Maize Genomics and Genetics 2025, Vol.16, No.2, 80-88 http://cropscipublisher.com/index.php/mgg 81 2 Overview of Haploid Breeding Technology 2.1 Haploid induction type Not all corns will spontaneously produce haploids under natural conditions. This situation is actually very rare, and the incidence rate is usually less than 0.1% (Liu et al., 2017; Zhou Liang, 2024). However, this type of "natural induction" does exist, although it is almost impossible to rely on it to promote breeding. In contrast, artificial means are obviously more reliable. The most commonly used one is a haploid induction system. For example, the type derived from Stock6 can increase the haploid production rate to 1%-2% (Liu et al., 2017). Its operation is actually not complicated: pollinate the target plant with pollen from the induction system, and haploids may appear in the offspring. If some molecular tools are used, such as manipulating the key protein CENH3, through overexpression and other methods, the induction efficiency can be further improved (Meng et al., 2022). It can be said that haploid induction has changed from "relying on nature" to "relying on technology". 2.2 Historical Development Corn haploid breeding is not a new invention in recent years. As early as the late 1950s, scientists discovered a key material-Stock6 inducer line (Kelliher et al., 2017). It was this material that opened the door to artificial induction of haploids. Since then, the in vivo induction system has gradually formed and been used, becoming a standard tool for many breeding programs. In the following decades, technological progress has continued, especially the introduction of double haploid (DH) technology, which allows people to obtain homozygous lines faster and greatly speeds up the pace of breeding (Chaikam et al., 2019; Meng et al., 2021). Further breakthroughs have appeared in the understanding of the mechanism. For example, the discovery of the MATRILINEAL (MTL) gene not only improves the induction efficiency, but also makes it clearer how this process occurs (Kelliher et al., 2017). 2.3 Current mainstream methods Today, when breeding haploids, everyone basically chooses those optimized inducer lines. Stock6 materials are still the main force. They are not only highly efficient in induction, but also highly adaptable and suitable for use in different environments and breeding goals (Chaikam et al., 2019; Trentin et al., 2020). Of course, breeding now relies on more than "traditional methods". Molecular technology is also used in many experiments. For example, methods such as genome editing and marker-assisted selection have been integrated into the haploid breeding process. There is also a system called IMGE, which combines CRISPR/Cas9 technology with haploid induction, which is more efficient and targeted (Wang et al., 2019). This set of solutions is now gradually recognized, especially when you want to quickly obtain edited materials for a certain trait (Andorf et al., 2019; Jacquier et al., 2020). 3 Production of Double Haploid Maize 3.1 Concept and advantages The term "pure line" is not unfamiliar in breeding, but after using double haploid (DH) technology, the speed of obtaining homozygous materials has been completely refreshed. Traditional methods often rely on self-pollination from generation to generation to slowly advance, while the DH method directly doubles the number of chromosomes in haploid cells to obtain completely homozygous individuals. The theory sounds complicated, but the purpose is actually very direct: to obtain genetically consistent materials as soon as possible (Figure 1) (Chaikam et al., 2019; Meng et al., 2021). However, it is not just "fast". In the breeding process, DH lines have several benefits that cannot be ignored. For example, they are genetically stable and do not require repeated screening; for example, planting management is also easier because they are uniform. For hybrid seed production, this consistency is particularly important. After all, if the parents are unstable, it is difficult to say how the offspring will perform (Chaikam et al., 2019; Jacquier et al., 2020). Of course, DH is not a panacea, but once used in combination with molecular markers, the genetic gain it brings is another bonus (Chaikam and Prasanna, 2020; Meng et al., 2021).
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