Field Crop 2025, Vol.8, No.4, 166-175 http://cropscipublisher.com/index.php/fc 169 4.2 Genetic admixture between landraces, improved varieties, and wild relatives In the world of corn, the bloodline is rarely "clean". Local varieties can be mixed. In the corn of the southeastern United States, traces of the northern Flint type can be found, as well as genes from tropical lowlands. The situation in the lowlands of South America is different. The varieties there often blend the background of the Andes Mountains and tropical lowlands. Highland corn in Peru is even more interesting-there is little difference among them, indicating that local varieties often hybridize with each other and gene flow is constantly taking place (Arbizu et al., 2025). Modern breeding has pushed this kind of "mixture" to the norm. Breeders will actively combine germplasms from different sources to ensure that superior strains contain more mixed components and bring about richer genetic diversity (Van Inghelandt et al., 2010). 4.3 Identification of core collections for conservation and breeding Identifying core germplasms with high genetic diversity is of great significance for both resource conservation and breeding efforts. Studies have shown that the germplasm from the Mexican Highlands and the Andes Mountains has not been fully utilized and may be very helpful for breeding projects (Bedoya et al., 2017). Core germplasm is usually determined through cluster analysis and population structure assessment, which ensures that they represent a wide range of genetic and phenotypic diversity in global maize germplasm (Adu et al., 2019b). These resources preserve beneficial genes such as drought tolerance, early maturity and disease resistance, which can provide support for the continuous improvement of corn (Badu-Apraku et al., 2021). 5 Factors Driving Population Differentiation 5.1 Environmental selection pressures and local adaptation mechanisms Environmental conditions such as temperature, precipitation and altitude can exert significant selective pressure on corn populations. This will cause local adaptations of corn in different places and genetic differentiation to occur. The regions with relatively obvious differentiation in the genome are often related to some adaptive traits, such as flowering time and plant type. These traits are of great significance for the survival of corn in different agricultural ecological zones (Chen et al., 2024b). Research has found that in tropical, subtropical and temperate corn, there are some specific genomic regions related to adaptability, which indicates that environmental selection has a significant impact on population structure (Wu et al., 2015). In addition, some environmental factors, such as the lowest temperature, can also affect the key developmental processes of corn, thereby further promoting differentiation. 5.2 Human-mediated selection through breeding and seed exchange networks Human activities can also influence population differentiation, including farmers' traditional choices, modern breeding and seed exchange. Farmers will select corn varieties that they like or are suitable for the local area. This choice will cause some quantitative traits to show significant differentiation even when there is a large flow of genes (Pressoir et al., 2004a; 2004b). Seed exchange networks are often related to society and culture, and they can affect the genetic structure of local populations. Different directions and frequencies of exchange can enhance or weaken differentiation (Orozco-Ramirez et al., 2016; Aragón-Martínez et al., 2023). Modern breeding programs also generate different heterosis groups and subpopulations, which further enriches the global diversity of corn (Beckett et al., 2017). 5.3 Genetic drift, gene flow, and historical demographic events shaping diversity Genetic drift, especially during the bottleneck period of corn domestication and some foundational events, has a significant impact on the diversity of corn. The domestication process will lead to a significant reduction in the effective population size, an increase in the proportion of harmful alleles, and also cause obvious differentiation. For instance, Andean corn has experienced a very strong founder effect (Beissinger et al., 2015). Gene flow, including the infiltration of genes from wild relatives such as ruminant, can bring new alleles and reduce the impact of harmful mutations, especially in highland populations (Wang et al., 2017). Historical population events, such as the expansion or reduction of distribution range, interact with selection and drift to form the complex differentiation pattern seen in global corn today (Van Heerwaarden et al., 2012).
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