6. Applications of haploid production
6.1. Development of pure homozygous lines
Homozygous, true breeding cultivars are highly important for screening of high yielding lines and to produce hybrid vigour as a method of crop improvement. Obtaining homozygous diploid plants by conventional methods is difficult in perennials. From several decades to over a hundred years are required to obtain a pure line by means of successive inbreeding throughout many generations. The seed set by inbreeding in many trees is very low, usually only a few of ten thousandth or sometimes no seed can be obtained at all; therefore, it is impractical to obtain pure lines by inbreeding (Chen, 1986). Moreover, conventional method of haploid production by inbreeding is impossible if the plant is strictly cross-pollinating in nature. On the other hand, homozygous diploid plants can be achieved in a single generation by diploidization of in vitro raised haploids by colchicine treatment.
6.2. Genetic studies
Because of the lack of accurate materials in research work, the progress in the study of genetics in trees is much slower than that in annual herbaceous plants. The genetics of a lot of important traits in economically important plant species has not been clearly demonstrated as yet. As a result, it is still unknown whether the desirable characters of the parents will appear in their progenies. Only when crossing between different homozygous diploid plants is carried out, we can gain a clear idea of dominance of genes controlling various characters and that these characters are either monogenic or polygenic (Chen 1986). Furthermore, if we can use the haploid plants as samples of gametes, then we can obtain directly the recombination value between genes. Moreover, we can also use the haploid plants to study chromosome homology within genome or between genomes.
6.3. Gametoclonal variation
The “gametoclonal variation” arises among plants regenerated from cultured gametic cells consisting of differences in morphological and biochemical characteristics as well as in chromosome number and structures that are observed. Besides yielding haploids, in vitro androgenesis helps in the screening of gametophytic variation at plant level. Pollen grains within an anther form a highly heterogeneous population because they are the product of meiosis which involves recombination and segregation. Therefore, each pollen plant is genetically different from the other. The gametoclonal variants being hemizygous in nature expresses also the recessive characters in the R0 plants (Bhojwani and Razdan, 1996). Different sources of variation can explain gametoclonal variation such as new genetic variation induced by cell culture procedures, from segregation and independant assortment, chromosome doubling procedures etc (Morrison and Evans, 1987; Huang, 1996).
6.4. Induction of mutations
In general, majority of induced mutations are recessive and, therefore, are not expressed in diploid cells due to the presence of dominant allele. Since, haploid plants have only one set of chromosomes, their dominant and recessive characters can be seen simultaneously on separate plants. It is extremely advantageous to provide a convenient system for the induction of mutations and selection of mutants with desirable traits in the absence of their dominant counterparts (Bhojwani and Razdan, 1996)
6.5. Obtaining new genotypes with alien chromosomes
The technique of interspecific and intergeneric hybridization can be combined with anther culture techniques (Thomas et al., 2003) for obtaining new genotypes with alien chromosomes. Thus, new genotypes with various reconstructed chromosome complements can be obtained after their successful chromosome doubling.
6.6. Genetic manipulation
As microspore culture is a single cell system, it makes selection at the single cell level possible and, furthermore, offers new prospects for genetic manipulation like mutagenesis and transformation. Direct gene transfer by microinjection offers the possibility of transgenic plant formation by using isolated pollen culture having high regeneration efficiency (Kasha and Maluszynski, 2003). Moreover, if transgenes can be incorporated into the haploid microspore genome, prior to DNA synthesis and chromosome doubling, the doubled haploids may also be homozygous for the transgenes. Thus, isolated microspores not only provide a good target for bombardment but, also are readily amenable to transgene in vitro selection. Jahne et al. (1994) were the first to achieve plants homozygous for the transgenes using biolistic bombardment of barley microspores.
6.7. Genomics
Doubled haploids play a vital role in genomics, especially, in the integration of genetic and physical maps, thereby, providing precision in targeting candidate genes (Kunzel et al., 2000; Wang et al., 2001). Doubled haploids combined with marker assisted selection provides a short cut in backcross conversion, a plant breeding method for improving an elite line defective in a particular trait (Toojinda et al., 1998). Expressed sequence tags may help in identification of genes that determine any trait of interest.