Maize Genomics and Genetics 2025, Vol.16, No.2, 70-79 http://cropscipublisher.com/index.php/mgg 73 variation can be known by calculating the PIC value. SNP markers are more powerful. They can not only find genetic differences, but also classify corn varieties according to hybrid advantages and grain types. But having said that, although these technologies are professional, their purpose is simple-to have a more comprehensive understanding of the genetic characteristics of fresh corn (Nguyen et al., 2023). With these data, breeders will be more confident in selecting new varieties. 3.3 Genetic differences in germplasm resources across geographic regions There are actually quite a lot of genetic differences between fresh corn varieties in different places. For example, researchers in Brazil and the United States have found some interesting phenomena. In Brazil, RAPD and SSR markers were used to detect that the similarity between different varieties was higher than the similarity within the same variety (Lopes et al., 2015). This is a bit counterintuitive, right? The situation in the United States is different. When SNP markers were used to detect American inbred lines, it was found that although there were some differences between the main variety groups, the degree of differentiation was only moderate. Interestingly, some unique genetic variants can be found in different breeding groups. In fact, these differences are not difficult to understand. After all, temperate and tropical/subtropical corn are originally divided into different groups. Breeding experts will also divide them into more detailed groups based on characteristics such as hybrid vigor and corn kernel color. So now everyone thinks that in order to breed better varieties, we must first understand the genetic characteristics of these local varieties, which is also important for protecting these germplasm resources. 4 Factors Influencing Genetic Diversity in Sweet Corn Germplasm 4.1 Effects of geography and ecological environments When it comes to the genetic diversity of fresh corn, it actually has a lot to do with where it grows. Think about it, the climate, soil and other environmental conditions in different regions are not the same. In order to adapt to these differences, corn has gradually formed unique genetic characteristics. Some studies have used microsatellite markers to analyze and found that the environment does affect the genetic structure inside and outside the corn population. Interestingly, researchers also found a phenomenon - artificially selecting the flowering time of corn in different places has not only changed its growth characteristics, but also important indicators such as yield (Beissinger et al., 2015). This fully shows that local environmental conditions have a great impact on the genetic diversity of corn. But then again, although environmental factors are very important, how they affect it may depend on the specific conditions of different regions. 4.2 Impacts of cultivation history and artificial selection pressure The genetic diversity of fresh corn is actually inseparable from the history of human cultivation. Think about it, since the earliest domestication, farmers have been selecting those corns that grow well for seeds, but the result has narrowed the gene pool (Da Silva et al., 2020). In particular, in pursuit of sweetness, mutant varieties such as su1 and sh2 are specially selected, making the differences between different inbred lines increasingly large. But then again, this kind of artificial selection is not without benefits. At least now using selection indexes for breeding can indeed improve the yield and quality of fresh corn. But the problem is that the "population bottleneck" effect during the domestication process caused a sharp drop in the corn population, resulting in the disappearance of many genetic variants. Thinking about this now, it's a pity. 4.3 Roles of genetic drift and gene flow The genetic diversity of fresh corn is quite interesting. It is mainly affected by two factors: genetic drift and gene flow. Let's talk about genetic drift first. Simply put, it is the random change of gene frequency - especially for varieties with small planting areas, this change is particularly obvious. Recent studies have found (Li, 2024) that corn has experienced a process of rapid population decline and expansion in history, which has affected the diversity pattern of the entire genome. But interestingly, gene flow can offset this effect. What is gene flow? It is the exchange of genes between different corn varieties. Through marker technologies such as RAPD and SSR (Choquette et al., 2023), we found that fresh corn has gene exchange both within varieties and between different varieties. What's more amazing is that artificial intelligence analysis can now predict what kind of genetic differences these gene flows will bring. To put it bluntly, gene flow is like constantly injecting fresh blood into the corn gene pool, which is particularly important for maintaining diversity.
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