1 Introducion

Hexaploid wheat (Triticum aestivum L.) is a crucial species in global agriculture, characterized by its complex genetic structure comprising three distinct genomes (AABBDD). This genetic composition results from the hybridization of tetraploid wheat (Triticum turgidum, AABB) with diploid wild goat grass (Aegilops tauschii, DD) (Rosyara et al., 2019; Zhang et al., 2021). The hexaploid nature of wheat provides a rich source of genetic diversity, which is essential for breeding programs aimed at improving various agronomic traits. Studies have shown that synthetic hexaploid wheat (SHW), created by crossing durum wheat with Aegilops tauschii, introduces novel genes and genomic regions that can enhance grain mineral concentrations and disease resistance (Bhatta et al., 2018; Szabó-Hevér et al., 2018). Additionally, the genetic characterization of hexaploid wheat through techniques such as complete sequencing of chloroplast DNA and haplotype analysis has provided insights into the genetic variations and evolutionary history of this species (Gogniashvili et al., 2021).

Wheat is one of the most important staple crops globally, providing a significant portion of the daily caloric intake for millions of people. It is cultivated on more land area than any other commercial crop and continues to be a critical component of food security. The adaptability of wheat to diverse environmental conditions and its ability to produce high yields make it indispensable in addressing global food demands. However, the increasing challenges posed by climate change, such as high temperatures and water scarcity, necessitate continuous improvement in wheat varieties to ensure stable and high yields (Guan et al., 2018). The genetic diversity inherent in hexaploid wheat is a valuable asset in breeding programs aimed at developing resilient and high-yielding wheat varieties (Tillett et al., 2022). 

The primary objective of this research is to explore the impact of hexaploid genetics on wheat breeding strategies. This study will conduct a detailed analysis of the genetic variations within hexaploid wheat and their potential implications for breeding programs. It aims to assess the potential of synthetic hexaploid wheat to introduce beneficial traits, such as enhanced grain mineral concentrations and disease resistance. Additionally, the research will evaluate the stability and yield-related traits of hexaploid wheat under various environmental conditions, with the goal of developing targeted breeding strategies that utilize the genetic diversity of hexaploid wheat to improve food security and crop resilience.

2 Understanding Hexaploid Wheat Genetics

2.1 Evolution and origin of hexaploid wheat

2.1.1 Ancestral Species and Hybridization Events

The formation of hexaploid wheat (Triticum aestivum, AABBDD) underwent two crucial allopolyploidization events, involving three diploid progenitors. The initial hybridization event between tetraploid wheat (Triticum turgidum, AABB) and diploid goatgrass (Aegilops tauschii, DD) led to the creation of hexaploid wheat^5^6^9. This process facilitated the transfer of genetic diversity from the parental species to hexaploid wheat, thereby enhancing its adaptability and agronomic traits^8^9. Figure 1 intricately depicts the complex journey of forming hexaploid wheat from multiple ancestor species through hybridization and polyploidization events, involving the dynamics of genetic variation and natural selection (Liu et al., 2021).

2.1.2 Genetic composition of hexaploid wheat (AABBDD)

The genetic composition of hexaploid wheat includes three distinct genomes: A, B, and D. This complex genome structure provides a rich source of genetic diversity, which is crucial for breeding and improving wheat varieties. The A and B genomes were derived from tetraploid wheat, while the D genome was contributed by Aegilops tauschii (Yuan et al., 2020; Liu et al., 2021; Li et al., 2021). This combination of genomes has endowed hexaploid wheat with unique traits, such as increased yield potential and resilience to environmental stresses (Wan et al., 2020; Wan et al., 2023)

2.2 Genetic and genomic characteristics

2.2.1 Chromosome structure and organization

Hexaploid wheat exhibits a complex chromosome structure, with 21 pairs of chromosomes (2n = 6x = 42). The chromosomes are organized into three homologous sets corresponding to the A, B, and D genomes. This polyploid nature has led to frequent numerical and structural chromosome changes, which play a significant role in expanding genetic diversity and facilitating wheat evolution (Yuan et al., 2020; Zhang et al., 2021). Additionally, polyploidization has been shown to enhance genetic recombination, particularly in the D genome, further contributing to the evolutionary potential of hexaploid wheat (Wan et al., 2020). 

