THE USE OF DOUBLED HAPLOID TECHNOLOGY FOR LINES DEVELOPMENT IN MAIZE BREEDING

In maize, doubled haploid (DH) technology is used in breeding programs to obtain lines, the primary input for developing new cultivars. This review aims to present the main points related to using this technology in maize, the steps for its application, and the advantages and implications of using DH lines in maize breeding programs.

The technology of doubled haploid (DH) lines is used to assist maize breeding programs in obtaining homozygous lines in as little as three generations, unlike the conventional method, which requires a minimum of six generations for the establishment of the line, reducing the time for the development of cultivars (Chaikam et al., 2019). Furthermore, this technique can spontaneously or artificially produce individual haploids followed by genome duplication . Since

HAPLOID INDUCTION
The haploid induction is the initial step for  (Prasanna et al., 2012).
The inducers developed at the beginning of the twenty-first century have obtained rates above 10% in the induction of haploid seeds (Forster et al., 2007;Rotarenco et al., 2010;Kebede et al., 2011;Mowers et al., 2018).
The gynogenetic haploidy process in In a recent study, Trampe et al. (2022) showed that the potential of different genotypes for haploid induction is a quantitative trait, strongly affected by environmental conditions. The use of doubled haploid technology for lines ...

IDENTIFICATION OF HAPLOIDS
In identifying positive haploids in maize, a non-destructive, easily applied, and accurate methodology is necessary to evaluate a large number of samples in a short time.
Nanda and Chase (1966) developed a system for identifying haploids in maize using the expression of the R1-navajo (R1-nJ) gene, which codifies the purple color through biosynthesis of anthocyanin in the endosperm and embryo of maize seeds. In this system, embryos with white color and a purple endosperm are haploids, having genes only from the source genotype. They can be selected for chromosome duplication to obtain DH lines. However, seeds with a purple embryo and purple endosperm are diploids, resulting from the effective crossing of the source genotype with the inducer, and they should be discarded.
Furthermore, a third category should also be discarded, formed by seeds in which expression of the R1-nj gene is inhibited, making haploid identification impossible ( Figure 1).
The system based on the R1-nj gene has been widely used to identify haploid maize due to its ease of use. However, visual inspection of each seed is laborious and time-consuming, requiring trained laborers for efficient selection. Another factor to be considered is the genetic variability in the expression of the R1-nj allele. Although the R1nj gene has dominant expression, its effect can be suppressed by anthocyanin inhibitor genes in the source genotype (Prasanna et al., 2012;Chaikam et al., 2015).
As described below, new methodologies have been tested for increasing acuity in selecting haploids and doubled haploids.

Near-infrared spectrometry (NIR):
This technique uses electromagnetic radiation with a 750-2500 nm wavelength frequency. When combined with multivariate calibration methods, it allows qualitative and quantitative evaluation in a rapid and non-destructive way and without using reagents (Jones et al., 2012). For example, in haploid seeds, Gustin et al. (2020) found an increase in protein content, and the seeds were  As the D0 seedlings have small ears and low pollen production, and some parts of the tassels are even sterile, the use of strategies to obtain seeds through self-fertilization is necessary. Therefore, in addition to careful isolation of ears to avoid contamination from undesirable pollen, it is crucial to perform self-fertilizations repeatedly on the same plant, whenever possible, to ensure the most significant number of seeds, and use pollination bags adapted to the reduced size of the tassels. necessary.

V)
It allows training, validation, and selection populations to be obtained for wide genomic selection.

VI)
The  (Bordes et al., 2006(Bordes et al., , 2007. Another possibility is that this more significant variability allows combinations of superior genes and that this is reflected in the combining ability of doubled haploid lines when obtained from segregating generations (Bernardo, 2009).
However, the supposed effect of narrowing the genetic base by the type of source population may preserve superior allele combinations accumulated over time when this recombination of genes occurs from elite maize lines (Bernardo, 2009). In addition, as the breeding program advances in obtaining DHs, the tendency is for the best DH lines to be used in crosses within each heterotic group to produce new sources for the extraction of lines.
This recycling process will eliminate the most deleterious alleles since in the D1 generation, the doubled haploid lines with very low vigor or that bear undesirable traits are eliminated.
Another great advantage of the doubled haploid technology is the possibility of integration among processes, such as wide with phenotypic data of hybrids in the training populations.
All the above shows the potential of the