1 Introducion

Genetic diversity is a crucial component for the improvement of any crop, including wheat (Triticum aestivum L.). It provides the raw material for breeding programs to develop new varieties that can withstand biotic and abiotic stresses, thereby ensuring food security in the face of climate change and growing population demands. Studies have shown that the genetic diversity within elite wheat cultivars is limited, which restricts the potential for further improvements (Feuillet et al., 2008; Sehgal et al., 2015). The exploitation of genetic diversity from various sources, including landraces and wild relatives, has been identified as a key strategy for sustaining crop genetic improvement (Sehgal et al., 2015; Upadhyay et al., 2020; Kumar et al., 2022).

Exotic germplasm, which includes landraces, wild relatives, and other non-cultivated gene pools, holds a wealth of genetic variation that is often absent in modern elite cultivars. This genetic variation can be harnessed to introduce novel alleles for traits such as drought and heat tolerance, disease resistance, and improved nutritional content (Sehgal et al., 2015; Ceoloni et al., 2017; Govindaraj et al., 2020). For instance, synthetic hexaploids and landraces have been found to possess higher genetic diversity indices compared to elite cultivars, making them valuable resources for pre-breeding programs (Sehgal et al., 2015). The use of advanced breeding techniques, such as chromosome engineering and molecular markers, has facilitated the incorporation of beneficial genes from exotic germplasm into cultivated wheat varieties (Feuillet et al., 2008; Ceoloni et al., 2017; Wang et al., 2017).

The primary objective of this study is to provide a comprehensive overview of the current state of research on the utilization of exotic germplasm for wheat improvement. Specifically, this study aims to highlight the importance of genetic diversity in wheat breeding programs, discuss the role of exotic germplasm in enhancing various wheat traits, summarize the challenges and opportunities associated with the use of exotic germplasm, and provide insights into the latest technological advancements that facilitate the exploitation of exotic genetic variation.

By addressing these objectives, this study seeks to underscore the potential of exotic germplasm in developing high-yielding, stress-resistant, and nutritionally superior wheat varieties, thereby improving the quality and yield of wheat and contributing to global food security.

2 Overview of Exotic Germplasm

2.1 Definition and sources of exotic germplasm

Exotic germplasm refers to genetic material from plant species or varieties that are not commonly used in current breeding programs. These sources often include wild relatives, landraces, and other underutilized species that possess unique genetic traits not found in modern cultivars. For instance, Aegilops tauschii, a wild relative of wheat, has been utilized to introduce genetic variations into wheat, enriching the germplasm pool with novel traits (Zhou et al., 2021). Similarly, wild emmer wheat has been explored for its potential to improve heat stress tolerance in wheat (Balla et al., 2022).

2.2 Historical use of exotic germplasm in wheat breeding

Historically, exotic germplasm has played a crucial role in wheat breeding by providing genetic diversity necessary for the development of new varieties with improved traits. For example, the development of the Australian Bread Wheat Nested Association Mapping (NAM) population involved crossing diverse exotic parents with elite Australian varieties to incorporate genetic diversity and dissect complex traits (Chidzanga et al., 2021) (Figure 1). Additionally, the introgression of Aegilops tauschii into wheat has been a significant step in overcoming the narrow genetic base of the wheat D genome, thereby enhancing its breeding potential (Zhou et al., 2021). The use of landraces, such as those from the A.E. Watkins collection, has also been instrumental in identifying unique loci for climate resilience and other desirable traits (Cheng et al., 2023).

Chidzanga et al. (2021) presents the phylogenetic diversity and geographical distribution of NAM (nested association mapping) exotic parents used to develop the OzNAM population. Figure 1a: Displays the phylogenetic trees showing the diversity in the panel from which 76 diverse exotic donor lines were selected. The highlighted NAM parents indicate the chosen lines, emphasizing their genetic variation. Figure 1b: Maps the geographical origins of these exotic parents, highlighting their global distribution, particularly from regions with dry and hot climates. The selected donor lines represent diverse germplasm with traits such as drought and heat tolerance and nitrogen use efficiency. This genetic diversity aims to enhance the OzNAM population's resilience and productivity under Australian agronomic conditions, leveraging locally adapted wheat varieties Gladius and Scout as reference parents. This approach facilitates the development of robust wheat varieties suited to challenging environmental conditions.

