Maize Genomics and Genetics 2025, Vol.16, No.2, 80-88 http://cropscipublisher.com/index.php/mgg 82 Figure 1 Model of the rapid breeding of DH lines by the use of inducer lines containing purple markers or fluorescent markers (Adopted from Meng et al., 2021) Image caption: The embryos that lack purple color or fluorescent signals are haploid. They are selected to put into N6 culture media for carrying out the subsequent doubling process with the chemical reagents. N6: short for N6 culture media (Adopted from Meng et al., 2021) 3.2 Key steps in production In the final analysis, making a DH line is actually a process of turning a "haploid" into a "homozygous", and the operation steps are relatively fixed. First, haploids must be induced, which is usually done with the help of a specific male genotype; then, haploids must be distinguished from normal diploids in seeds or seedlings, and this step cannot be wrong. Commonly used methods include identification systems such as red root markers and high oil markers. Although there are many means, accuracy is always the first priority (Chaikam et al., 2019; Jacquier et al., 2020). The identified haploids will undergo chromosome doubling treatment, commonly using chemical agents such as colchicine. This step is critical because only after the number of chromosomes is doubled can the plant be restored to fertility. After that, we can get the homozygous lines we want through self-pollination (Chaikam et al., 2019; Chaikam and Prasanna, 2020). Of course, the current technology is much better than before. For example, the new generation of induced lines not only has high induction efficiency, but also can adapt to tropical or local materials; and the emergence of automated identification systems has also made some repetitive tasks much easier. Improvements in chromosome doubling have also played a significant role in saving costs and time (Chaikam et al., 2019; Jacquier et al., 2020). 3.3 Current applications Today, DH technology has become an indispensable part of the corn breeding process. Especially in the development of hybrid varieties, quickly obtaining homozygous lines is almost equivalent to locking in the possibility of excellent combinations in advance. Once the cycle is shortened, the breeding efficiency will naturally increase (Wang et al., 2019; Meng et al., 2021). Some breeding projects even use DH directly in recurrent selection, with good results (Gallais and Bordes, 2007). In recent years, DH technology has been continuously "upgraded". For example, some teams have tried to combine it with genome editing technology to come up with the so-called IMGE (haploid induction-mediated genome editing) method-this is actually to use CRISPR/Cas9 directly on haploids, bypassing those tedious
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