Haploids, dihaploids and doubled haploids are powerful tools in genetics and plant breeding. They let geneticists observe allele effects directly, produce completely homozygous lines quickly, and shorten breeding cycles. This chapter explains definitions, methods of production, applications, examples in crops, and significance — written for B.Sc. students.
Haploids are organisms (usually plants in the breeding context) that contain a single set of chromosomes (n), i.e., half the normal diploid number. Since only one allele exists per locus, recessive alleles are unmasked.
- Androgenesis: development from male gametophyte (pollen/anther or microspore culture).
- Gynogenesis: development from unfertilized egg cells or ovules.
- Wide crosses with chromosome elimination: interspecific hybridization followed by elimination of one parent's chromosomes (example: Hordeum × Hordeum bulbosum).
- Anther and microspore culture: tissue culture techniques that induce embryos from microspores.
Haploids are usually sterile or have reduced fertility and show direct expression of recessive traits because each gene is present in a single allele copy.
- Study of gene expression and dominance: dominance relationships can be directly observed.
- Detection of recessive mutations: recessive mutants are expressed without masking.
- Chromosome analysis and mapping: simplifies karyotype study and linkage group assignment.
- Source for doubled haploids: chromosome doubling of haploids yields homozygous diploids (DHs).
- Mutagenesis and variability: mutagen-treated haploids immediately reveal recessive changes.
Dihaploids are diploid plants (2n) that originated from haploids and therefore are homozygous at all loci. Sometimes the term is used especially in context of polyploid crops (e.g., deriving diploid progeny from a tetraploid).
- Chromosome doubling of haploid tissue (chemical induction or spontaneous).
- In polyploid crops, reductional processes may produce dihaploid progeny (example: potato breeding).
- Genetic purity: dihaploids are completely homozygous and useful as fixed lines.
- Mapping and inheritance: simplify segregation analyses because loci are fixed.
- Polyploid research: help study behavior and inheritance in crops with higher ploidy levels.
Doubled haploids are plants produced by doubling the chromosome number of a haploid, creating fully homozygous diploids (2n) in a single generation. These are widely used in modern plant breeding.
- Anther/microspore culture to obtain haploid regenerants, followed by chromosome doubling.
- Wide hybridization with chromosome elimination to obtain haploids, then doubling.
- Chemical doubling: colchicine, oryzalin or other anti-mitotic agents cause chromosome doubling in haploid tissue.
- Spontaneous doubling: sometimes doubling occurs during tissue-culture regeneration.
- Complete homozygosity in one generation (saves the 6–8 generations of selfing in conventional methods).
- Rapid fixation of desirable alleles.
- Uniform and stable lines ideal for hybrid parent lines or genetic studies.
- Accelerates marker-assisted selection and QTL mapping because genotypes are fixed and reproducible.
- Pure line development for hybrid breeding.
- Genetic mapping and QTL analysis: DH populations are excellent mapping populations (stable and reproducible).
- Mutation screening: recessive effects show directly in DH-derived lines.
- Marker assisted selection: fixed genotypes allow straightforward selection by markers.
- Pre-breeding and trait introgression: DH lines speed up incorporation of traits like disease resistance.
| Feature | Haploid | Dihaploid | Doubled Haploid (DH) |
|---|---|---|---|
| Chromosome set | n | 2n (from haploid origin) | 2n (haploid doubled) |
| Homozygosity | Yes (hemizygous; single allele) | Yes (completely homozygous) | Yes (completely homozygous) |
| Fertility | Usually low/sterile | Often fertile | Fertile |
| Primary uses | Mutation detection, expression studies, source for DH | Genetic studies in polyploids, mapping | Rapid pure line development, mapping, MAS, hybrid parent production |
- Barley: haploids produced via Hordeum × H. bulbosum crosses; DH lines used for mapping and breeding.
- Wheat: haploids obtained from wide crosses and anther culture; DH lines used for disease resistance and variety development.
- Rice: anther culture is a common method for producing DH lines for breeding programs.
- Maize: DH technology is used commercially to create inbred parental lines much faster than conventional selfing.
- Potato: Dihaploids derived from tetraploid potato are used for diploid breeding and genetic mapping.
- Genotype dependency: success of anther/microspore culture and haploid induction depends strongly on genotype.
- Low frequency and technical skill: haploid production and chromosome doubling require tissue culture facilities and expertise.
- Somaclonal variation: tissue culture steps may introduce variation unrelated to target genetics.
- Fertility issues: some haploid/doubled haploid regenerants may show reduced fertility or abnormal development.
Haploids and doubled haploids have transformed plant genetics and breeding by producing completely homozygous lines in a single generation. This speeds up breeding, improves the efficiency of marker-assisted selection and mapping, and makes genetic analyses (especially of recessive traits) far easier. While technical and genotype-dependent challenges remain, DH technology is standard practice in many crop improvement programs.
- Plant Tissue Culture manuals and crop-specific breeding guides (look for sections on anther culture and doubled haploids).
- Research articles on doubled haploid production in maize, wheat, rice and barley for practical protocols and genotype effects.
- Haploid (n): organism with single chromosome set.
- Diploid (2n): organism with two chromosome sets (normal somatic number).
- Doubled haploid (DH): a haploid whose chromosomes have been doubled to produce a homozygous diploid.
- Androgenesis: development of an embryo from a pollen grain or microspore.
- Gynogenesis: development from an unfertilized egg or ovule.
- Explain how haploids help in the detection of recessive mutations. Give an example.
- Describe two methods for producing haploids in crop plants.
- Why are doubled haploids valuable in QTL mapping studies?
- List three limitations of DH technology and suggest possible solutions or mitigations.