2.3 Key characteristics of exotic germplasm

Exotic germplasm is characterized by its rich genetic diversity, which includes alleles and haplotypes that are often absent in modern cultivars. This diversity can be harnessed to improve various agronomic traits such as drought tolerance, heat stress adaptation, and photosynthetic capacity. For instance, the genetic variability in wild emmer wheat has been linked to significant improvements in heat stress tolerance (Balla et al., 2022). Similarly, the use of Aegilops tauschii has been shown to increase the single nucleotide polymorphism (SNP) rate in wheat by 62%, highlighting its potential to alleviate genetic bottlenecks (Joynson et al., 2020). Moreover, large-scale diversity analyses of wheat accessions have revealed unexplored genetic footprints that can be targeted for future breeding efforts (Sansaloni et al., 2020).

By leveraging the unique genetic traits found in exotic germplasm, wheat breeding programs can develop new varieties that are better equipped to meet the challenges posed by climate change and increasing global food demands.

3 Genetic Diversity in Exotic Germplasm

3.1 Assessment of genetic diversity

Morphological characterization is a fundamental approach to assess genetic diversity in wheat germplasm. This method involves evaluating various phenotypic traits such as plant height, spike length, grain yield, and other agronomic characteristics. For instance, a study on 20 wheat genotypes revealed significant differences in traits like the number of fertile spikes per plant and yield per plot, indicating substantial genetic variability (Negisho et al., 2021). Similarly, another research involving 35 diverse wheat genotypes showed highly significant differences in traits such as days to 50% flowering, days to maturity, and grain yield, underscoring the importance of morphological traits in genetic diversity assessment (Saleh et al., 2021).

Molecular markers and genomic approaches provide a more precise and comprehensive assessment of genetic diversity. Techniques such as genotyping-by-sequencing (GBS) and single nucleotide polymorphism (SNP) arrays are commonly used. For example, the molecular diversity of 1 423 spring bread wheat accessions was investigated using GBS loci, revealing thousands of new SNP variations, particularly in landraces adapted to drought and heat stress environments (Upadhyay et al., 2020). Another study utilized the 35 K Axiom Wheat Breeder’s Array to genotype 483 wheat genotypes, resulting in 14 650 quality-filtered SNPs, which provided a detailed genetic diversity profile (Chidzanga et al., 2021). These advanced genomic tools enable the identification of novel alleles and facilitate the introgression of beneficial traits from exotic germplasm into elite breeding lines.

3.2 Comparison with domestic germplasm

Comparing exotic germplasm with domestic varieties often reveals greater genetic diversity in the former. For instance, synthetic hexaploids were found to be genetically more diverse (DI=0.284) compared to elite varieties (DI=0.267) and landraces (DI=0.245) (Upadhyay et al., 2020). Another study on durum wheat germplasm showed large genetic variation across different geographical origins, with principal component analysis explaining 71% of the cumulative variation (El-rawy, 2020). These findings highlight the broader genetic base of exotic germplasm, which can be crucial for breeding programs aiming to enhance genetic diversity.

Exotic germplasm often possesses complementary traits that are absent or rare in domestic varieties. For example, landraces from regions like Iraq, Iran, and India were found to harbor new allelic variations for vernalization and glutenin genes, which are valuable for breeding programs targeting specific environmental adaptations and quality traits (Upadhyay et al., 2020). Additionally, synthetic-derived wheat lines have been shown to contribute alleles that increase grain yield under various environmental conditions, outperforming elite parent cultivars. These complementary traits from exotic germplasm can be harnessed to improve the resilience and productivity of domestic wheat varieties.

In summary, the assessment of genetic diversity in exotic germplasm through both morphological and molecular approaches reveals significant variability and novel traits that can be leveraged for wheat improvement. Comparing this diversity with domestic germplasm underscores the potential of exotic sources to enhance genetic variation and introduce complementary traits, thereby contributing to the development of superior wheat cultivars.

4 Utilization of Exotic Germplasm in Wheat Breeding

4.1 Successful case studies

The introgression of exotic germplasm has been pivotal in enhancing disease resistance in wheat. For instance, the introgression of chromosomal segments from wild relatives such as Aegilops tauschii has been shown to introduce novel haplotypes that confer resistance to various diseases (Zhou et al., 2021). Additionally, the use of synthetic hexaploid wheats (SHWs) has been effective in incorporating disease resistance traits into bread wheat, as demonstrated by the successful development of varieties with improved resistance profiles (Hao et al., 2019).

