Haploid plants are characterized by possessing only a single set of chromosomes (gametophytic number of chromosomes i.e. n) in the sporophyte. This is in contrast to diploids which contain two sets (2n) of chromosomes. Haploid plants are of great significance for the production of homozygous lines (homozygous plants) and for the improvement of plants in plant breeding programmes.
Haploids may be divided into two broad categories:
1. Monoploids (monohapioids):These are the haploids that possess half the number of chromosomes from a diploid species e.g. maize, barley.
2. Polyhaploids:The haploids possessing half the number of chromosomes from a polyploid species are regarded as polyhaploids e.g. wheat, potato. It may be noted that when the term haploid is generally used it applies to any plant originating from a sporophyte (2n) and containing half the number (n) of chromosomes.
In Vivo and in Vitro Approaches:
The importance of haploids in the field of plant breeding and genetics was realised long ago. Their practical application, however, has been restricted due to very a low frequency (< 0.001%) of their formation in nature.
The process of apomixis or parthenogenesis (development of embryo from an unfertilized egg) is responsible for the spontaneous natural production of haploids. Many attempts were made, both by in vivo and in vitro methods to develop haploids. The success was much higher by in vitro techniques.
In vivo techniques for haploid production:
There are several methods to induce haploid production in vivo.
Some of them are listed below:
Development of an egg cell containing male nucleus to a haploid is referred to as androgenesis. For a successful in vivo androgenesis, the egg nucleus has to be inactivated or eliminated before fertilization.
An unfertilized egg can be manipulated (by delayed pollination) to develop into a haploid plant.
3. Distant hybridization:
Hybrids can be produced by elimination of one of the parental genomes as a result of distant (interspecific or inter-generic crosses) hybridization.
4. Irradiation effects:
Ultra violet rays or X-rays may be used to induce chromosomal breakage and their subsequent elimination to produce haploids.
5. Chemical treatment:
Certain chemicals (e.g., chloramphenicol, colchicine, nitrous oxide, maleic hydrazide) can induce chromosomal elimination in somatic cells which may result in haploids.
In vitro techniques for haploid production:
In the plant biotechnology programmes, haploid production is achieved by two methods.
Haploid production occurs through anther or pollen culture, and they are referred to as androgenic haploids.
Ovary or ovule culture that results in the production of haploids, known as gynogenic haploids.
In androgenesis, the male gametophyte (microspore or immature pollen) produces haploid plant. The basic principle is to stop the development of pollen cell into a gamete (sex cell) and force it to develop into a haploid plant. There are two approaches in androgenesis— anther culture and pollen (microspore) culture. Young plants, grown under optimal conditions of light, temperature and humidity, are suitable for androgenesis.
The selected flower buds of young plants are surface-sterilized and anthers removed along with their filaments. The anthers are excised under aseptic conditions, and crushed in 1% acetocarmine to test the stage of pollen development.
If they are at the correct stage, each anther is gently separated (from the filament) and the intact anthers are inoculated on a nutrient medium. Injured anthers should not be used in cultures as they result in callusing of anther wall tissue.