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#doubled haploids

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Plant Breeding 4

Main advantage of DH in breeding is:

Primary bottleneck in conventional line breeding is time required to attain homozygosity through repeated selfing, typically six generations to reach acceptable uniformity and distinctness for release. Doubled haploid technology converts heterozygous material into completely homozygous lines immediately, compressing 5-6 year inbreeding cycle into less than 18 months including haploid induction and doubling steps. Haploid inducer lines or anther culture produce haploid embryos, chromosome doubling yields instant true-breeding lines that can be multiplied for yield trials in next season with high uniformity. This rapid fixation accelerates varietal release, parent line development for hybrids, and genetic stock production for research. Additionally DH populations allow early generation selection with high heritability because lines are uniform, facilitating accurate evaluation of combining ability and hybrid prediction using genomic selection. Labor saved on repeated selfing, field maintenance, and note keeping outweighs tissue culture costs, explaining widespread adoption in maize, canola, barley, and vegetables where speed to market provides competitive advantage and genetic gain per year increases substantially over conventional methods.

Ref: Geiger HH & Gordillo GA. 2009 Plant Breeding Reviews – DH rapid homozygosity; Weyen 2008 J

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Plant Breeding 4

Wide hybridization followed by genome elimination is used in DH production of:

Wide hybridization with chromosome elimination exploits post-zygotic incompatibility where chromosomes of one parent are selectively lost during early embryonic divisions due to centromere incompatibility. In cereal DH production, wheat Triticum aestivum crossed as female to maize Zea mays as pollinator leads to fertilization but maize chromosomes fail to attach to wheat spindle due to centromere divergence and asynchronous mitosis, becoming micronuclei and eliminated within few divisions. Resulting embryo contains only haploid wheat chromosomes, rescued via embryo culture on artificial medium before endosperm abortion because endosperm also degenerates. Similarly barley × Hordeum bulbosum system uses bulbosum elimination to produce barley haploids. This method achieves broad genotype independence, avoids albinism problem common in anther culture of cereals, and produces stable haploids that are doubled with colchicine into fertile DH. In wheat breeding it is standard for DH population development for mapping and variety extraction, producing hundreds of homozygous lines per year efficiently. Embryo rescue media typically contain high sucrose, auxins, and cytokinins to support weak haploid embryos that lack functional endosperm; after rescue, haploid seedlings verified by morphological markers like reduced vigor and sterility, then treated with colchicine to double chromosomes and restore fertility for selfing and seed multiplication.

Ref: Laurie DA & Bennett MD 1988 – Wheat × maize DH; Kasha KJ & Kao KN – Bulbosum method mechanism

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Plant Breeding 4

Doubled haploids achieve homozygosity in:

Conventional pedigree inbreeding reduces heterozygosity by half each selfing generation, requiring F6-F7 to attain >98% homozygosity, a process taking 5-6 years. Doubled haploid approach collapses this timeline to single step through manipulation of gametic pathway. Haploid plant carrying n chromosomes is induced via anther culture, isolated microspore culture, ovule culture, or wide hybridization-induced chromosome elimination such as wheat × maize system. Haploid set containing single allele per locus is then doubled chemically with colchicine or spontaneously through endomitosis, producing 2n chromosome complement where both copies are identical duplication of same genome. Resulting disomic is fully homozygous, 100% inbred in one generation, termed true-breeding. Two generations may be needed counting haploid induction plus doubling, but genetic homozygosity per se is achieved immediately upon doubling, without progressive fixation. Breeders obtain stable inbred lines from heterozygous F1 within 12-18 months versus 5-6 years, dramatically shortening breeding cycles in barley, wheat, maize, and vegetables for rapid cultivar release. Flow cytometry and chromosome counting confirm ploidy status after doubling, while molecular markers verify homozygosity absence of heterozygous alleles; DH technology combined with genomic selection and speed breeding forms modern accelerated

Ref: Snape JW. Doubled haploid breeding; Dwivedi et al. 2015 – One-generation homozygosity review

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