Abstract
Maize doubled haploid (DH) lines are usually created in vivo, through crosses with maternal haploid inducers. These inducers have the inherent ability of generating seeds with haploid embryos when used to pollinate other genotypes. The resulting haploid plants are treated with a doubling agent and self-pollinated, producing completely homozygous seeds. This rapid method of inbred line production reduces the length of breeding cycles and, consequently, increases genetic gain. Such advantages explain the wide adoption of this technique by large, well-established maize breeding programs. However, a slower rate of adoption was observed in medium to small-scale breeding programs. The high price and/or lack of environmental adaptation of inducers available for licensing, or the poor performance of those free of cost, might explain why smaller operations did not take full advantage of this technique. The lack of adapted inducers is especially felt in tropical countries, where inducer breeding efforts are more recent. Therefore, defining optimal breeding approaches for inducer development could benefit many breeding programs which are in the process of adopting the DH technique. In this manuscript, we review traits important to maize maternal haploid inducers, explain their genetic basis, listing known genes and quantitative trait loci (QTL), and discuss different breeding approaches for inducer development. The performance of haploid inducers has an important impact on the cost of DH line production.
Highlights
The doubled haploid (DH) technology is the preferred method of inbred line development in many crop species [1]
While there is no information in the literature on how different breeding companies produce haploid seeds, it seems reasonable to assume that well-established breeding programs, which produce a large number of DH lines every year, employ centralized isolation fields for haploid induction, whereas smaller maize breeding programs, which generate a limited number of DH lines annually, perform hand-pollinations in induction nurseries
To describe a typical breeding approach for inducer development, we propose the scenario of a tropical breeding program interested in developing its own, adapted inducer from a licensed, exotic inducer
Summary
The doubled haploid (DH) technology is the preferred method of inbred line development in many crop species [1]. When inducers are used as the pollen-source parent, they are called maternal haploid inducers, because only the genome of the maternal donor plant is transmitted to haploid embryos. When inducers are used as the seed-bearing parent, they are called paternal haploid inducers, because only the nuclear genome of the paternal donor plant is passed to haploid embryos. In contrast to maternal haploid inducers, paternal inducers transmit their cytoplasmic genome to haploid seeds, which may or may not be desirable. Haploid seeds are selected, sown and the resulting plants pollinated by the same inbred line. This process restores the diploid state of the resulting embryos, while the male sterile cytoplasm is introgressed from the inducer. The indeterminate gametophyte (ig) gene is responsible for haploid induction as a single gene trait [2]
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