2.2.2 Key genetic markers and QTLs

Key genetic markers and quantitative trait loci (QTLs) are essential for identifying and selecting desirable traits in wheat breeding programs. Synthetic hexaploid wheat (SHW) has been utilized to introduce favorable genes from tetraploid and diploid donors, leading to the development of high-yield and disease-resistant cultivars (Brammer et al., 2021; Wan et al., 2023). For instance, QTLs related to stripe rust resistance and big-spike traits have been successfully pyramided into new wheat varieties using SHW (Wan et al., 2023). Moreover, advanced technologies such as genome-wide association studies (GWAS) and high-throughput genotyping have facilitated the identification of critical genetic markers, enabling precision breeding and genetic improvement of wheat (Li et al., 2021). 

3 Breeding Strategies in Hexaploid Wheat

3.1 Traditional breeding methods

3.1.1 Selection and crossbreeding

Traditional breeding methods in hexaploid wheat primarily involve selection and crossbreeding. These methods rely on phenotypic selection of desirable traits such as yield, disease resistance, and stress tolerance. For instance, synthetic hexaploid wheat (SHW) has been developed by crossing durum wheat with wild relatives like Aegilops tauschii to introduce beneficial traits into modern wheat varieties (Aberkane et al., 2020). This approach has led to the creation of high-yield cultivars with improved resistance to diseases and environmental stresses (Aberkane et al., 2020; Wan et al., 2023).

3.1.2 Limitations and challenges

Despite their successes, traditional breeding methods face several limitations. One major challenge is the time-consuming nature of these methods, as multiple generations are often required to stabilize desirable traits. Additionally, the genetic diversity available within the primary gene pool of wheat is limited, which can restrict the potential for further improvements (Aberkane et al., 2020; Okada et al., 2020). Moreover, traditional methods may not always effectively address complex traits controlled by multiple genes, such as drought tolerance and heat resistance (Liu et al., 2020; Alghabari et al., 2023).

3.2 Modern breeding techniques

3.2.1 Marker-assisted selection (MAS)

Marker-Assisted Selection (MAS) is a modern breeding technique that uses molecular markers to select for desirable traits at the seedling stage, thereby accelerating the breeding process. MAS has been particularly effective in identifying and incorporating quantitative trait loci (QTLs) associated with important traits such as drought tolerance and disease resistance (Liu et al., 2020). For example, QTL analysis in SHW has identified several markers linked to root traits under drought stress, facilitating the development of drought-tolerant wheat varieties (Liu et al., 2020).

3.2.2 Genomic selection (GS)

Genomic Selection (GS) is another advanced technique that uses genome-wide markers to predict the breeding value of individuals. This method allows for the selection of superior genotypes based on their genetic potential rather than just their phenotypic performance. GS has been shown to be effective in improving complex traits such as yield, quality, and stress tolerance in hexaploid wheat (Walkowiak et al., 2020). The use of multiple wheat genomes has revealed extensive genetic diversity, which can be harnessed through GS to develop next-generation wheat cultivars (Walkowiak et al., 2020).

3.3 Advanced genetic engineering

3.3.1 CRISPR/Cas9 and gene editing

CRISPR/Cas9 and other gene-editing technologies have revolutionized wheat breeding by enabling precise modifications of specific genes. These techniques allow for the targeted introduction or deletion of genes to enhance desirable traits such as disease resistance, yield, and stress tolerance. For instance, gene editing has been used to improve the heat tolerance of SHW by rapidly expressing heat shock proteins under stress conditions (Truong et al., 2020). This approach offers a faster and more efficient way to develop improved wheat varieties compared to traditional breeding methods.

3.3.2 Transgenic approaches

Transgenic approaches involve the introduction of foreign genes into the wheat genome to confer new traits. This method has been used to develop wheat lines with enhanced resistance to biotic and abiotic stresses. For example, transgenic SHW lines have been created to express antifungal enzymes and pathogen-related genes, providing resistance to leaf rust and other diseases (Truong et al., 2020). These transgenic lines also exhibit improved yield and quality traits, making them valuable resources for wheat breeding programs (Truong et al., 2020).

