Molecular Plant Breeding 2024, Vol.15, No.4, 198-208 http://genbreedpublisher.com/index.php/mpb 200 These shifts reflect a broader trend towards leveraging multi-disciplinary technologies to achieve sustainable genetic improvement in maize breeding programs. 3 Genetic Basis of Exotic Varieties 3.1 Sources of exotic germplasm Exotic germplasm, which includes genetic material from outside the breeder's target area, has been a valuable resource for broadening the genetic base of local maize populations. Sources of exotic germplasm include temperate, tropical, and sub-tropical regions. For instance, the Germplasm Enhancement of Maize (GEM) project has been instrumental in incorporating exotic germplasm into U.S. maize breeding programs by selecting progeny lines from crosses between elite temperate lines and exotic parents (Rogers et al., 2022). Additionally, landraces from regions such as Sahel and Coastal West Africa have shown significant genetic diversity, offering novel alleles for enriching elite maize germplasm (Nelimor et al., 2020). 3.2 Genetic characteristics and traits Exotic germplasm is characterized by a wide range of genetic traits that can be beneficial for maize improvement. For example, teosinte alleles have been found to increase oil and carotenoid traits in maize kernels, highlighting their potential for nutritional enhancement (Figure 2) (Fang et al., 2019). Furthermore, hybrids derived from temperate and tropical germplasm have shown high grain yield potential and stability across diverse agro-ecologies, as well as early maturity (Nyoni et al., 2021). The genetic diversity in exotic germplasm also includes traits such as resistance to lodging and higher number of leaves above the cob, which are crucial for improving yield under high plant density conditions (Ndou et al., 2021). 3.3 Methods of identifying useful exotic traits Identifying useful traits in exotic germplasm involves several methods, including phenotypic characterization, quantitative trait loci (QTL) mapping, and genomic prediction. Phenotypic characterization helps in elucidating variation in agronomic traits, as demonstrated by the assessment of 196 maize landraces for 26 agronomic traits, which revealed significant genetic variability (Nelimor et al., 2020). QTL mapping has been used to identify loci associated with nutritional traits, such as oil and carotenoid content, providing insights into the genetic basis of these traits (Fang et al., 2019). Genomic prediction models, like those developed for the GEM project, have shown promise in predicting genetic gain and enhancing the efficiency of breeding programs by leveraging genotype data (Rogers et al., 2022; Tibbs-Cortes et al., 2022). Additionally, haplotype-trait association mapping has been employed to discover beneficial haplotypes for complex traits, making native diversity accessible for elite germplasm improvement (Mayer et al., 2020). By integrating these methods, breeders can effectively harness the genetic potential of exotic germplasm to improve maize varieties, ensuring better yield, nutritional quality, and resilience to environmental stresses. 4 Techniques for Incorporating Exotic Varieties Incorporating exotic varieties into maize breeding programs is essential for enhancing genetic diversity and improving various agronomic traits. Several techniques have been developed to effectively integrate these exotic germplasms into elite maize lines. This section discusses four primary techniques: introgression and backcrossing, marker-assisted selection (MAS), genomic selection (GS), and hybrid breeding strategies. 4.1 Introgression and backcrossing Introgression and backcrossing are traditional methods used to introduce desirable traits from exotic germplasm into elite lines. This process involves crossing an elite line with an exotic donor and then backcrossing the progeny with the elite parent to recover the elite genetic background while retaining the desired exotic traits. For instance, the introgression of temperate maize germplasm into tropical elite lines has shown significant improvements in grain yield and ear prolificacy, demonstrating the effectiveness of this technique in enhancing adaptability to different environments (Musundire et al., 2021).
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