Impact of Spontaneous Haploid Genome Doubling in Maize Breeding

Abstract

Doubled haploid (DH) technology has changed the maize-breeding landscape in recent years. Traditionally, DH production requires the use of chemical doubling agents to induce haploid genome doubling and, subsequently, male fertility. These chemicals can be harmful to humans and the plants themselves, and typically result in a doubling rate of 10%-30%. Spontaneous genome doubling and male fertility of maize haploids, without using chemical doubling agents, have been observed to a limited extent, for nearly 70 years. Rates of spontaneous haploid genome doubling (SHGD) have ranged from less than 5% to greater than 50%. Recently, there has been increased interest to forgo chemical treatment and instead utilize this natural method of doubling. Genetic-mapping studies comprising worldwide germplasm have been conducted. Of particular interest has been the detection of large-effect quantitative trait loci (QTL) affecting SHGD. Having a single large-effect QTL with an additive nature provides flexibility for the method of introgression, such as marker-assisted backcrossing, marker-assisted gene pyramiding, and systematic design. Moreover, it allows implementation of new methodologies, such as haploid-inducer mediated genome editing (HI-edit) and promotion of alleles by genome editing. We believe the use of SHGD can further enhance the impact of DH technology in maize.

Keywords: chemical doubling agent; colchicine; doubled haploid; genome doubling; haploid male fertility; maize; spontaneous haploid genome doubling.

Figure 1

General outline of a pipeline for producing doubled haploids in maize by using a chemical doubling agent for genome doubling.

Figure 2

Genetic map showing location of quantitative trait loci (QTL) contributing to spontaneous haploid genome doubling (SHGD) from Ren et al. [23] (orange, lowercase hmf), Ren et al. and Trampe et al. [21,22] (blue), Yang et al. [38] (pink, uppercase HMF) and SNPs contributing to SHGD from Chaikam et al. [34] (green), and from Ma et al. [33] (purple). Map length depicted is from Ren et al. [23], totaling 1484.5 cM.

Figure 3

Pipeline for doubled haploid production, using SHGD as an alternative to artificial genome doubling agents. Green shading indicates stages of a traditional doubled haploid (DH) pipeline that are replaced by direct seeding into the field when utilizing SHGD.

Figure 4

Schematic for phenotypic backcross introgression of SHGD into non-SHGD germplasm, resulting in DH lines possessing SHGD.