Maize Genomics and Genetics 2025, Vol.16, No.2, 89-97 http://cropscipublisher.com/index.php/mgg 91 efficient in marker development and high-density genetic map construction, which is necessary for marker-assisted selection (Jones et al., 2007; Chen et al., 2016). 3.2 Phenotypic evaluation and trait assessment Not all germplasms can perform well in adversity, so phenotypic evaluation is particularly important. Faced with stresses such as drought and disease, breeders are more concerned about who can "bear it". For example, the drought resistance-related markers included in the 55K SNP chip are very valuable for maize cultivation in water-stressed areas (Xu et al., 2017). On the other hand, the double haploid lines in the Iowa BSSS population also showed good stress resistance potential, and the changes in their allele frequencies indicated that selection pressure was quietly taking effect (Ledesma et al., 2023). Of course, stress resistance is not the only breeding goal, and yield and quality are the results most often pursued by breeders. The Maize6H-60K chip is not only useful in identifying stress resistance genes, but also plays a key role in the study of the genetic basis of yield-related traits (Tian et al., 2020). For commercialization, sometimes a hybrid combination with stable high yield is more popular than a "drought-resistant" variety. SNP-based trait positioning has helped to select many excellent lines (Jones et al., 2007). However, "invisible characteristics" such as nutrition and quality have also begun to be valued. For example, SNPs related to seed oil synthesis have been identified, providing new clues for improving the nutritional content of corn (Xu et al., 2017). For example, the study of inbred lines in the mid-altitude humid ecological zone of Ethiopia also found unique genetic variation, which has the potential to be used for improving nutritional quality (Chen et al., 2016). The ultimate goal of this kind of evaluation of germplasm is to serve the needs of consumer taste and the food industry. 4 Multiple Ways to Use Maize Germplasm Resources in Breeding In maize breeding, the role of germplasm resources cannot be overemphasized. However, there is often more than one way to use these resources. In addition to using them directly, the key is how to combine, how to select, and how to introduce them. This section will discuss the use of these resources from several common but not completely overlapping perspectives - including pre-breeding, development of new breeding lines, hybridization strategies, and some molecular methods that have been popular in recent years. 4.1 About pre-breeding and introduction of wild traits When it comes to expanding the genetic base, people's first reaction may not be "pre-breeding", but it is indeed an inconspicuous but critical link. Many times, some wild relatives or local old varieties have useful traits, but they are not easy to use directly in modern breeding materials. This requires "pre-breeding" to make a transition. For example, the SeeD project did a thorough job. They used genomic selection methods to discover those multi-gene-controlled traits from local varieties and introduced them step by step into core breeding materials (SeeD, reference 3). In the final analysis, even resources that look "wild" may become part of future excellent varieties under the correct screening and introduction. 4.2 How to develop diverse germplasm into good breeding materials Not all "diversity" can be directly realized. The value of diverse germplasm depends on whether you can transform it into a usable breeding line. This step requires strategy, such as which traits are worth retaining and which can be accurately introduced with the help of markers. Interestingly, the successful development of drought-resistant hybrids in the American Corn Belt is a typical example. The combination of drought-resistant traits is very complex, and they have used many methods, such as predicting breeding value through phenotypic and genetic information, and even using molecular markers to lock in key drought-resistant traits. The hybrids that were finally bred performed very well under drought conditions, which is inseparable from the clever integration of these "diverse germplasm" resources. 4.3 Hybridization and hybrid vigor: not a simple addition problem Hybrid vigor sounds like a "strong combination", but the science behind it is not so straightforward. The good performance of hybrid offspring depends on the complementarity between different germplasms. You can't just pick two parents and expect their offspring to have a good harvest. A study involving 724 tropical and temperate inbred lines found that some combinations, such as Reid × SPT and Lancaster × LRC, performed well in terms of
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