4 Impacts of Hexaploid Genetics on Breeding Outcomes

4.1 Yield improvement

4.1.1 Genetic basis of yield traits

Hexaploid wheat (Triticum aestivum) has a complex genome that provides a rich source of genetic variation for yield improvement. The genetic basis of yield traits in hexaploid wheat involves numerous quantitative trait loci (QTLs) that influence grain weight and number. Key genes such as TaGNI, TaGW2, TaCKX6, TaGS5, TaDA1, WAPO1, and TaRht1 have been identified to impact these traits, with some genes increasing grain weight while others affect grain number, highlighting the trade-offs in yield components (Tillett et al., 2022). Additionally, synthetic hexaploid wheat (SHW) derived from crossing tetraploid wheat with Aegilops tauschii has been shown to enhance early biomass traits, which are crucial for yield improvement (Yang et al., 2020).

4.1.2 Breeding for high yield varieties

Breeding strategies utilizing SHW have led to the development of high-yield wheat cultivars. For instance, the 'large population with limited backcrossing method' and the 'recombinant inbred line-based breeding method' have been employed to pyramid yield-related QTLs/genes from SHW into new cultivars, resulting in record-breaking high-yield wheat in southwestern China (Wan et al., 2023). The use of SHW has also facilitated the transfer of desirable traits such as early vigour and enhanced resource utilization efficiency, contributing to yield (Yang et al., 2020).

4.2 Disease resistance

4.2.1 Genetic resistance to major wheat diseases

Hexaploid wheat possesses a diverse array of resistance genes that provide protection against major wheat diseases. Advances in genomics have enabled the identification and characterization of these resistance genes, such as those involved in resistance to Fusarium head blight (FHB) and leaf rust. For example, the Sm1 gene associated with insect resistance and other nucleotide-binding leucine-rich repeat proteins have been identified in hexaploid wheat (Walkowiak et al., 2020). Additionally, SHW lines have been developed with resistance to leaf rust and heat stress, demonstrating the potential of SHW in enhancing disease resistance (Truong et al., 2020).

4.2.2 Incorporating resistance genes through breeding

Breeding programs have successfully incorporated resistance genes from hexaploid wheat into new cultivars. The creation of a wheat resistance gene atlas has been proposed to facilitate the rapid deployment of resistance genes against a wide range of pathogens, ensuring durable resistance in wheat crops (Hafeez et al., 2021). Furthermore, the development of synthetic hexaploid wheat lines with resistance to diseases such as powdery mildew and FHB has been achieved through the pyramiding of resistance genes, resulting in cultivars with both high yield potential and disease resistance (Zhu et al., 2022; Han et al., 2023).

4.3 Abiotic stress tolerance

4.3.1 Drought and heat tolerance

Hexaploid wheat has been a valuable resource for breeding drought-tolerant varieties. SHW lines possess numerous genes for drought tolerance, and QTL mapping has identified specific loci associated with root traits that contribute to drought resistance (Liu et al., 2020). Additionally, SHW lines have been developed with enhanced heat tolerance, as demonstrated by the rapid expression of heat shock proteins under stress conditions (Truong et al., 2020).

4.3.2 Breeding for climate resilience

Breeding for climate resilience in wheat involves the incorporation of genes that confer tolerance to abiotic stresses such as drought and heat. The use of SHW has been instrumental in this regard, as it combines the genetic diversity of wild relatives with the adaptability of modern wheat varieties. For instance, SHW lines have been developed with traits such as early vigour and enhanced biomass, which are crucial for resilience under changing climatic conditions (Yang et al., 2020). The integration of these traits into breeding programs aims to produce wheat cultivars that can withstand the challenges posed by climate change (Aberkane et al., 2020).