Exotic germplasm has also been utilized to improve wheat's tolerance to abiotic stresses. For example, recombinant lines of durum wheat with Thinopyrum ponticum segments have shown remarkable stability and improved performance under heat and water-deficit conditions (Giovenali et al., 2019). Similarly, synthetic-derived wheats have been identified with genomic regions associated with drought adaptability, enhancing their productivity under water-limited conditions (Afzal etal., 2019).

The introgression of exotic germplasm has led to significant yield improvements and enhanced quality traits in wheat. The use of synthetic hexaploid wheat in breeding programs has resulted in varieties with higher yield potential compared to those developed through conventional breeding methods (Hao et al., 2019). Moreover, the introgression of wild-relative genes has contributed to the adaptive diversity of modern bread wheat, enhancing both yield and quality traits (He et al., 2019).

4.2 Breeding strategies

Introgression breeding involves the transfer of specific traits from exotic germplasm into elite cultivars. This strategy has been successfully employed to introduce beneficial traits such as disease resistance, abiotic stress tolerance, and yield improvement. For instance, the introgression of Aegilops tauschii genetic variations into wheat has enriched the germplasm pool and facilitated the development of high-yielding varieties (Zhou et al., 2021). Additionally, targeted introgression of chromosome segments from exotic germplasm has been shown to improve yield and other agronomic traits in soybean, suggesting its potential applicability in wheat breeding (Ru and Bernardo, 2019).

Pre-breeding and advanced backcrossing are essential strategies for incorporating exotic germplasm into wheat breeding programs. These methods involve the initial crossing of exotic germplasm with elite cultivars, followed by multiple backcrosses to recover the desirable traits while minimizing linkage drag. The use of double top-cross (DTC) and two-phase selection procedures has been demonstrated to be effective in developing high-yielding wheat varieties with introgressed traits from synthetic hexaploid wheat (Hao et al., 2019). Moreover, advanced backcrossing has been employed to introduce and pyramid specific chromosome segments from exotic germplasm into elite cultivars, achieving significant genetic gains (Ru and Bernardo, 2019).

4.3 Genomic tools and techniques

Marker-assisted selection (MAS) has revolutionized the process of introgressing exotic traits into wheat. By utilizing molecular markers linked to desirable traits, breeders can efficiently select for these traits in the early generations of breeding programs. For example, the identification of quantitative trait loci (QTLs) associated with abiotic stress tolerance has facilitated the use of MAS in breeding programs aimed at improving stress resilience in major cereals, including wheat (Raj and Nadarajah, 2022).

Genomic selection (GS) is another powerful tool that leverages genome-wide marker information to predict the breeding value of individuals. This approach has been successfully applied in wheat breeding to enhance the selection process for complex traits such as yield and stress tolerance. The integration of GS with traditional breeding methods has the potential to accelerate the development of improved wheat varieties with introgressed exotic traits (Hao et al., 2020) (Figure 2).

Hao et al. (2020) demonstrates the double top-cross (DTC) and two-phase selection (2PS) strategies for introgressing synthetic wheat (SHW) into elite germplasm. The DTC strategy involves sequential crosses with three elite varieties, retaining 12.5% of SHW nuclear DNA. The 2PS strategy is implemented in two phases: the first phase (F2 and F3 generations) eliminates undesirable agronomic traits like tough glumes, late maturity, tall stature, and yellow rust susceptibility. The second phase (F4 onwards) focuses on improving yield. This method has successfully produced new wheat cultivars such as Shumai 580, Shumai 969, and Shumai 830, with a fourth, Shumai 114, expected soon. The effectiveness of the DTC-2PS strategy is evident in producing high-yielding, agronomically superior varieties with desirable traits from SHW, showcasing its potential for wheat breeding programs.

CRISPR and other gene-editing technologies offer unprecedented precision in the manipulation of genetic material. These tools have been employed to introduce specific genes or alleles from exotic germplasm into wheat, thereby enhancing its genetic diversity and adaptive potential. The use of CRISPR/Cas9 system, for instance, has enabled the targeted modification of genes associated with abiotic stress tolerance, providing new avenues for wheat improvement (Raj and Nadarajah, 2022).

In summary, the utilization of exotic germplasm in wheat breeding has led to significant advancements in disease resistance, abiotic stress tolerance, and yield improvement. The integration of advanced breeding strategies and genomic tools has further enhanced the efficiency and effectiveness of these efforts, paving the way for the development of resilient and high-yielding wheat varieties.

