Summaryhaploid inducer line can be transferred (DH) technology can not only shorten the breeding process but also increase genetic gain. Haploid induction and subsequent genome doubling are the two main steps required for DH technology. Haploids have been generated through the culture of immature male and female gametophytes, and through inter‐ and intraspecific via chromosome elimination. Here, we focus on haploidization via chromosome elimination, especially the recent advances in centromere‐mediated haploidization. Once haploids have been induced, genome doubling is needed to produce DH lines. This study has proposed a new strategy to improve haploid genome doubling by combing haploids and minichromosome technology. With the progress in haploid induction and genome doubling methods, DH technology can facilitate reverse breeding, cytoplasmic male sterile (CMS) line production, gene stacking and a variety of other genetic analysis.

Four QTL related to haploid male fertility were detected by a segregation distortion method and the key QTL qhmf4 was fine mapped to an interval of ~800 kb. Doubled haploid (DH) technology enables rapid development of homozygous lines in maize breeding programs. However, haploid genome doubling is a bottleneck for the commercialization of DH technology and is limited by haploid male fertility (HMF). This is the first study reporting the quantitative trait locus (QTL) analysis of HMF in maize. Four QTL, qhmf1, qhmf2, qhmf3, and qhmf4, controlling HMF have been identified by segregation distortion (SD) loci detection in the selected haploid population derived from 'Yu87-1/Zheng58'. Three loci, qhmf1, qhmf2, and qhmf4, were also detected in the selected haploid population derived from '4F1/Zheng58'. The QTL qhmf4 showed the strongest SD in both haploid populations. Based on the sequence information of 'Yu87-1' and 'Zheng58', thirteen markers being polymorphic between the two lines were developed to saturate the qhmf4 region. A total of 8168 HBC (haploid backcross generation) plants produced from 'Yu87-1' and 'Zheng58' were screened for recombinants. All the 48 recombinants were backcrossed to 'Zheng58' to develop HBC progeny. The heterozygous HBC individuals were crossed with CAU5 to induce haploids. In each HBC progeny, haploids were genotyped and evaluated for anther emergence score (AES). Significant (or no significant) difference (P < 0.05) between haploids with or without 'Yu87-1' donor segment indicated presence or absence of qhmf4 in the donor segment. The analysis of the 48 recombinants narrowed the qhmf4 locus down to an ~800 kb interval flanked by markers IND166 and IND1668.

Doubled haploid (DH) lines have become widely used in maize (Zea mays L.) breeding. Haploid genome doubling is an important step in developing DH lines. The low rate of spontaneous genome doubling, which causes low haploid male fertility (HMF), seriously limits the largescale application of DH breeding without colchicine treatment. Our objective was to gain new insights into the genetics controlling HMF to improve the rate of HMF in DH breeding procedures. Haploid populations of 20 inbreds and their 31 single crosses derived from Chinese elite maize germplasm were screened for four traits related to HMF: anther emergence rate, pollen production rate, anther emergence score, and pollen production score. Haploid male fertility was compared between single crosses and their parents. Genotype effects were significant (p < 0.01) for all traits among Chinese elite maize lines and their single crosses, and interactions between genotype and environment were also significant (p < 0.05) for anther performance. Heritabilities ranged from 0.68 to 0.91 for these four traits. Haploid male fertility was controlled by additive effects with two or more genes. Anther emergence score proved to be the best trait for describing HMF and is the most practical trait for breeders. We propose that the potential use of HMF in breeding programs could reduce the need for toxic and costly artificial doubling treatments.

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Common rust is one of the major foliar diseases in maize, leading to significant grain yield losses and poor grain quality. To dissect the genetic architecture of common rust resistance, a genome-wide association study (GWAS) panel and a bi-parental doubled haploid (DH) population, DH1, were used to perform GWAS and linkage mapping analyses. The GWAS results revealed six single-nucleotide polymorphisms (SNPs) significantly associated with quantitative resistance of common rust at a very stringent threshold of P-value 3.70 × 10–6 at bins 1.05, 1.10, 3.04, 3.05, 4.08, and 10.04. Linkage mapping identified five quantitative trait loci (QTL) at bins 1.03, 2.06, 4.08, 7.03, and 9.00. The phenotypic variation explained (PVE) value of each QTL ranged from 5.40 to 12.45%, accounting for the total PVE value of 40.67%. Joint GWAS and linkage mapping analyses identified a stable genomic region located at bin 4.08. Five significant SNPs were only identified by GWAS, and four QTL were only detected by linkage mapping. The significantly associated SNP of S10_95231291 detected in the GWAS analysis was first reported. The linkage mapping analysis detected two new QTL on chromosomes 7 and 10. The major QTL on chromosome 7 in the region between 144,567,253 and 149,717,562 bp had the largest PVE value of 12.45%. Four candidate genes of GRMZM2G328500, GRMZM2G162250, GRMZM2G114893, and GRMZM2G138949 were identified, which played important roles in the response of stress resilience and the regulation of plant growth and development. Genomic prediction (GP) accuracies observed in the GWAS panel and DH1 population were 0.61 and 0.51, respectively. This study provided new insight into the genetic architecture of quantitative resistance of common rust. In tropical maize, common rust could be improved by pyramiding the new sources of quantitative resistance through marker-assisted selection (MAS) or genomic selection (GS), rather than the implementation of MAS for the single dominant race-specific resistance gene.

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