Doubled Haploid Technology in Maize Breeding
Shailendra K Jha, Scientist (Plant Breeding) at ICAR-IARI, New Delhi, Plant Breeder
Lekshmy S, Scientist (Plant Physiology) at ICAR-IARI, New Delhi, Plant Physiologist working in the area of nitrogen use efficiency of crop plants.
Kumar Durgesh, Scientist (Plant Breeding) at ICAR-IARI, New Delhi, Plant Breeder
Maize is one of the major cultivated cereal crops in the world having very high productivity. The yield potential and productivity of maize is high compared to other cereals because of systematic exploitation of heterosis. The maximum exploitation of heterosis can be perpetuated through use of single cross hybrid, having a prerequisite of superior inbred lines. The conventional method of inbred line development by combing superior parents or extraction of inbred from unimproved/improved populations require several generations of selfing to achieve homozygosity. The uniformity of parental lines is one of the key requirement to have uniform hybrids. The use of doubled haploid in breeding, which gives complete homozygosity on chromosome doubling of haploid is having several advantages. It gives completely uniform and homozygous parental line just after seed setting in doubled haploid plant.
Doubled haploid line in maize can be produced through in vitro or in vivo methods. In vitro method of double haploid production can be worked through culture of haploid tissues like anther, pollen or ovule. The in vitro methods require sophisticated laboratory and skilled human resource. In addition, all the genotypes not respond through this method because of totipotency. So, the method is genotype specific. In contrast to in vitro method, in vivo method uses haploid inducer lines (HILs) for production of haploid. HILs are special genetic stocks, which upon crossing with diploid plats gives haploid embryo in normal seed. Initially the haploid induction was dependent on spontaneous haploid production. Identification of “Stock 6”, a haploid inducer line with 2-3% of haploid induction rate has boosted the incorporation of DH technique in maize breeding.
The DH production through in vivo method initiates with recombination of parental lines followed by pollination by pollen from HILs. The resulting seeds are screened for presence of haploid embryo containing normal seeds. The haploid seeds are identified by screening for morphological traits like pigmentation in seed, coleoptile or root. Recently a high throughput method based on oil content has been also been designed for rapid identification of haploid seed. The haploid seed after germination treated with chromosome doubling agents like colchicine. Each D 0 plant (developed from haploid embryo) upon chromosome doubling followed by selfing gives seed of D1 lines. Each D1 line will give a different inbred parent, which after evaluation and identification can be used as superior parent in hybrid development.
The success of DH program depends on haploid induction rate and pollen production ability of HILs. Recently HILs with high rate of haploid induction has been identified which are used in development of haploid induction hybrids with high pollen production ability. Further, the frequency of superior and desirable doubled haploid parental lines can be enhanced through haploid induction in selected superior F2 plants or in a superior recombinant plant from improved population.
The use of DH in parental line development compress the breeding cycle to 3-4 season to get sufficient seed of a fixed and completely homozygous line to be incorporated in hybrid breeding program. This saves the several generations of logistics and time required for selfing of segregating lines for development of near homozygous inbred lines through conventional method. In addition, the inbred lines developed through DH fulfills the perfect requirement of DUS because of complete homozygosity.
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