5 Challenges and Solutions

5.1 Barriers to the use of exotic germplasm

One of the primary challenges in utilizing exotic germplasm for wheat improvement is genetic incompatibility. Exotic germplasm often contains genetic variations that are not easily compatible with the genetic makeup of elite wheat cultivars. This incompatibility can result in reduced fertility, poor agronomic performance, and other undesirable traits when attempting to introgress beneficial alleles from exotic sources into modern wheat varieties (Zhou et al., 2021; Chidzanga et al., 2021; Sharma et al., 2021).

Linkage drag is another significant barrier, where undesirable traits are co-inherited with beneficial alleles due to their close proximity on the chromosome. This phenomenon complicates the breeding process as it requires additional steps to separate the beneficial traits from the undesirable ones, thereby prolonging the breeding cycle and reducing efficiency (Chidzanga et al., 2021; Sharma et al., 2021; Cheng et al., 2023).

5.2 Strategies to overcome challenges

To address genetic incompatibility and linkage drag, advanced breeding techniques such as marker-assisted selection (MAS) and genomic selection (GS) are employed. These techniques allow for the precise identification and selection of beneficial alleles while minimizing the co-introduction of undesirable traits. For instance, the use of nested association mapping (NAM) populations has been shown to effectively incorporate genetic diversity and dissect complex traits, thereby facilitating the introgression of beneficial alleles into elite wheat backgrounds (Molero et al., 2018; Chidzanga et al., 2021 Cheng et al., 2023).

Genomic and bioinformatics approaches play a crucial role in overcoming the challenges associated with the use of exotic germplasm. High-throughput sequencing and genotyping technologies enable the detailed characterization of genetic diversity and the identification of key genomic regions associated with desirable traits. For example, the construction of high-quality reference genomes and the development of genomic variation landscapes provide valuable resources for gene discovery and breeding (Sansaloni et al., 2020; Zhou et al., 2021; Yang et al., 2022). Additionally, bioinformatics tools such as ggComp facilitate the precise evaluation of germplasm resources, enabling the identification of beneficial alleles and their strategic introgression into modern wheat varieties (Joynson et al., 2020; Yang et al., 2022).

By leveraging these advanced techniques and approaches, the barriers to the use of exotic germplasm can be effectively mitigated, thereby enhancing the genetic diversity and overall resilience of wheat cultivars.

6 Future Prospects and Research Directions

6.1 Emerging trends in germplasm utilization

The utilization of exotic germplasm in wheat breeding is gaining momentum as researchers recognize the potential of wild relatives to enhance genetic diversity and improve crop resilience. Recent studies have demonstrated the successful introgression of Aegilops tauschii into wheat, enriching the genetic pool and providing valuable traits for breeding programs (Gorafi et al., 2018; Aberkane et al., 2020; Zhou et al., 2021). The development of synthetic hexaploid wheat (SHW) has been particularly noteworthy, with significant contributions to yield potential and stress tolerance (Hao et al., 2019; Afzal et al., 2019; Rosyara et al., 2019). The integration of advanced genomic tools, such as genome-wide association studies (GWAS) and haplotype-based approaches, is further refining the precision of germplasm utilization, enabling the identification of beneficial alleles and their targeted incorporation into breeding lines (Afzal et al., 2019 Balla et al., 2022).

6.2 Potential of synthetic wheats

Synthetic wheats, particularly those derived from Aegilops tauschii, have shown immense potential in wheat improvement. These synthetic derivatives have been instrumental in introducing novel genetic variations that enhance drought tolerance, disease resistance, and overall yield (Afzal et al., 2019; Aberkane et al., 2020; Zhou et al., 2021). The creation of synthetic octoploid wheat pools and the development of multiple synthetic derivatives (MSD) have provided robust platforms for exploring and harnessing genetic diversity (Gorafi et al., 2018 Zhou et al., 2021). The success of these synthetic wheats in breeding programs underscores their value in addressing global food security challenges, especially in the face of climate change (Hao et al., 2019; Rosyara et al., 2019; Aberkane et al., 2020).

