Molecular Plant Breeding 2024, Vol.15, No.4, 198-208 http://genbreedpublisher.com/index.php/mpb 199 provide a comprehensive understanding of how exotic varieties can be utilized to enhance the genetic base of maize, ultimately contributing to increased agricultural productivity and food security. 2 Historical Context 2.1 Early use of exotic varieties in maize breeding The utilization of exotic germplasm in maize breeding has a long history, primarily aimed at broadening the genetic base of local populations. Early efforts focused on integrating genes from temperate, tropical, and sub-tropical regions to enhance the genetic diversity and adaptability of maize. For instance, the Germplasm Enhancement of Maize (GEM) project, initiated in 1993, was a significant early effort to incorporate exotic germplasm into U.S. maize breeding programs. This project involved crosses between elite temperate lines and exotic parents, leading to the release of numerous useful breeding lines (Rogers et al., 2022). Similarly, the International Maize and Wheat Improvement Center (CIMMYT) has been actively involved in breeding elite tropical maize germplasm with tolerance to various abiotic and biotic stresses, leveraging exotic varieties to improve resilience in stress-prone environments (Prasanna et al., 2021). 2.2 Historical milestones and key contributions Several key milestones have marked the historical journey of exotic germplasm utilization in maize breeding. The GEM project stands out as a pivotal initiative, demonstrating the potential of exotic germplasm to enhance genetic diversity and yield potential in U.S. maize crops (Rogers et al., 2022). Another significant contribution comes from the CIMMYT’s efforts over the past four decades, which have led to the development and deployment of stress-tolerant maize cultivars across sub-Saharan Africa, Asia, and Latin America (Figure 1) (Prasanna et al., 2021). Additionally, the integration of temperate germplasm into sub-tropical breeding programs in Africa has shown promising results, with hybrids demonstrating high yield potential and stability across diverse agro-ecologies (Nyoni et al., 2023). Figure 1 Schematic depiction of the maize breeding pipeline of CIMMYT for developing and deploying elite multiple stress-tolerant tropical maize germplasm for sub-Saharan Africa, Asia, and Latin America (Adopted from Prasanna et al., 2021) 2.3 Shifts in breeding strategies over time Over time, maize breeding strategies have evolved significantly, incorporating advanced technologies and methodologies to enhance genetic gains. The transition from conventional breeding to more sophisticated approaches such as genomic selection, doubled haploidy, and genome optimization marks a significant shift in breeding strategies. For example, the proposed "genomic design breeding" pipeline integrates these advanced techniques to optimize maize genomes for desired traits, representing a move towards more precise and efficient breeding practices (Jiang et al., 2019). Furthermore, the resurgence of introgression breeding, facilitated by genomic tools, has renewed interest in utilizing exotic genes to drive genetic advances in maize (Hao et al., 2020).
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