A double haploid (DH) is a plant derived from a haploid cell (containing a single set of chromosomes, 'n'), whose chromosome number has been artificially doubled (2n). This process results in a completely homozygous individual, meaning it has two identical sets of chromosomes, with all genes existing in a homozygous state.

Applications in Plant Breeding:
Double haploids have revolutionized plant breeding by significantly accelerating the breeding process:

1. Accelerated Breeding: Conventionally, achieving homozygosity requires 6-8 generations of self-pollination. DH technology achieves this in a single generation (haploidization followed by chromosome doubling), drastically shortening breeding cycles.

2. Efficient Selection: Homozygosity allows for immediate and direct selection of desired traits. There is no hidden genetic variation, making phenotypic selection highly efficient and reliable.

3. Development of Inbred Lines: DH technology is crucial for rapidly developing completely homozygous inbred lines, which are essential parents for producing superior F1 hybrid varieties in crops like maize and rice.

4. Genetic Mapping and QTL Analysis: The completely homozygous nature of DH lines simplifies genetic analysis, making it easier to identify and map genes (QTLs) associated with complex traits.

Production Methods of Haploids:
Haploids, which are then doubled to produce DHs, are primarily produced through in vitro tissue culture techniques:

1. Androgenesis (Male Gamete Culture):

   • Anther Culture: Immature anthers (containing microspores) are excised from a donor plant and cultured on a specialized nutrient medium. Under specific conditions, microspores can differentiate into haploid plantlets.

   • Microspore Culture: Similar to anther culture, but microspores are isolated from anthers and then cultured directly. This often yields higher frequencies of haploids.

2. Gynogenesis (Female Gamete Culture):

   • Ovule/Ovary Culture: Unfertilized ovules or ovaries are extracted from a donor plant and cultured in vitro. The unfertilized egg cell can then develop into a haploid embryo and eventually a plantlet. This method is used when androgenesis is ineffective.

3. Distant Hybridization (Chromosome Elimination):

   • Involves making a wide cross between two different species (e.g., Triticum aestivum x Hordeum bulbosum). After fertilization, the chromosomes of one parent (H. bulbosum) are preferentially eliminated during early embryo development, resulting in a haploid embryo of the desired parent (T. aestivum).

Once haploid plants are produced, their chromosome number is typically doubled using colchicine treatment to obtain fertile, homozygous double haploid plants.