Triticeae Genomics and Genetics, 2024, Vol.15, No.3, 152-161 http://cropscipublisher.com/index.php/tgg 154 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.
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