**1. Introduction**

With the recent development of life science technology, the importance of molecular breeding technology as well as hybridization and selection breeding is growing rapidly in seed production. In addition, it is necessary to establish a breeding system to maintain international competitiveness in the vegetable seed market by shortening the development time of new varieties through molecular breeding. However, there is a lack of biotechnology-based varieties due to the difficulty in marker development and insufficient information on genetic resources. The embryo development of flowering plants using microspores is a fascinating system in that it can dramatically promote the process of plant breeding.

The Brassicaceae includes important plants, such as *Arabidopsis thaliana* L., *Brassica napus* L., *Brassica carinata* L., *Brassica juncea* L., and *Brassica rapa* L. These plants are being used as model genomes for molecular biology research of Brassicaceae. In particular, comparative genomic studies of *Arabidopsis* and cabbage have been actively conducted, and many research papers on cabbage, radishes, and rapeseed have recently been published [1–3]. In the Brassicaceae, the radish has been relatively understudied on the genomics level, and research has focused on traits related to F1 breeding, including male sterility. Genomics research on radishes is currently conducted mainly in Korea, China, Japan, and the United States. The genome size of radishes is known to be approximately 530–573 Mb [4,5]. Recently, 402 Mb genome sequences of the Japanese cultivar 'Aokubi' have been published. A total of 1345 scaffolds were assigned to the linkage map, spanning 116.0 Mb. However, the sequences are draft scaffolds with a small size of N50 (46.3 kb) and only 116 Mb of these sequences been assigned to chromosomes showing incomplete genome assembly [6]. A continuum-level genome draft has been reported for the North American wild radish in the United States [3]. The assembly size is 254 Mb and N50 is 10.1 kb. In Korea, the assembly of pseudomolecule-level assemblies, rather than contigs or scaffolds, has been announced, which is assembled at 426.2 Mb and the size of scaffold N50 is 1.22 Mb. The scaffold mapped to the chromosome in the standard dielectric is 344 Mb, forming nine pseudomolecules [7]. This shows the level of maturity of the recent *B. rapa* and *B. oleracea* standards. The first genetic maps in radish were made using populations created by cross-breeding between *R. sativus* and *R. raphanustrum* to enhance DNA polymorphism, and the RFLP molecular markers identified in *Brassica* spp. were used. Over time, genetic mapping has been done using various types of markers through intraspecific hybridization.

Despite the diversity and importance of radishes, little genetic information has been released about this plant. Most horticultural or vegetable crops have relatively long breeding cycles, from six months to a year or more [8]. Conventionally, only one generation per year of radishes is produced. Research on molecular markers related to shortening the breeding cycle is still ongoing, and the introduction of a breeding system using modern breeding technologies based on molecular markers or genomic information should be developed. For radish breeding and genetic research, it is important to shorten the breeding cycle and then create pure lines in a short time frame. Owing to the high self-incompatibility of the radish, it is necessary to secure homozygous genetic resources with fixed traits for the development of gene functional molecular markers [9]. Because the brassica crops have pronounced self-incompatibility, pollination is difficult [10,11]. For this reason, bud pollination, the process of opening immature pots, and attaching flower pots of mature flowers are utilized [12,13]. In addition, it requires a lot of time and effort to develop fixed lines since at least it takes 6–7 generations of single seed descent. In order to solve this problem, a haploid breeding method to obtain a homozygous plant in a short period of time by doubling chromosomes through the induction of a plant from haploid cells has attracted attention [14,15]. Haploids can quickly produce homozygotes through chromosomal duplications, which can significantly shorten the breeding age through self-pollination [16]. Haploid plants are also used in many fields, as they are advantageous for screening phenotypes of superior recessive traits that are not expressed in diploids, as well as for use in interspecific cross-breeding. Haploid is a general term describing sporophytes with a single set of parental chromosomes. Normal haploids are poorly grown and have low values due to difficulties such as infertility, but the embryoid body grown by doubled haploid lines are characterized by high purity.

In traditional breeding, the selection of these quantitative traits is usually performed by phenotypic testing, mainly by the grower's experience, but these traits are easily influenced by the external environment. Therefore, by creating a genetic map of quantitative trait loci (QTL) with molecular markers that are closely related to the target trait, marker-assisted selection (MAS) can be used to simplify the breeding process and improve efficiency. These molecular markers have become an essential tool for plant genetics and breeding studies. Starting with RFLP (Restriction Fragment Length Polymorphism) markers, various labels, such as RAPD (Random Amplification of Polymorphic DNA), SSR (Simple Sequence Repeat), and SNP (Single Nucleotide Polymorphism), have been developed. Among them, SNPs have been developed. They are molecular markers that are suitable for genome analysis, mapping, and marker-assisted breeding.

In this study, to improve the molecular breeding efficiency of radishes, we generated linkage maps with an F1 population and performed a QTL study associated with the regeneration rate by microspore culture, which is considered to be important in radish. Using an F1 population derived from an intra-specific cross between the high regeneration rate radish lines and low regeneration rate radish lines, 'GX71' and 'GX50', respectively, genetic linkage maps were constructed with SNPs. In order to obtain a large amount of SNP markers, low-cost and high-efficiency genotype-by-sequencing (GBS) was performed, and the SNP markers were obtained by filtering several steps to secure reliable associations between genotypes and phenotypes. The results will be utilized to develop molecular markers associated with the efficiency of microspore culture so that early stage screening is possible for the doubled haploid breeding programs in radishes.