6.3 Integrating exotic germplasm into modern breeding programs

Integrating exotic germplasm into modern wheat breeding programs requires strategic approaches to overcome challenges such as linkage drag and adaptation issues. The double top-cross (DTC) strategy and two-phase selection procedures have proven effective in incorporating synthetic hexaploid wheat into elite lines, resulting in high-yielding varieties with enhanced genetic diversity (Hao et al., 2019). The use of advanced genomic tools, such as SNP-GWAS and haplotype-GWAS, has facilitated the identification of key genomic regions associated with desirable traits, enabling more precise breeding efforts (Afzal et al., 2019; Balla et al., 2022). Additionally, the establishment of core germplasm collections and the systematic evaluation of exotic lines under diverse environmental conditions are critical for maximizing the benefits of exotic germplasm in breeding programs (Gorafi et al., 2018; Zhou et al., 2021; Balla et al., 2022).

In conclusion, the future of wheat improvement lies in the strategic utilization of exotic germplasm, particularly through the development and integration of synthetic wheats. Continued advancements in genomic technologies and breeding strategies will be essential in harnessing the full potential of these genetic resources to meet the growing demands for food security and climate resilience.

7 Concluding Remarks

The research on harnessing genetic diversity for wheat improvement using exotic germplasm has yielded several significant findings. The use of non-denaturing fluorescence in situ hybridization (ND-FISH) has revealed extensive chromosomal polymorphisms and genetic diversity among wheat lines, particularly highlighting the polymorphism of the B-genome over the A- and D-genomes. Studies have identified substantial genetic variability and diversity in wheat germplasm, which is crucial for breeding programs aimed at improving yield and adaptation to environmental fluctuations. A haplotype-based approach has been developed to enhance the precision of wheat breeding, enabling the identification of novel haplotypes with potential for trait improvement. The introgression of Aegilops tauschii genome into wheat has been shown to enrich the wheat germplasm pool and improve breeding efficiency. Additionally, wild emmer wheat diversity has been exploited to improve wheat's heat stress adaptation, identifying several quantitative trait loci (QTL) associated with heat tolerance. The development of nested association mapping (NAM) populations has provided a valuable genetic resource for breeding under dry and hot climates. Advances in genomics and phenomics have facilitated trait discovery in polyploid wheat, overcoming previous challenges related to its large genome and limited genetic diversity. Genome-wide analyses have identified adaptive traits in synthetic-derived wheats, highlighting the role of crop-wild introgressions in improving drought tolerance. Exome sequencing has underscored the significant contribution of wild-relative introgression to the adaptive diversity of modern bread wheat. Finally, the integration of exotic material has been shown to alleviate the genetic bottleneck in the D genome, enhancing photosynthetic capacity and overall genetic diversity.

The findings from these studies have profound implications for wheat improvement. The identification of extensive genetic diversity and specific chromosomal polymorphisms provides a rich resource for breeding programs aimed at enhancing wheat yield, stress tolerance, and adaptability to changing environmental conditions. The development of haplotype-based approaches and the introgression of genomes from wild relatives like Aegilops tauschii and wild emmer wheat offer new avenues for incorporating beneficial traits into elite wheat cultivars, thereby improving their agronomic performance. The creation of NAM populations and the application of advanced genomic and phenomic techniques enable more precise and efficient breeding strategies, addressing the challenges posed by climate change and the need for sustainable food production. The identification of adaptive traits through genome-wide analyses and exome sequencing further enhances our understanding of the genetic basis of important agronomic traits, facilitating the development of wheat varieties with improved drought tolerance, heat stress adaptation, and overall resilience. The strategic integration of exotic germplasm into breeding programs not only increases genetic diversity but also uncovers hidden variations that can be harnessed to optimize complex traits like photosynthetic capacity.

Future research should focus on several key areas to further harness genetic diversity for wheat improvement. First, expanding the use of advanced genomic tools and techniques, such as ND-FISH, haplotype-based approaches, and genome-wide association studies (GWAS), will be crucial for identifying and exploiting novel genetic variations. Second, continued efforts to introgress genomes from wild relatives and exotic germplasm into elite wheat varieties should be prioritized, with a focus on enhancing traits related to stress tolerance, yield, and adaptability. Third, the development and utilization of NAM populations and other genetic resources should be expanded to facilitate the dissection of complex traits and the incorporation of diverse genetic material into breeding programs. Fourth, integrating diverse data types, including genomic, epigenetic, and phenomic data, will be essential for leveraging big data approaches and machine learning to gain deeper insights into trait biology and improve breeding efficiency. Finally, collaborative efforts among researchers, breeders, and policymakers will be necessary to ensure the successful translation of these scientific advancements into practical applications that address global food security challenges and promote sustainable agriculture.

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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