Double haploids : A promising tool in plant Breeding
Authors: Prashant Raghunath Shingote, Dipti Raghunath Dhumale

Sporophytes having gametic chromosome number famously known as haploids, while those having somatic chromosome number are called diploids. Haploid and doubled haploid plants have gametophytic (n) and sporophytic (2n) chromosome numbers. Haploid plants are usually derived from male or female gametic cells moreover, a haploid resulting from a diploid is known as monoploid, while a polyploid derived haploid called poly-haploid. The first haploid angiosperm was discovered in 1920, but effective application in breeding was given thought experimental induction of haploidy in Datura anoxia through anther culture (Guha and Maheswari, 1964). Since then, induction of haploidy has been reported in many species and extensively used in plant breeding programs (Dunwell, 2010; Asif, 2013). Considering the importance of haploidy, this method is becoming more admirable for plant biotechnologist and breeders. To cope up the demand for large-scale production of double haploids, handy and reliable procedures needs to be developed in various plant species. These efforts resulted in the discovery of different methods viz. parthenogenesis, sparse pollination, distant hybridization, and pollen irradiation (Asif, 2013).

Development of haploid in plants mainly depends on different factors like genotype, explant selection, medium composition, chromosome doubling, pretreatments etc. Recently used haploid production methods are androgenesis, gynogenesis parthenogenesis and distant hybridization. These methods are being explored in different crops for haploid production (Dunwell, 2010).

Androgenesis: A phenomenon of embryo or zygote development which encompasses only male parent chromosomes. In this method direct or indirect callusing is induced through inoculation of anthers or microspores.

Gynogenesis: A phenomenon of embryo development from an ovule, embryo sac or unfertilized ovary which encompasses only female parent chromosomes.

Parthenogenesis: A Phenomenon of the development of embryo through an asexual mode of reproduction. In this method, the embryo is developed via apogamy, pseudogamy or semigamy of the unfertilized egg cell.

Distant hybridization: A method of crossing between two distantly related species or genera. In this method donor parents chromosomes are removed due to nonhomology with recipient parent, thus the ensuing embryo hold chromosomes only from the recipient parent followed by doubling.

Application in Plant Breeding

Doubled haploids are competent and valuable research tool for acquiring fast homozygosity in a heterozygous offspring in a single step. Doubled haploid offers a great perspective in plants having the effective role in plant selection and other breeding programs.

Double haploid as a mapping population has immense consequences in generating a genetic map, molecular analysis, and study of plant genetics. Doubled haploid plants have been a valuable resource in a number of crop species for generation of recombinant inbred lines and inbred lines for hybrid production, also useful for identification of different molecular markers, gene mapping, and finding of QTLs. Double haploids are more helpful and effective in the identification of polygenic traits which are difficult to identify through single plant basis, creation and study of fixed traits.

Similar to self-pollinated plants use of haploidy in cross-pollinated diploid crops is also based on the use of Double haploid lines. Nevertheless, due to inbreeding depression, only parental inbred lines are used for the production of hybrids. The barriers for repeated self-pollination in cross-pollinated plants has been overcome through haploids production. Owning to haploid production the time duration is considerably reduced in biennial and long duration crops. This is the only way for developing haploidy inbred lines in these crops. Shen et al. in autotetraploid Chinese cabbage reported inoculation of isolated microspore culture for obtaining primary trisomics (Shen et al. 2006).

The breeding programs were practically used in production Double haploid lines which have widespread application in identification of SNP and quantitative genetics (Pink et al. 2008, Trick et al. 2009). Specific recent examples of the utilisation of Double haploid lines in QTL mapping in plants like maize, barley, rice and wheat for different agronomical traits are reviewed in Dwivedi et al. (2015).

Haploids are also incredibly valuable in genome sequencing. Several plant genome sequencing projects were based on the assembly of haploid genomes. For example, use of ‘Lovell’ a peach haploid cultivar for construction physical maps and genome sequencing. In Coffea (C. canephora) double haploid genotype was used for physical mapping and construction of two BAC libraries (as reviewed in Dunwell, 2010). International Citrus Genomic Consortium selected in situ gynogenesis induced clementine haploid as a reference line for sequencing.

Hybrid seed production depends on the availability of inbred lines. Uniformity in inbred lines is useful in developing hybrids with uniform traits and performance. Inbreeds can also be used in developing synthetics and composites. In cross-pollinated crops like onion, there is a high degree of heterozygosity and severe inbreeding depression. Because of inbreeding depression, the inbred lines thus developed remain quite heterozygous and allow changes in genetic constitution over time by drift or selection which makes breeding for varieties or hybrids with uniform traits for yield, quality and colour become difficult. Hence, developing inbreds through in vitro haploid production will be a method of choice to obtain homozygous pure lines.

References:

1. Asif, Muhammad. Progress and opportunities of doubled haploid production. No. QK981. A85. Springer, 2013.
2. Dunwell, Jim M. "Haploids in flowering plants: origins and exploitation." Plant Biotechnology Journal 8.4 (2010): 377-424.
3. Dwivedi, Sangam L., et al. "Haploids: constraints and opportunities in plant breeding." Biotechnology advances 33.6 (2015): 812-829.
4. Guha, Sipra, and S. C. Maheshwari. "In vitro production of embryos from anthers of Datura." Nature 204 (1964): 497.
5. Pink, David, et al. "Double haploids, markers and QTL analysis in vegetable brassicas." Euphytica 164.2 (2008): 509-514.
6. Shuxing, Shen, Hou Xilin, and Zhang Chenghe. "A study on obtaining primary trisomics by the isolated microspore culture of autotetraploid Chinese cabbage." Acta Horticulturae Sinica 33.6 (2006): 1209.



About Author / Additional Info:
I have completed my graduation and post graduation in Agricultural biotechnology. From last 4-5 years I have been working on different aspects of plant molecular genetics and functional genomics. I have published nearby 10 publication maximum of which are in peer reviewed journals having impact factors.