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Homozygous Lines - The chromosome number of haploid plants is readily doubled by using colchicine; the plants so obtained are called doubled haploids (DH). DH plants are completely homozygous, and are produced in only two years.

Homozygous lines are regularly produced during breeding of both self and cross pollinated crops by selfing or close inbreeding; this approach takes at least 6 years.

Thus DH lines represent a saving of 4 years for obtaining homozygous lines, and this is the chief practical application of haploid plants. Haploid plants have been used in breeding programmes of rice, wheat, barley, Brassica spp. tobacco, potato, rye, etc. mainly in China, Japan, Canada and Europe, and resulted in many commercial varieties.

In general, anthers from F1 hybrids of selected or desirable crosses are cultured to produce haploids. In case of the H. bulbosum route, the F1 hybrids are crossed with H. bulbosum and the resulting haploid progeny are rescued by embryo culture.

The chromosome number of these haploids is then doubled to obtain homozygous doubled haploid plants, which are subjected to selection. This approach is often referred to as hybrid sorting, since the heterozygous gene combinations present in the hybrid are quickly resolved into homozygous combinations.

Hybrid sorting not only saves, valuable time in the production of homozygous lines, but it also improves the effectiveness of selection which is now based on homozygous genotypes only. A doubled haploid strain may be released directly as a variety if it, performs will in yield trials.

Alternatively, it may be ordered into hybridization programme as a parent if it has some useful characteristics to offer. The hybrid so obtained may again be subjected to hybrid sorting.

The performance of DH lines has been compared with those derived by pedigree selection (PS) or single seed descent (SSD; without selection) in barley, wheat, tobacco, etc. In general, DH lines are either comparable or inferior to SSD, but in some cases they may be as good as PS lines.

Further, the DH lines represent a nonrandom sample from the possible genotypes recoverable from the concerned hybrids, and there is a considerable unconscious selection, usually in the desirable direction, during the production of haploid plants. These conclusions apply equally well to both anther culture and H. bulosul routes of haploid production.
 

 
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