5 Case Studies and Applications

5.1 Successful breeding programs

5.1.1 High-yielding varieties

The development of synthetic hexaploid wheat (SHW) has significantly contributed to the creation of high-yielding wheat varieties. For instance, the breeding strategy involving the large population with limited backcrossing method' has successfully pyramided stripe rust resistance and big-spike-related QTLs/genes from SHW into new high-yield cultivars. This approach has led to the creation of record-breaking high-yield wheat in southwestern China (Wan et al., 2023). Additionally, the derivatives of goat grass (Aegilops tauschii) have been used to widen the genetic base for wheat breeding, resulting in high yield potential and good quality attributes in SHW-derived lines (Aberkane et al., 2020).

5.1.2 Disease-resistant varieties

SHW has also been instrumental in developing disease-resistant wheat varieties. For example, the SynDT line, a SHW developed in Korea, exhibits resistance to leaf rust by inducing the expression of antifungal enzymes and pathogen-related genes (Truong et al., 2020). Moreover, SHW has shown effective resistance to tan spot, a significant foliar disease, with 233 out of 443 SHW plants evaluated showing resistant reactions (Lozano-Ramírez et al., 2022). These disease-resistant varieties are crucial for maintaining wheat production in the face of biotic stress.

5.2 Role of international collaboration

5.2.1 Collaborative research projects

International collaboration has played a pivotal role in the development and dissemination of SHW. The International Maize and Wheat Improvement Center (CIMMYT) has been at the forefront, distributing over 10,000 samples of SHW to 110 institutions in 40 countries between 2000 and 2018. This collaborative effort has led to the release of at least 86 SHW-derived varieties in 20 countries, demonstrating the global impact of SHW on wheat breeding (Aberkane et al., 2020). Additionally, genome-wide association studies involving multiple international research teams have identified significant marker-trait associations for disease resistance, further enhancing the utility of SHW in breeding programs (Lozano-Ramírez et al., 2022).

5.2.2 Global genebank utilization

The utilization of global genebanks has been essential in the development of SHW. For instance, 629 unique accessions from 15 countries were used for pre-breeding, producing 1577 primary SHWs. These genebank resources have been crucial in transferring desirable traits from wild relatives into modern wheat varieties, thereby enhancing genetic diversity and resilience (Aberkane et al., 2020). The extensive use of genebank collections underscores the importance of international cooperation in leveraging genetic resources for wheat improvement.

6 Challenges and Future Perspectives

6.1 Genetic bottlenecks and diversity loss

6.1.1 Causes of genetic erosion

Genetic erosion in wheat breeding is primarily caused by the narrow genetic base of modern wheat varieties, which results from intensive selection and breeding practices aimed at improving specific traits such as yield, disease resistance, and quality. This has led to the loss of genetic diversity, making wheat crops more vulnerable to biotic and abiotic stresses (Sansaloni et al., 2020; Yang et al., 2022). The domestication and modern breeding of wheat have significantly reduced the genetic variation present in wild relatives and landraces, which are crucial reservoirs of alleles for stress tolerance and other beneficial traits (Aberkane et al., 2020; Ullah et al., 2020).

The principal component analysis shows the polymorphism across approximately 1,200 varieties, highlighting the significant reduction in genetic diversity among modern wheat varieties compared to wild relatives and landraces (Figure 2). Furthermore, the dendrogram in Figure 2b, depicting gene presence/absence variation (PAV) among different varieties, reveals the genetic similarities and differences, indicating how the narrowing of the genetic base may be attributed to selective pressures that fix specific advantageous genes while potentially losing others (Walkowiak et al., 2020). These visual analyses confirm how wheat genomes undergo genetic erosion under the influence of intensified selection and breeding practices, which is critical for developing future breeding strategies aimed at enhancing crop resilience and adaptability.

6.1.2 Strategies to enhance genetic diversity

To counteract genetic erosion, several strategies can be employed to enhance genetic diversity in wheat breeding. One effective approach is the utilization of synthetic hexaploid wheat (SHW), which is created by crossing tetraploid wheat with diploid wild relatives such as Aegilops tauschii. This method introduces new alleles and increases genetic recombination, thereby broadening the genetic base of wheat (Wan et al., 2020; Wan et al., 2023). Additionally, the incorporation of diverse germplasm from genebanks and the use of advanced genomic tools to identify and introgress beneficial alleles from wild relatives and landraces can further enhance genetic diversity (Aberkane et al., 2020; Sansaloni et al., 2020).

