Maize Genomics and Genetics 2025, Vol.16, No.2, 89-97 http://cropscipublisher.com/index.php/mgg 90 genetic traits (Nelimor et al., 2020). Some wild relatives such as teosinte, although not used much in agriculture, have become indispensable "raw materials" in research and breeding because they retain the early genetic characteristics of corn (Flint-Garcia et al., 2009). As for the improved varieties that are widely used now, most of them are the result of continuous screening by the breeding team under established goals, such as increasing yields, disease resistance and drought resistance (Nyoni et al., 2023). However, over-reliance on such improved varieties can easily make genetic diversity monotonous. 2.2 Global collection and conservation efforts In fact, many people are not aware of how complicated the preservation and collection of corn resources are. Since the mid-20th century, many research institutions have realized that preserving these diverse germplasm resources is a kind of "insurance" for future agricultural security. Organizations such as the International Maize and Wheat Improvement Center (CIMMYT) not only collect local varieties, but also wild species and modern varieties. There is more than one way to preserve them - seed banks are the most familiar form, but it is equally important to keep varieties in their original habitats (the so-called "in situ conservation"), because this allows them to continue to evolve naturally and adapt to changing environments (Rajpal et al., 2023). These two methods have their own uses, one is stable and the other is active, and they cannot replace each other (Yu et al., 2020). 2.3 Genetic diversity and its role in breeding If breeding only relies on a few existing improved varieties, it is actually difficult to make a breakthrough. The truly promising genetic variation is often hidden in those less noticed local varieties and wild species (Nelimor et al., 2020). For example, some local varieties may be naturally drought-resistant or disease-resistant, which happen to be lacking in modern high-yield varieties. Such genetic resources are actually of great significance in expanding the genetic basis of breeding materials (Rajpal et al., 2023). Now the technology has also caught up, such as double haploid technology, genome typing and other methods, which can make these diverse "resources" used more quickly, improve breeding efficiency, and also help to breed new varieties with stronger adaptability and more stable performance (Prasanna, 2010; Wu and Li, 2024). 3 Progress in Characterization and Evaluation of Germplasm Resources 3.1 Use of molecular technology in characterization of germplasm resources The characterization of corn germplasm resources is no longer limited to traditional methods. The addition of molecular technology, especially SNP genotyping, has given us a more detailed means to see the genetic differences and population structure between varieties. In a study of the National Maize Inbred Line Germplasm Bank in the United States, more than 680 000 SNP markers were identified through sequencing (Romay et al., 2013). These data not only allowed researchers to see the genetic distance between different germplasms, but also exposed obvious population stratification. Of course, SNP chips are also constantly being upgraded. The launch of Maize6H-60K chip and 55K chip has greatly improved the genome coverage and genotyping efficiency. Especially when dealing with rare variants in tropical germplasm, the latter performs more prominently (Xu et al., 2017). This type of chip not only helps identify unique alleles, but also provides considerable convenience in the construction of germplasm fingerprints and assisted selection breeding (Jones et al., 2007; Lu et al., 2009). If SNP technology has changed the "clarity" of germplasm identification, then GWAS (genome-wide association analysis) has rewritten the way we look for trait genes. Its usefulness has become increasingly evident in recent years - more than once, it has revealed gene regions related to agronomic traits such as corn flowering period, seed coat color and even sweetness (Romay et al., 2013). Especially after combining with high-density SNP chips, GWAS can more accurately locate quantitative trait loci (QTL), providing a more powerful reference for variety improvement (Xu et al., 2017; Tian et al., 2020). It is worth mentioning that next-generation sequencing (NGS) brings another perspective to observe genetic diversity. Analysis of 265 maize inbred lines by genotyping sequencing (GBS) revealed significant population structure among breeding materials (Ertiro et al., 2017). Compared with traditional typing methods, NGS is more
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