MICROSPORES AND THEIR APPLICATIONS IN BASIC AND APPLIED PLANT SCIENCES pp. 217-234
Authors: (Mehran E. Shariatpanahi, Alisher Touraev, Max F. Perutz Laboratories, Vienna University, Vienna, Austria, and others)
Abstract: Haploid is the general term for plants (sporophytes) that contain the gametic chromosome number (n). The haploid could also be called either monoploid (x) in a diploid sporophytic (2n) species in which only one set of chromosomes exists or polyhaploids in polyploidy species as they have more than one set of chromosomes. The haploid plant obtained from an autotetraploid (4x) with four sets of one genome was originally called a dihaploid (because 2n=2x). When the chromosome number of a haploid is doubled, it is called a doubled haploid (DH). It should be mentioned that a doubled haploid is different from dihaploid. The dihaploid is not homozygous since it represents two chromosome sets selected from four sets in the autotetraploid, whereas the doubled haploid from a monoploid or an allohaploid should be completely homozygous [Kasha and Maluszynski 2003]. The main advantage of doubled haploids in breeding is the reduction of the time required to develop new cultivars [Snape et al., 1986; Touraev et al., 2001; Thomas et al., 2003]. For annual self-pollinated crops it generally takes 10-15 years to produce a cultivar through a conventional plant breeding program such as the pedigree method, which includes selfing and subsequent selection. The time delay is costly and prevents breeders from responding rapidly to end users need. The production of haploids followed by chromosome doubling to produce homozygous lines, from which superior lines are selected, can reduce the time required for cultivar development by 3-4 years. For cross-pollinated heterozygous crops, doubled haploids are a rapid method to produce homozygous pure breeding lines, which can be used in the development of synthetic varieties or hybrids. The production of doubled haploids can improve selection efficiency as the phenotype of the plant is not masked by dominance effects. Traits encoded by recessive genes can be easily identified. A smaller population of doubled haploids is required when screening for desirable recombinants than would be the case for conventional diploid populations [Touraev et al., 2001].Over 200 varieties have been produced by deploying various DH methods [Thomas et al., 2003]. The vast majority of varieties derived from doubled haploidy are in barley (96), followed by rapeseed (47), wheat (20) and the rest [Thomas et al., 2003].