6.2 Integrating new technologies

6.2.1 Advances in genomics and biotechnology

Recent advances in genomics and biotechnology have revolutionized wheat breeding by providing tools for precise genetic analysis and manipulation. High-throughput sequencing technologies and the development of comprehensive wheat genome assemblies have facilitated the identification of genetic variations and the mapping of quantitative trait loci (QTLs) associated with important agronomic traits (Liu et al., 2020; Walkowiak et al., 2020). Techniques such as CRISPR/Cas9 and RNA interference (RNAi) allow for targeted gene editing, enabling the modification of specific genes to improve traits such as disease resistance, drought tolerance, and grain quality (Yang et al., 2022).

6.2.2 Future directions in wheat breeding

The future of wheat breeding lies in the integration of traditional breeding methods with modern genomic and biotechnological approaches. The development of SHW and the use of diverse wild relatives will continue to play a crucial role in introducing new genetic variation into wheat breeding programs (Zhang et al., 2021; Wan et al., 2023). Additionally, the application of genomic selection, which uses genome-wide markers to predict the performance of breeding lines, can accelerate the breeding process and improve the efficiency of selecting superior genotypes (Walkowiak et al., 2020; Sansaloni et al., 2020). Collaborative efforts among global research institutions and the sharing of genetic resources and data will be essential to address the challenges posed by climate change and the growing demand for wheat production (Aberkane et al., 2020; Michikawa et al., 2020).

7 Concluding Remarks

The research on hexaploid genetics has significantly advanced our understanding of wheat breeding strategies. Synthetic hexaploid wheat (SHW) has emerged as a crucial genetic resource, enabling the transfer of favorable genes from tetraploid and diploid donors to common wheat, thereby enhancing yield and resistance to various stresses. The creation of SHW by crossing goat grass (Aegilops tauschii) with durum wheat has widened the genetic base for wheat breeding, leading to the development of high-yield, disease-resistant varieties.

Studies have shown that SHW can tolerate aneuploidy, which is beneficial for maintaining genetic stability and diversity. The introduction of the U-genome from Aegilops umbellulata into synthetic hexaploids has resulted in significant phenotypic variations, which are valuable for breeding programs. Furthermore, SHW genotypes have demonstrated resilience under drought and heat stress conditions, making them suitable for breeding in challenging environments.

Genomic studies have revealed extensive structural rearrangements and introgressions from wild relatives in hexaploid wheat, which contribute to its adaptability and resistance to biotic and abiotic stresses. Quantitative trait loci (QTL) analysis has identified several QTLs associated with root traits under drought conditions, providing essential information for breeding drought-tolerant wheat varieties. Additionally, dynamic and reversible DNA methylation changes have been observed in polyploid wheat, which correlate with altered gene expression and transposable element activity, offering insights into polyploid genome evolution.

The findings from these studies underscore the potential of synthetic hexaploid wheat in future wheat breeding strategies. The broad genetic base provided by SHW can be leveraged to develop new wheat varieties with enhanced yield, disease resistance, and stress tolerance. The ability of SHW to tolerate aneuploidy and the phenotypic variations introduced by the U-genome highlight the importance of incorporating diverse genetic resources into breeding programs.

Future breeding strategies should focus on utilizing the genetic diversity and recombination potential of SHW to address global food security challenges. The identification of QTLs associated with desirable traits under stress conditions provides a roadmap for developing resilient wheat varieties. Moreover, understanding the epigenetic changes in polyploid wheat can inform breeding practices aimed at optimizing gene expression and transposable element activity for improved crop performance.

In conclusion, the integration of synthetic hexaploid wheat into breeding programs holds promise for meeting the increasing global demand for wheat production in the face of environmental challenges. By harnessing the genetic and epigenetic potential of SHW, breeders can develop next-generation wheat cultivars that are well-equipped to thrive in diverse and changing environments.

Acknowledgments

The author extends sincere thanks to two anonymous peer reviewers for their feedback on the manuscript of this study.

Conflict of Interest Disclosure

The author affirms 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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