions. NIR: Northwest Inland region in China; NSER: Northern-Specific Early-Maturity region; YRR: Yellow River region; YZRR: Yangtze River region; and Amerasian: 27 accessions primarily introduced from six different countries (USA, Azerbaijan, Israel, Kyrgyzstan, Tajikistan, and Uzbekistan); (C) Breeding stage distribution of the GWAS panel; Unknown: accessions that were not found among the pedigrees (Adopted from Feng et al., 2022)

5.3 Development of stress-resistant boll-weight varieties

Developing stress-resistant cotton varieties that maintain high boll weight under adverse conditions is crucial for sustaining yield. Strategies include the selection of genotypes with favorable alleles for stress tolerance and the use of advanced breeding techniques. For instance, the identification of donor parents with favorable alleles for traits like boll weight and seed cotton yield can aid in developing stress-resistant varieties. Studies have shown that genotypes such as Surabhi and RAH 1004 possess high genetic affinity and favorable alleles for improving boll weight and yield under stress conditions (Balakrishna et al., 2023).

In Africa, breeding programs have successfully developed heat-tolerant cotton varieties with high boll weight. For example, inter-program crosses between germplasm from Texas A&M AgriLife Research and Burkina Faso have led to the development of hybrids with improved agronomic traits, including boll weight. Hybrids such as E32×15-3-416 and FK64×15-3-416 have shown significant improvements in boll weight and other yield components, demonstrating the potential of these breeding strategies to enhance cotton production in heat-stressed environments (Bourgou et al., 2022).

6 Challenges and Limitations in Boll Weight and Size Improvement

6.1 Trade-offs between genetic and phenotypic traits

Improving boll weight and size in cotton often involves trade-offs with other yield-related traits. For instance, while increasing boll weight can enhance overall yield, it may negatively impact other traits such as boll number and lint percentage. Studies have shown that boll weight and boll number often exhibit a negative genetic and phenotypic correlation, indicating that selecting for larger bolls could reduce the number of bolls per plant, thereby affecting total yield (Mei et al., 2013; Song et al., 2015). Additionally, traits like seed index and lint percentage, which are crucial for fiber quality, may also be adversely affected by the selection for larger boll size (Abbas et al., 2015; Zhu et al., 2021). Therefore, breeders must carefully balance these trade-offs to optimize both yield and quality.

6.2 Environmental variability

Environmental factors such as temperature, water availability, and soil nutrients significantly impact boll weight and size. Research indicates that the genetic effects of yield traits, including boll weight, are highly susceptible to environmental interactions (Jia et al., 2014). For example, variations in temperature and water stress can lead to significant fluctuations in boll weight across different growing seasons and locations (Mei et al., 2013; Feng et al., 2022). Soil nutrient availability also plays a crucial role, as nutrient deficiencies can limit the potential genetic gains in boll size and weight (Alishah, 2021). These environmental variabilities necessitate the development of cotton varieties that are resilient and adaptable to diverse growing conditions.

6.3 Technical and resource limitations

The high costs associated with genomic tools and resource constraints pose significant challenges for breeding programs, especially in low-income regions. Advanced genomic techniques such as genome-wide association studies (GWAS) and marker-assisted selection (MAS) are essential for identifying and utilizing genetic markers linked to desirable traits like boll weight (Zhu et al., 2021; Feng et al., 2022). However, the implementation of these technologies requires substantial financial investment and technical expertise, which may not be readily available in resource-limited settings (Hibbiny, 2020). Consequently, breeding programs in these regions may struggle to keep pace with advancements in genetic improvement, limiting their ability to achieve significant gains in boll weight and size.

7 Future Directions for Boll Weight and Size Improvement

7.1 Integration of multi-omics approaches

Leveraging multi-omics approaches, such as transcriptomics, proteomics, and metabolomics, can significantly enhance our understanding of the mechanisms underlying boll development in cotton. These technologies allow for the comprehensive analysis of gene expression, protein interactions, and metabolic pathways, providing a holistic view of the factors influencing boll size and weight. For instance, the integration of genomics with other omics has been shown to elucidate growth and yield responses to various stresses in crops, including cotton (Yang et al., 2021). By employing these advanced techniques, researchers can identify key regulatory genes and pathways that can be targeted for genetic improvement, ultimately leading to the development of cotton varieties with enhanced boll weight and size.

7.2 Precision breeding for adaptability

Precision breeding strategies tailored to specific ecological regions can optimize boll weight and size in cotton. This involves the use of advanced breeding techniques, such as marker-assisted selection (MAS) and genomic selection, to develop cotton varieties that are well-adapted to local environmental conditions. For example, the identification of stable elite haplotypes and candidate genes associated with boll weight across multiple environments can inform breeding programs aimed at increasing yield through MAS (Feng et al., 2022). Additionally, the evaluation of genetic variation and heritability of agronomic traits in diverse cotton populations can guide the selection of parent lines for breeding programs targeting specific regions (Percy et al., 2006).

7.3 Germplasm sharing and international collaboration

Promoting international cooperation for the sharing of germplasm resources and technologies is crucial for the global improvement of boll weight and size in cotton. Collaborative efforts can facilitate the exchange of genetic materials and breeding techniques, enabling researchers to access a broader genetic pool and accelerate the development of superior cotton varieties. For instance, the identification of donor parents containing favorable alleles for improving target cotton hybrids highlights the importance of utilizing diverse genetic resources for breeding programs (Balakrishna et al., 2023). By fostering international partnerships, researchers can leverage collective expertise and resources to address common challenges and achieve significant advancements in cotton breeding.

7.4 Sustainability and climate adaptation

Developing new cotton cultivars optimized for boll weight traits to address climate change challenges is essential for ensuring sustainable cotton production. Climate-smart breeding approaches that incorporate resilience to abiotic stresses, such as drought and heat, can enhance the adaptability of cotton varieties to changing environmental conditions. Studies have shown that the application of macronutrients, such as nitrogen, phosphorus, and potassium, can improve boll development and physiological activities in cotton, suggesting that nutrient management can play a role in climate adaptation strategies (Ahmad et al., 2019). Additionally, the genetic analysis of dry matter weight in boll-leaf systems can provide insights into the developmental processes that contribute to boll weight, informing breeding efforts aimed at enhancing resilience to climate stress (Wen, 2012).

Acknowledgments

Many thanks to Dr Fang for their support and assistance in literature review and data analysis.

Conflict of Interest Disclosure

The authors affirm that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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